The CHARA Array has added a 7th telescope to extend the existing 6 telescope array. The CHARA Michelson Array Pathfinder (CMAP) includes a 1m Planewave RC Telescope mounted in a custom designed mobile trailer and pier system. The telescope and trailer can be placed at multiple locations around the Mount Wilson Observatory site; each site consisting of a flat concrete pad with a novel pier design. Optical fibers will connect each site to the CHARA optical delay and combiner lab. This enables new short baselines of ∼17m for imaging the surfaces of large stars and new long baselines on the order of ∼600m for resolving small stars. There are two sites developed at the array for this telescope. In the future, there are plans to expand the array to greater than 1 km maximum baselines. These baselines will be used in conjunction with the existing 15 baselines that range from 34 to 331m. Moving such a telescope around the observatory presents some unique challenges. The telescope can make use of the same optical delay lines and beam combiners as the other CHARA Array telescopes.
KEYWORDS: Cameras, Telescopes, Sensors, Stars, Data acquisition, Signal to noise ratio, K band, Mirrors, Design, Infrared cameras, Data acquisition boards, Astronomical telescopes
The newly installed Silmaril beam combiner at the CHARA array is designed to observe previously inaccessible faint targets, including Active Galactic Nuclei and T-Tauri Young Stellar Objects. Silmaril leverages cuttingedge optical design, low readout noise, and a high-speed C-RED1 camera to realize its sensitivity objectives. In this presentation, we offer a comprehensive overview of the instrument’s software, which manages critical functions, including camera data acquisition, fringe tracking, automatic instrument alignment, and observing interfaces, all aimed at optimizing on-sky data collection. Additionally, we offer an outline of the data reduction pipeline, responsible for converting raw instrument data products into the final OIFITS used by the standard interferometry modeling software. Finally, a thorough analysis of the camera and instrument characterization results will be presented, evaluating instrument performance in terms of sensitivity. The purpose of this paper is to provide a solid reference for studies based on Silmaril data.
Silmaril is a new 3-telescope beam combiner at the CHARA Array. In this presentation, we present the current design of the instrument, its on-sky measured and theoretical best performance, and its future development. Its design is specifically made to push for sensitivity. It combines 3 beams of the CHARA Array, allowing closure phase measurement but limiting the loss in sensitivity. It limits the number of optical elements to the minimum needed for combining the 3 beams. Using a CRED ONE camera allows for sub-electron readout noise, which means that our sensitivity is limited by the thermal background. Long focal-length cylindrical mirrors limit the thermal background by using an f/20 cold stop in front of the camera. We use a low spectral resolution prism to allow fringe tracking without compromising the instrument’s sensitivity. Thanks to an ingenious edge filter design, we can observe both H- and K-band simultaneously, with a low thermal background on the H-band side of the detector. In the future, we intend to extend the instrument with a second set of 3 beams and add a Narcissus mirror to lower the thermal background and improve the sensitivity even more.
The Center for High Angular Resolution Astronomy (CHARA) Array is a six-element interferometer with baselines ranging from 34 to 331m. The Array has had many upgrades in recent years including new beam combiners: MYSTIC is a 6T combiner for K-band; SPICA is a 6T combiner for the visible R-band; and SILMARIL is a 3T combiner for high sensitivity in the H- and K-bands. A seventh, mobile telescope is now on site for use with fiber optics for beam transport. Observing time is available to the community through a program funded by NSF. The observing programs are solicited and peer-reviewed by NSF’s NOIRLab. Here we summarize the scientific work and the on-going technical advances of the CHARA Array.
The Center for High Angular Resolution Astronomy (CHARA) Array currently consists of 6 telescopes at fixed positions, connected by vacuum pipes to the delay lines. The CHARA Michelson Array Pathfinder (CMAP) project includes two major components: 1) a mobile telescope that can be placed at a number of locations, and 2) a fiber optic relay system to transport light to the beam combining facility. The telescope will be equipped with a custom-built instrument bench with adaptive optics and fiber injection. The light will be transported by optical fibers to the existing CHARA delay lines. In this contribution, we present the progress on the various subsystems needed to integrate the new telescope and beam transport method into the existing CHARA environment. We will also describe our efforts to find internal fringes with the new fibers.
We report progress on Project Prime (PRecision Interferometry with MIRC for Exoplanets) to detect exoplanets using precision closures using MIRC-X and MYSTIC at CHARA. Our investigations include modeling systematics caused by OPD drifts, differential dispersion, beamtrain birefringence, and flatfielding errors. Injection tests suggest we can recover hot Jupiter companions as faint at 1/5000 of the host star brightness with 4 nights of observing and we will present some results of our recent searches for the hot Jupiters. Our upper limits are starting to constrain current-generation Global Circulation Models (GCMs). We propose the addition of modest nulling (10:1) to today’s interferometers in order to vastly increase the ease of this work and to open up many more targets for detections.
MIRC-X and MYSTIC are six-telescope near-infrared beam (1.08-2.38μm) combiners at the CHARA Array on Mt Wilson CA, USA. Ever since the commissioning of MIRC-X (J and H bands) in 2018 and MYSTIC (K bands) in 2021, they have been the most popular and over-subscribed instruments at the array. Observers have been able to image stellar objects with sensitivity down to 8.1mag in H and 7.8mag in K-band under the very best conditions. In 2022 MYSTIC was upgraded with a new ABCD mode using the VLTI/GRAVITY 4-beam integrated optics chip, with the goal of improving the sensitivity and calibration. The ABCD mode has been used to observe more than 20T Tauri stars; however, the data pipeline is still being developed. Alongside software upgrades, we detail planned upgrades to both instruments in this paper. The main upgrades are: 1) Adding a motorized filter wheel to MIRC-X along with new high spectral resolution modes 2) Updating MIRC-X optics to allow for simultaneous 6T J+H observations 3) Removing the warm window between the spectrograph and the warm optics in MYSTIC 4) Adding a 6T ABCD mode to MIRC-X in collaboration with CHARA/SPICA 5) Updating the MIRC-X CRED-ONE camera funded by Prof. Kraus from U. Exeter 6) Carrying out science verification of the MIRC-X polarization mode 7) Developing new software for ABCD-mode data reduction and more efficient calibration routines. We expect these upgrades to not only improve the observing experience, but also increase the sensitivity by 0.4mag in J+H-bands, and 1mag in K-band.
The Michigan Young Star Imager at CHARA (MYSTIC) is a K-band interferometric beam combining instrument funded by the U.S. National Science Foundation, designed primarily for imaging sub-au scale disk structures around nearby young stars and to probe the planet formation process. Installed at the CHARA Array in July 2021, with baselines up to 331 m, MYSTIC provides a maximum angular resolution of λ / 2B ∼ 0.7 mas. The instrument injects phase-corrected light from the array into inexpensive, single-mode, polarization maintaining silica fibers, which are then passed via a vacuum feedthrough into a cryogenic dewar operating at 220 K for imaging. MYSTIC uses a high frame rate, ultra-low read noise SAPHIRA detector and implements two beam combiners: a six-telescope image plane beam combiner, based on the MIRC-X design, for targets as faint as 7.7 Kmag, as well as a four-telescope integrated optic beam-combiner mode using a spare chip leftover from the GRAVITY instrument. MYSTIC is co-phased with the MIRC-X (J + H band) instrument for simultaneous fringe-tracking and imaging and shares its software suite with the latter to allow a single observer to operate both instruments. We present the instrument design, review its operational performance, present early commissioning science observations, and propose upgrades to the instrument that could improve its K-band sensitivity to 10th magnitude in the near future.
MIRC-X and MYSTIC are six telescope all-in-one beam combiners at the CHARA array, Mount Wilson Observatory, USA. These two instruments make simultaneous cophasing observations in J + H + K-band wavelengths and deliver λ/2B~0.5 mas angular resolution imaging utilizing the world's largest baseline lengths ranging from 30-330 m. MIRC-X and MYSTIC are operated in two observing modes based on the target spectrum and their over-resolved nature: (i) primary-secondary and (ii) combined mode. In the primary-secondary mode, we use one instrument as a fringe tracker and the other as a science combiner to enable high spectral resolution interferometry. Higher visibility fringes are selected depending on their baseline length and wavelengths for robust fringe tracking for over-resolved objects in the combined mode. We installed four accelerometers to study vibrations on the beam combiner table to improve fringe tracking. We here report the instrument concepts and the first on-sky science results.
With a possible angular resolution down to 0.1-0.2 millisecond of arc using the 330 m baselines and the access to the 600-900 nm spectral domain, the CHARA Array is ideally configured for focusing on precise and accurate fundamental parameters of stars. CHARA/SPICA (Stellar Parameters and Images with a Cophased Array) aims at performing a large survey of stars all over the Hertzsprung-Russell diagram. This survey will also study the effects of the different kinds of variability and surface structure on the reliability of the extracted fundamental parameters. New surface-brightness-colour relations will be extracted from this survey, for general purposes on distance determination and the characterization of faint stars. SPICA is made of a visible 6T fibered instrument and of a near-infrared fringe sensor. In this paper, we detail the science program and the main characteristics of SPICA-VIS. We present finally the initial performance obtained during the commissioning.
Optical interferometry is a powerful technique to achieve high angular resolution. However, its main issue is its lack of sensitivity, compared to other observation techniques. Efforts have been made in the previous decade to improve the sensitivity of optical interferometry, with instruments such as PIONIER and GRAVITY at VLTI, or MIRC-X and MYSTIC at CHARA. While those instruments pushed on sensitivity, their design focus was not the sensitivity but relative astrometric accuracy, imaging capability, or spectral resolution. Our goal is to build an instrument specifically designed to optimize for sensitivity. This meant focusing our design efforts on different parts of the instrument and investigating new technologies and techniques. First, we make use of the low noise C-RED One camera using e-APD technology and provided by First Light Imaging, already used in the improvement of sensitivity in recent new instruments. We forego the use of single-mode fibers but still favor an image plane design that offers more sensitivity than a pupil plane layout. We also use a minimum number of optical elements to maximize the throughput of the design, using a long focal length cylindrical mirror. We chose to limit our design to 3 beams, to have the capability to obtain closure phases, but not dilute the incoming flux in more beam combinations. We also use in our design an edge filter to have the capability to observe Hand K-band at the same time. We use a low spectral resolution, allowing for group delay fringe tracking but maximizing the SNR of the fringes for each spectral channel. All these elements will lead to a typical limiting magnitude between 10 and 11 in both H- and K-bands.
We report the ultrafast laser inscription (ULI) of a 2-telescope integrated optic (IO) beam combiner for K-band interferometry in commercial Infrasil glass. The ULI setup used for this work is based on a 1030 nm femtosecond laser which is paired with a spatial-light-modulator (SLM). The SLM controls the numerical aperture of the focused beam used to write waveguides in the substrate. The optimum ULI parameters were found to inscribe straight single-mode waveguides exhibiting an insertion loss of 1.1 ± 0.1 dB for a 17 mm long chip over the entire K-band. To develop optimal directional couplers, we focused our efforts on investigating the effect of varying the core-to-core separation and the effect of detuning the waveguide parameters in the coupler. By doing so, we have identified fabrication parameters that are suitable for the fabrication of a beam combiner integrating an achromatic 3 dB directional coupler and two photometric taps with a splitting ratio of 80:20. These results demonstrate the capability of the ULI fabrication technique to inscribe efficient achromatic directional couplers in the K-band range. A final fabrication step will involve simple assembly of the beam combiner with input/output fibers in preparation for on-sky testing at the CHARA array planned for July 2022.
The Michigan Young STar Imager at CHARA (MYSTIC) is a K-band interferometric beam combining instrument funded by the United States National Science Foundation, designed primarily for imaging sub-au scale disk structures around nearby young stars and to probe the planet formation process. Installed at the CHARA array in July 2021, with baselines up to 331 meters, MYSTIC provides a maximum angular resolution of λ/2B ∼ 0.7 mas. The instrument injects phase corrected light from the array into inexpensive, single-mode, polarization maintaining silica fibers, which are then passed via a vacuum feedthrough into a cryogenic dewar operating at 220 K for imaging. MYSTIC utilizes a high frame rate, ultra-low read noise SAPHIRA detector, and implements two beam combiners: a 6-telescope image plane beam combiner, based on the MIRC-X design, for targets as faint as 7.7 Kmag, as well as a 4-telescope integrated optic beam-combiner mode using a spare chip leftover from the GRAVITY instrument. MYSTIC is co-phased with the MIRC-X (J+H band) instrument for simultaneous fringe-tracking and imaging, and shares its software suite with the latter to allow a single observer to operate both instruments. Herein, we present the instrument design, review its operational performance, present early commissioning science observations, and propose upgrades to the instrument that could improve its K-band sensitivity to 10th magnitude in the near future.
Much research has been done to show the possibilities of using long transport fibers in optical interferometry. The CHARA Michelson Array Pathfinder will extend the spatial coverage of the CHARA Array by adding a mobile 1-meter telescope connected by optical fibers. The pathfinder will operate in H-band and will explore baselines up to approximately 1 km, giving an angular resolution of 0.2 mas. The new telescope will be placed at short baselines to image the surfaces of large stars and at long baselines to resolve small stars. Here we describe the project and our progress on various subsystems.
SPICA-FT is part of the CHARA/SPICA instrument which combines a visible 6T fibered instrument (SPICAVIS) with a H-band 6T fringe sensor. SPICA-FT is a pairwise ABCD integrated optics combiner. The chip is installed in the MIRC-X instrument. The MIRC-X spectrograph could be fed either by the classical 6T fibered combiner or by the SPICA-FT integrated optics combiner. SPICA-FT also integrates a dedicated fringe tracking software, called the opd-controller communicating with the main delay line through a dedicated channel. We present the design of the integrated optics chip, its implementation in MIRC-X and the software architecture of the group-delay and phase-delay control loops. The final integrated optics chip and the software have been fully characterized in the laboratory. First on-sky tests of the integrated optics combiner began in 2020. We continue the on-sky tests of the whole system (combiner + software) in Spring and Summer 2022. We present the main results, and we deduce the preliminary performance of SPICA-FT.
In this thesis work, we exploit the unique capabilities of long baseline interferometry to fill two gaps in exoplanet parameter space: 1) the discovery of new planets around stars more massive than the Sun (Project ARMADA), and 2) the characterization of known planets that are extremely close to their host star (Project PRIME). Current detection methods struggle to find exoplanets around hot (A/B-type) stars. We are pushing the astrometric limits of ground-based optical interferometers to carry out a survey of sub-arcsecond A/B-type binary systems with ARMADA. We are achieving astrometric precision at the few tens of micro-arcsecond level in short observations at CHARA/MIRC-X and VLTI/GRAVITY. This incredible precision allows us to probe the au-regime for giant planets orbiting individual stars of the binary system. We present the status of our survey, including our newly implemented etalon wavelength calibration method at CHARA, detection of new stellar mass companions, and non-detection limits down to a few Jupiter masses in some cases. With Project PRIME, we show that ground-based optical interferometry can be used to measure the orbit-dependent spectra of close-in “hot Jupiter”-type exoplanets with precision closure phases. Detecting the infrared spectra of such planets allows us to place useful constraints on atmosphere circulation models. We perform injection tests with MIRC-X and MYSTIC at CHARA for the hot Jupiter exoplanet Ups And b to show that we are reaching down to a contrast of 2e-4. The promise of both these methods demonstrate that optical interferometers are a valuable tool for probing unique regimes of exoplanet science.
The Center for High Angular Resolution Astronomy (CHARA) Array is a six-element interferometer with baselines ranging from 34 to 331 m. Three new beam combiners are entering operation: MYSTIC is a 6-telescope combiner for K-band; SPICA is a 6-telescope combiner for the visible R-band; and SILMARIL is a 3-telescope combiner for high sensitivity in H and K-bands. A seventh, portable telescope will use fiber optics for beam transport and will increase the baselines to 1 km. Observing time is available through a program funded by NSF. The programs are solicited and peer-reviewed by NSF’s National Optical-Infrared Astronomy Research Laboratory. The open community access has significantly expanded the range of astronomical investigations of stars and their environments. Here we summarize the scientific work and the on-going technical advances of the CHARA Array.
We present a new polarimetric mode for the MIRC-X 6-telescope beam combiner at CHARA. Utilizing the extensive u − v coverage afforded by CHARA this mode will be able to resolve and constrain scattered light in environs at milliarcsecond separations of target stars, a largely unexplored parameter space to-date in astronomy. Notably, this upgrade will allow for the investigation of the scattering properties of the inner dust wall at the sublimation radius of Herbig Ae/Be star disks, dust shells surrounding evolved stars, and gas-rich disks around Be stars. Our design adds a series of rotating half-wave plates, achromatic across J- and H-bands, and a polarizing beamsplitter into the MIRC-X beam path. In this work, we also preview on-sky observations, discussing ongoing work calibrating instrumental polarization effects in the CHARA beam path as well as upgrades to the MIRC-X data reduction pipeline.
The CHARA Array is an optical/infrared interferometer that combines the light from six 1-meter telescopes. With baselines ranging from 34 to 331 meters, CHARA provides sub-milliarcsecond resolution to measure stellar diameters, image stellar surfaces, resolve close binary companions, and study circumstellar environments. In this paper, we present recent highlights from the CHARA Array, focusing on the implementation of adaptive optics, the ongoing development of next generation beam combiners, an update on the community access program, and a discussion about future developments.
In this contribution we report on our work to increase the spectral range of the Michigan Infrared Combiner- eXeter (MIRC-X) instrument at the CHARA array to allow for dual H and J band interferometric observations. We comment on the key science drivers behind this project and the methods of characterisation and correction of instrumental birefringence and dispersion. In addition, we report on the first results from on-sky commissioning in November 2019.
The CHARA Array is the longest baseline optical interferometer in the world. Operated with natural seeing, it has delivered landmark sub-milliarcsecond results in the areas of stellar imaging, binaries, and stellar diameters. However, to achieve ambitious observations of faint targets such as young stellar objects and active galactic nuclei, higher sensitivity is required. For that purpose, adaptive optics are developed to correct atmospheric turbulence and non-common path aberrations between each telescope and the beam combiner lab. This paper describes the AO software and its integration into the CHARA system. We also report initial on-sky tests that demonstrate an increase of scientific throughput by sensitivity gain and by extending useful observing time in worse seeing conditions. Our 6 telescopes and 12 AO systems with tens of critical alignments and control loops pose challenges in operation. We describe our methods enabling a single scientist to operate the entire system.
We describe versatile turbulence simulator for testing and calibration of new techniques for high-resolution imaging of objects outside the Earth’s atmosphere using ground-based instrumentation. Examples here include: dynamic aperture diversity, wave front sensing using multi-aperture phase retrieval, and free-space beam propagation for rapidly re-configurable interferometers. Used in the testing of all of these, the simulator uses a high resolution spatial light modulator in tandem with a lower resolution deformable mirror to simulate atmospheric phase distortions over a wide range of turbulence conditions.
We describe a new concept for future interferometric observations. Our laboratory experiment simulates an interferometer with two telescopes observing through different volumes of atmospheric turbulence. We simulate both vertical and horizontal propagation through the atmosphere; the latter mimics free-space beam propagation without the need for vacuum pipes or fiber optics. Practically, we simulate the effects of atmospheric turbulence using numerically calculated Kolmogorov phase screens injected onto a spatial light modulator. We correct this distorted wavefront using an adaptive optics system to determine the range of turbulence conditions over which we can detect fringes from the two telescopes. Our experiment lays the groundwork for investigating the potential of having movable telescopes in which light propagates from the telescopes to the beam combiner through free space.
CHARA/SPICA (Stellar Parameters and Images with a Cophased Array) is currently being developed at Observatoire de la Cote d’Azur. It will be installed at the visible focus of the CHARA Array by the end of 2021. It has been designed to perform a large survey of fundamental stellar parameters with, in the possible cases, a detailed imaging of the surface or environment of stars. To reach the required precision and sensitivity, CHARA/SPICA combines a low spectral resolution mode R = 140 in the visible and single-mode fibers fed by the AO stages of CHARA. This setup generates additional needs before the interferometric combination: the compensation of atmospheric refraction and longitudinal dispersion, and the fringe stabilization. In this paper, we present the main features of the 6-telescopes fibered visible beam combiner (SPICA-VIS) together with the first laboratory and on-sky results of the fringe tracker (SPICA-FT). We describe also the new fringe-tracker simulator developed in parallel to SPICA-FT.
We report the ultrafast laser inscription (ULI) and characterization of 3 dB directional achromatic couplers for K-band between 2 and 2.4 μm. The couplers were fabricated in commercial Infrasil glass using 1030 nm femtosecond laser pulses. Straight waveguides inscribed using optimal fabrication parameters exhibit an average propagation loss of ∼1.21 dB over full range of K-band with a single-mode behavior for a length of 17 mm. Directional couplers with different interaction lengths and waveguide widths were fabricated and characterized. We demonstrate that 3 dB achromatic directional couplers for K-band can be fabricated using ULI. These results show that ULI can fabricate highquality couplers for future applications in astronomical interferometry. Our eventual aim is to develop a two-telescope K-band integrated optical beam combiner to replace JouFLU at CHARA.
MIRC-X is a six telescope beam combiner at the CHARA array that works in J and H wavelength bands and provides an angular resolution equivalent to a B=331m diameter telescope. The legacy MIRC combiner has delivered outstanding results in the fields of stellar astrophysics and binaries. However, we required higher sensitivity to make ambitious scientific measurements of faint targets such as young stellar objects, binary systems with exoplanets, and active galactic nuclei. For that purpose, MIRC-X is built and is offered to the community since mid-2017. MIRC-X has demonstrated up to two magnitudes of improved faint magnitude sensitivity with the best-case H <= 8. Here we present a review of the instrument and present early science results, and highlight some of our ongoing science programs.
The CHARA array is an optical/near infrared interferometer consisting of six 1-meter diameter telescopes, the longest baseline of which is 331 meters. With sub-millisecond angular resolution, the CHARA array is able to spatially resolve nearby stellar systems and reveal their detailed structures. To improve the sensitivity and scientific throughput, the CHARA array was funded by NSF-ATI in 2011 for an upgrade of adaptive optics (AO) systems to all six telescopes. This first grant covered Phase I of the adaptive optics system, which includes an on-telescope Wavefront Sensor (WFS) and non-common-path (NCP) error correction. Phase II of the program was funded by the NSF/MRI in 2016, and includes purchasing and installing the deformable mirrors at each telescope to complete the system. In this paper we will discuss both phases of the program, how the challenge of AO differs for interferometry, and the first results of the full system.
We present the design for MYSTIC, the Michigan Young STar Imager at CHARA. MYSTIC will be a K-band, cryogenic, 6-beam combiner for the Georgia State University CHARA telescope array. The design follows the image-plane combination scheme of the MIRC instrument where single-mode fibers bring starlight into a nonredundant fringe pattern to feed a spectrograph. Beams will be injected in polarization-maintaining fibers outside the cryogenic dewar and then be transported through a vacuum feedthrough into the 220K cold volume where combination is achieved and the light is dispersed. We will use a C-RED One camera (First Light Imaging) based on the eAPD SAPHIRA detector to allow for near-photon-counting performance. We also intend to support a 4-telescope mode using a leftover integrated optics component designed for the VLTI-GRAVITY experiment, allowing better sensitivity for the faintest targets. Our primary science driver motivation is to image disks around young stars in order to better understand planet formation and how forming planets might in influence disk structures.
MIRC-X is an upgrade of the six-telescope infrared beam combiner at the CHARA telescope array, the world's largest baseline interferometer in the optical/infrared, located at the Mount Wilson Observatory in Los Angeles. The upgraded instrument features an ultra-low noise and fast frame rate infrared camera (SAPHIRA detector) based on e-APD technology. We report the MIRC-X sensitivity upgrade work and first light results in detail focusing on the detector characteristics and software architecture.
The CHARA Array is a six-element, optical/NIR interferometer, which currently has the largest operational baselines in the world. The Array is operated by Georgia State University and is located at the Mount Wilson Observatory in California. The Array thrives thanks to members of the CHARA consortium that includes LESIA (Observatoire de Paris), Observatoire de la Cote dAzur, University of Michigan, Sydney University, Australian National University, and University of Exeter. Here we give a brief introduction to the Array infrastructure with a focus on a developing Adaptive Optics (AO) program, the new community access program funded by the NSF, and recent science results.
The Planet Formation Imager (PFI) is a near- and mid-infrared interferometer project with the driving science goal of imaging directly the key stages of planet formation, including the young proto-planets themselves. Here, we will present an update on the work of the Science Working Group (SWG), including new simulations of dust structures during the assembly phase of planet formation and quantitative detection efficiencies for accreting and non-accreting young exoplanets as a function of mass and age. We use these results to motivate two reference PFI designs consisting of a) twelve 3m telescopes with a maximum baseline of 1.2km focused on young exoplanet imaging and b) twelve 8m telescopes optimized for a wider range of young exoplanets and protoplanetary disk imaging out to the 150K H2O ice line. Armed with 4 x 8m telescopes, the ESO/VLTI can already detect young exoplanets in principle and projects such as MATISSE, Hi-5 and Heimdallr are important PFI pathfinders to make this possible. We also discuss the state of technology development needed to make PFI more affordable, including progress towards new designs for inexpensive, small field-of-view, large aperture telescopes and prospects for Cubesat-based space interferometry.
We present the recent developments preparing the construction of a new visible 6T beam combiner for the CHARA Array, called SPICA. This instrument is designed to achieve a large survey of stellar parameters and to image surface of stars. We first detail the science justification and the general idea governing the establishment of the sample of stars and the main guidance for the optimization of the observations. After a description of the concept of the instrument, we focus our attention on the first important aspect: optimizing and stabilizing the injection of light into single mode fibers in the visible under partial adaptive optics correction. Then we present the main requirements and the preliminary design of a 6T-ABCD integrated optics phase sensor in the H-band to achieve long exposures and reach fainter magnitudes in the visible.
MIRC-X is a new beam combination instrument at the CHARA array that enables 6-telescope interferometric imaging on object classes that until now have been out of reach for milliarcsecond-resolution imaging. As part of an instrumentation effort lead by the University of Exeter and University of Michigan, we equipped the MIRC instrument with an ultra-low read-noise detector system and extended the wavelength range to the J and H- band. The first phase of the MIRC-X commissioning was successfully completed in June 2017. In 2018 we will commission polarisation control to improve the visibility calibration and implement a 'cross-talk resiliant' mode that will minimise visibility cross-talk and enable exoplanet searches using precision closure phases. Here we outline our key science drivers and give an overview about our commissioning timeline. We comment on operational aspects, such as remote observing, and the prospects of co-phased parallel operations with the upcoming MYSTIC combiner.
The Planet Formation Imager (PFI) project aims to image the period of planet assembly directly, resolving structures as small as a giant planet's Hill sphere. These images will be required in order to determine the key mechanisms for planet formation at the time when processes of grain growth, protoplanet assembly, magnetic fields, disk/planet dynamical interactions and complex radiative transfer all interact - making some planetary systems habitable and others inhospitable. We will present the overall vision for the PFI concept, focusing on the key technologies and requirements that are needed to achieve the science goals. Based on these key requirements, we will define a cost envelope range for the design and highlight where the largest uncertainties lie at this conceptual stage.
The Planet Formation Imager (PFI) Project has formed a Technical Working Group (TWG) to explore possible facility architectures to meet the primary PFI science goal of imaging planet formation in situ in nearby starforming regions. The goals of being sensitive to dust emission on solar system scales and resolving the Hill-sphere around forming giant planets can best be accomplished through sub-milliarcsecond imaging in the thermal infrared. Exploiting the 8-13 micron atmospheric window, a ground-based long-baseline interferometer with approximately 20 apertures including 10km baselines will have the necessary resolution to image structure down 0.1 milliarcseconds (0.014 AU) for T Tauri disks in Taurus. Even with large telescopes, this array will not have the sensitivity to directly track fringes in the mid-infrared for our prime targets and a fringe tracking system will be necessary in the near-infrared. While a heterodyne architecture using modern mid-IR laser comb technology remains a competitive option (especially for the intriguing 24 and 40μm atmospheric windows), the prioritization of 3-5μm observations of CO/H2O vibrotational levels by the PFI-Science Working Group (SWG) pushes the TWG to require vacuum pipe beam transport with potentially cooled optics. We present here a preliminary study of simulated L- and N-band PFI observations of a realistic 4-planet disk simulation, finding 21x2.5m PFI can easily detect the accreting protoplanets in both L and N-band but can see non-accreting planets only in L band. We also find that even an ambitious PFI will lack sufficient surface brightness sensitivity to image details of the fainter emission from dust structures beyond 5 AU, unless directly illuminated or heated by local energy sources. That said, the utility of PFI at N-band is highly dependent on the stage of planet formation in the disk and we require additional systematic studies in conjunction with the PFI-SWG to better understand the science capabilities of PFI, including the potential to resolve protoplanetary disks in emission lines to measure planet masses using position-velocity diagrams. We advocate for a specific technology road map in order to reduce the current cost driver (telescopes) and to validate high accuracy fringe tracking and high dynamic range imaging at L, M band. In conclusion, no technology show-stoppers have been identified for PFI to date, however there is high potential for breakthroughs in medium-aperture (4-m class) telescopes architecture that could reduce the cost of PFI by a factor of 2 or more.
The Planet Formation Imager (PFI) project aims to provide a strong scientific vision for ground-based optical astronomy beyond the upcoming generation of Extremely Large Telescopes. We make the case that a breakthrough in angular resolution imaging capabilities is required in order to unravel the processes involved in planet formation. PFI will be optimised to provide a complete census of the protoplanet population at all stellocentric radii and over the age range from 0.1 to ~100 Myr. Within this age period, planetary systems undergo dramatic changes and the final architecture of planetary systems is determined. Our goal is to study the planetary birth on the natural spatial scale where the material is assembled, which is the "Hill Sphere" of the forming planet, and to characterise the protoplanetary cores by measuring their masses and physical properties. Our science working group has investigated the observational characteristics of these young protoplanets as well as the migration mechanisms that might alter the system architecture. We simulated the imprints that the planets leave in the disk and study how PFI could revolutionise areas ranging from exoplanet to extragalactic science. In this contribution we outline the key science drivers of PFI and discuss the requirements that will guide the technology choices, the site selection, and potential science/technology tradeoffs.
The CHARA Array, operated by Georgia State University, is located at Mount Wilson Observatory just north of Los Angeles in California. The CHARA consortium includes many groups, including LIESA in Paris, Observatoire de la Cote d’Azur, the University of Michigan, Sydney University, the Australian National University, the NASA Exoplanet Science Institute, and most recently the University of Exeter. The CHARA Array is a six-element optical/NIR interferometer, and for the time being at least, has the largest operational baselines in the world. In this paper we will give a brief introduction to the array infrastructure with a focus on our Adaptive Optics program, and then discuss current funding as well as opportunities of funding in the near future.
KEYWORDS: Interferometers, Interferometry, Astronomy, Prototyping, Signal to noise ratio, Infrared radiation, Modulators, Signal detection, Frequency conversion, Nonlinear dynamics
The interferometric concept named ALOHA (Astronomical Light Optical Hybrid Analysis) offers an alternative for high resolution imaging in the mid-infrared domain by shifting the astronomical light to shorter wavelength where optical guided components from telecommunications are available and efficient. A prototype with two arms converting a signal from 1.55 μm to 630 nm is used to validate the concept in laboratory and on-sky. Thanks to collaboration with the CHARA team, photometric tests were achieved with a single arm of the interferometer and have allowed to predict instrument performance in its interferometric configuration in order to obtain first fringes in H band.
The Optical interferometry DataBase (OiDB) aims at facilitating the access to science-ready data provided by various existing or decommissioned interferometers. The first version of OiDB has been released in June 2015. Today it contains more than 5000 OIFITS datafiles including the full collection of PIONIER data since 2011. All these reduced data are made publicly available and easily downloadable from OiDB. After presenting the characteristics of OiDB, we analyse how the community made use of it during this first year of operation and how we will improve it.
We describe a back-end Adaptive Optics system for the CHARA Array called Lab-AO intended to compensate for non-common path errors between the AO system at the telescopes and the final beam combining area some hundreds of meters away. The system is an on-axis, very small field of view, low order system that will work on star light if enough is present, or will make use of a blue light beacon sent from the telescope towards the laboratory if not enough star light is available. The first of six of these system has been installed and has recently been tested on the sky. Another five will be built for the remaining telescopes later this year.
Complex non-linear and dynamic processes lie at the heart of the planet formation process. Through numerical simulation and basic observational constraints, the basics of planet formation are now coming into focus. High resolution imaging at a range of wavelengths will give us a glimpse into the past of our own solar system and enable a robust theoretical framework for predicting planetary system architectures around a range of stars surrounded by disks with a diversity of initial conditions. Only long-baseline interferometry can provide the needed angular resolution and wavelength coverage to reach these goals and from here we launch our planning efforts. The aim of the Planet Formation Imager" (PFI) project is to develop the roadmap for the construction of a new near-/mid-infrared interferometric facility that will be optimized to unmask all the major stages of planet formation, from initial dust coagulation, gap formation, evolution of transition disks, mass accretion onto planetary embryos, and eventual disk dispersal. PFI will be able to detect the emission of the cooling, newlyformed planets themselves over the first 100 Myrs, opening up both spectral investigations and also providing a vibrant look into the early dynamical histories of planetary architectures. Here we introduce the Planet Formation Imager (PFI) Project (www.planetformationimager.org) and give initial thoughts on possible facility architectures and technical advances that will be needed to meet the challenging top-level science requirements.
The CHARA array is an optical interferometer with six 1-meter diameter telescopes, providing baselines from 33 to 331 meters. With sub-milliarcsecond angular resolution, its versatile visible and near infrared combiners offer a unique angle of studying nearby stellar systems by spatially resolving their detailed structures. To improve the sensitivity and scientific throughput, the CHARA array was funded by NSF-ATI in 2011 to install adaptive optics (AO) systems on all six telescopes. The initial grant covers Phase I of the AO systems, which includes on-telescope Wavefront Sensors (WFS) and non-common-path (NCP) error correction. Meanwhile we are seeking funding for Phase II which will add large Deformable Mirrors on telescopes to close the full AO loop. The corrections of NCP error and static aberrations in the optical system beyond the WFS are described in the second paper of this series. This paper describes the design of the common-path optical system and the on-telescope WFS, and shows the on-sky commissioning results.
KEYWORDS: Sensors, Electronics, Staring arrays, Analog electronics, Digital filtering, Prisms, Signal processing, Radio astronomy, Signal detection, Electrons
As part of a new collaboration between CHARA and the Max Planck Institute for Radio Astronomy, we have developed a new detector system for the CLASSIC/CLIMB beam combiner of the CHARA Array. This detector is based on the Rockwell HAWAII-1 HgCdTe focal plane array and has lower readout noise (∼5 electrons) than the current PICNIC based system. Presently, CLASSIC/CLIMB observations at different wavelength bands can be made only successively by selecting individual filters in a filter wheel. Therefore, another upgrade goal is to install a non-deviating prism in order to image the H- and K’-band light onto separate detector pixels and to simultaneously observe in the H and K’ bands. The detector control electronics were built at the Max Planck Institute for Radio Astronomy. The goal was to achieve the lowest possible readout noise and electronic pick-up noise. The detector readout noise can be significantly reduced by the following approach: First, the analog detector output signal is processed by a moving boxcar filter consisting of an analog approximation of a finite impulse response filter with a response time adapted to the 10 MHz sample rate of an analog-to-digital converter. Second, a digital filter averages up to 1024 samples for each addressed pixel. This hybrid (analog plus digital) filter approach gives a unique flexibility of a programmable bandwidth for optimum noise reduction.
Among the most fascinating and hotly-debated areas in contemporary astrophysics are the means by which planetary systems are assembled from the large rotating disks of gas and dust which attend a stellar birth. Although important work has already been, and is still being done both in theory and observation, a full understanding of the physics of planet formation can only be achieved by opening observational windows able to directly witness the process in action. The key requirement is then to probe planet-forming systems at the natural spatial scales over which material is being assembled. By definition, this is the so-called Hill Sphere which delineates the region of influence of a gravitating body within its surrounding environment. The Planet Formation Imager project (PFI; http://www.planetformationimager.org) has crystallized around this challenging goal: to deliver resolved images of Hill-Sphere-sized structures within candidate planethosting disks in the nearest star-forming regions. In this contribution we outline the primary science case of PFI. For this purpose, we briefly review our knowledge about the planet-formation process and discuss recent observational results that have been obtained on the class of transition disks. Spectro-photometric and multi-wavelength interferometric studies of these systems revealed the presence of extended gaps and complex density inhomogeneities that might be triggered by orbiting planets. We present detailed 3-D radiation-hydrodynamic simulations of disks with single and multiple embedded planets, from which we compute synthetic images at near-infrared, mid-infrared, far-infrared, and sub-millimeter wavelengths, enabling a direct comparison of the signatures that are detectable with PFI and complementary facilities such as ALMA. From these simulations, we derive some preliminary specifications that will guide the array design and technology roadmap of the facility.
The CHARA Array has been a PI led, low budget, and low manpower operation, and has followed a fairly unconventional path in its development. In this, the third paper of a series of three, we discuss some of the engineering and design decisions made along the way, some right and some wrong, with a focus on the choice between in-house development and the purchase of pre-built, or sub-contracted, subsystems. Along with these issues we will also address a few parts of the system that we might have done differently given our current knowledge, and those that somehow turned out very well.
The reviewers of our first NSF proposal asked us to prepare a more ambitious plan, and we did. When it was funded, the scope of the resources made available was far below the scope of the project. What to do? The only way to proceed within budget was to eliminate the entire professional engineering component of the proposal team, and we did so. This left the CHARA staff and a few consultants. The story of building the CHARA Array is largely the story of how to build a facility and instrument with no engineers, no managers, and no meetings. How was this possible?
The Fiber Linked Unit for Optical Recombination (FLUOR) is a precision interferometric beam combiner operating at the CHARA Array on Mt. Wilson, CA. It has recently been upgraded as part of a mission known as “Jouvence of FLUOR” or JouFLU. As part of this program JouFLU has new mechanic stages and optical payloads, new alignment systems, and new command/control software. Furthermore, new capabilities have been implemented such as a Fourier Transform Spectrograph (FTS) mode and spectral dispersion mode. These upgrades provide new capabilities to JouFLU as well as improving statistical precision and increasing observing efficiency. With these new systems, measurements of interferometric visibility to the level of 0.1% precision are expected on targets as faint as 6th magnitude in the K band. Here we detail the upgrades of JouFLU and report on its current status.
This discussion, the first of three describing how the CHARA Array came to be, focuses on the establishment of the Center for High Angular Resolution Astronomy at Georgia State University, our site selection saga, and some apparently brilliant decisions stumbled into. The technical and scientific achievements of the CHARA Array to date are far more than just an argument for perseverance. CHARA's success stands upon audacity, risk taking, luck, and, above all else, a core team of wonderfully talented and dedicated individuals who made it all turn out well.
We initiated a multi-technique campaign to understand the physics and properties of the massive binary system MWC 314. Our observations included optical high-resolution spectroscopy and Johnson photometry, nearinfrared spectrophotometry, and K′−band long-baseline interferometry with the CHARA Array. Our results place strong constraints on the spectroscopic orbit, along with reasonable observations of the phase-locked photometric variability. Our interferometry, with input from the spectrophotometry, provides information on the geometry of the system that appears to consist of a primary star filling its Roche Lobe and loosing mass both onto a hidden companion and through the outer Lagrangian point, feeding a circumbinary disk. While the multi-faceted observing program is allowing us to place some constraints on the system, there is also a possibility that the outflow seen by CHARA is actually a jet and not a circumbinary disk.
We propose an exotic use of sum frequency generation process (SFG) to develop a new kind of high resolution interferometer for astronomical imaging. SFG is well known to be intrinsically a noiseless non linear process of upconversion which permits a wavelength shift. Thereby we propose to shift astronomical MIR and FIR radiation to shorter wavelength where optical fibers and optical components are available and efficient. In order to demonstrate the validity of this method for high resolution imaging, we plan to set up a two-arm upconversion interferometer on the CHARA telescope array (California). Each arm would include an upconversion stage at the focus of telescope. The success of such a project is obviously conditioned by the quality of nonlinear components (waveguided PPLN) in term of efficiency and noise biases. Moreover, coherence study requires the use of identical non linear components which implies manufacturing constraints. To ensure the feasibility of this project, several studies have been conducted. By implementing an upconversion interferometer in laboratory we have recently demonstrated our ability to analyze the coherence properties of a 1550nm signal at visible wavelength. We also have successfully converted astronomical light using one arm of this interferometer at the Hawaï observatory. It showed the capability of our instrument to astronomical observing conditions in photon counting regime. A preliminary mission at CHARA observatory allowed us to check the compatibility of our instrument with the environment onsite and expected photometric levels. From these data we estimate to be able to study the coherence of astronomical target at 1550nm using such an instrument.
The CHARA Array is a six telescope optical/IR interferometer run by the Center for High Angular Resolution
Astronomy of Georgia State University and is located at Mount Wilson Observatory just to the north of Los Angeles
California. The CHARA Array has the largest operational baselines in the world and has been in regular use for
scientific observations since 2004. In 2011 we received funding from the NSF to begin work on Adaptive Optics for our
six telescopes. Phase I of this project, fully funded by the NSF grant, consists of designing and building wavefront
sensors for each telescope that will also serve as tip/tilt detectors. Having tip/tilt at the telescopes, instead of in the
laboratory, will add several magnitudes of sensitivity to this system. Phase I also includes a slow wavefront sensor in the
laboratory to measure non-common path errors and small deformable mirrors in the laboratory to remove static and
slowly changing aberrations. Phase II of the project will allow us to place high-speed deformable mirrors at the
telescopes thereby enabling full closed loop operation. We are currently seeking funding for Phase II. This paper will
describe the scientific rational and design of the system and give the current status of the project.
Rotation plays a crucial role in the shaping and evolution of a star. Widely incorporated into early and late-stage stellar models, rotational effects remain poorly understood in main-sequence stars, mainly due to the absence of observations challenging contemporary models. The Precision Astronomical Visible Observations (PAVO) instrument, located at the Center for High Angular Resolution Astronomy (CHARA) array, provides the highest angular resolution yet achieved (0.3 mas) for stars V=8 magnitude and brighter. We describe instrumental techniques and advances implemented in PAVO@CHARA to observe heavily resolved targets and yield well calibrated closure phases which are key milestones on the pathway to delivery of the first-ever image in the visible of fast-rotating main-sequence star.
FLUOR, which has been operational on CHARA since 2002, is an infrared fiber beam combiner. The telescope array will
soon be fitted with an adaptive optics system, which will enhance the interferometer performance. In this framework,
FLUOR has been entirely redeveloped and will be able to measure visibilities with higher accuracy and better sensitivity. The technical upgrades consist of improving some existing systems and developing new features. The bench, which is now remotely operable, primarily offers spectral dispersion (long fringes scanning), a more sensitive camera and a Fourier Transform Spectrometer mode. This paper presents the detailed opto-mechanical design of JouFLU (FLUOR rejuvenation), and the current instrument status.
The CHARA Array possesses the longest baselines in the world for infrared and visible interferometry, while the Michigan Infrared Combiner (MIRC) is the most advanced beam combiner for imaging. CHARA+MIRC has allowed imaging the surfaces of rapid rotators, interacting binary stars, and magnetically-active stars all for the first time. In this presentation, I will give an overview of the discoveries made by MIRC over the past five years and discuss technical and scientific lessons learned.
KEYWORDS: Visibility, Interference (communication), Telescopes, Fringe analysis, Data analysis, Mirrors, Signal to noise ratio, Interferometry, Calibration, Camera shutters
The CHARA Array is a six telescope optical/IR interferometer run by the Center for High Angular Resolution
Astronomy of Georgia State University and is located at Mount Wilson Observatory just to the north of Los
Angeles California. The CHARA Array has the largest operational baselines in the world and has been in regular
use for scientific observations since 2004. Our most sensitive beam combiner capable of measuring closure phases
is the CLassic Interferometry with Multiple Baselines beam combiner known as CLIMB. In this paper we discuss
the design and layout of CLIMB with a particular focus on the data analysis methodology. This analysis is
presented in a very general form and will have applications in many other beam combiners. We also present
examples of on sky data showing the precision and stability of both amplitude and closure phase measurements.
Since our first detection of a resolved near-infrared emission around the main sequence star Vega, which we
identified as the signature of hot dust grains close to the sublimation limit, we have been systematically searching
for similar signatures around a magnitude-limited sample of nearby main sequence stars with the FLUOR
instrument at the CHARA array. About 40 targets with spectral types ranging from A to K have been observed
within the last 6 years, leading to first statistical trends on the occurence of the bright exozodi phenomenon as a
function of spectral type. Our target sample is balanced between stars known to harbour cold dust populations
from space-based missions (e.g., Spitzer, Herschel) and stars without cold dust, so that the occurence of abundant
hot dust can also be correlated with the presence of large reservoirs of cold planetesimals. In this paper, we
present preliminary conclusions from the CHARA/FLUOR survey. We also discuss the first results obtained in
2011/2012 with the new PIONIER visiting instrument at the VLTI, which is now used to extend our survey
sample to the Southern hemisphere and to fainter targets. A first measurement of the exozodi/star flux ratio
as a function of wavelength within the H band is presented, thanks to the low spectral resolution capability of
PIONIER. Finally, we also briefly discuss our plans for extending the survey to fainter targets in the Northern
hemisphere with an upgraded version of the FLUOR beam combiner.
The CHARA Array is a six-telescope optical/IR interferometer managed by the Center for High Angular Resolution
Astronomy of Georgia State University and located at Mount Wilson Observatory in the San Gabriel Mountains
overlooking Pasadena, California. The CHARA Array has the longest operational baselines in the world and has been in
regular use for scientific observations since 2005. In this paper we give an update of instrumentation improvements,
primarily focused on the beam combiner activity. The CHARA Array supports seven beam combiners: CHARA
CLASSIC, a two-way high-sensitivity K/H/J band system; CLIMB, a three-way K/H/J open-air combiner; FLUOR, a
two-way K-band high-precision system; MIRC, a four/six-way H/K-band imaging system; CHAMP, a six-way K-band
fringe tracker; VEGA, a four-way visible light high spectral resolution system; and PAVO, a three-way visible light high
sensitivity system. We also present an overview of science results obtained over the last few years, including some recent imaging results.
In this paper, we review the current performance of the VEGA/CHARA visible spectrograph and make a review of
the most recent astrophysical results. The science programs take benefit of the exceptional angular resolution, the
unique spectral resolution and one of the main features of CHARA: Infrared and Visible parallel operation. We
also discuss recent developments concerning the tools for the preparation of observations and important features
of the data reduction software. A short discussion of the future developments will complete the presentation,
directed towards new detectors and possible new beam combination scheme for improved sensitivity and imaging
capabilities.
The CHARA-Michigan Phasetracker (CHAMP) successfully tracks fringes in 4-telescope and 6-telescope modes when observing high-visibility targets. We have found that our primary targets (Young Stellar Objects) have unexpectedly low visibility fringes (<20%) for most baselines at CHARA, generally below our tracking thresholds. We have undertaken an upgrade cycle in 2011-2012 to re-optimize CHAMP to allow group-delay tracking on the faintest fringes possible. We describe our multi-pronged strategy using special dicroics, new piezo scanners, and our first attempts to explore CHARA J-band made possible by using special metrology-blocking laser filters. CHAMP can now be used with all the combiners at CHARA.
The efficiency of the CHARA Array has proven satisfactory for a wide variety of scientific programs enabled by the
first-generation beam combination and detector systems. With multi-beam combination and more ambitious scientific
goals, improvements in throughput and efficiency will be highly leveraged. Engineering data from several years of
nightly operations are used to infer atmospheric characteristics and raw instrumental visibility in both classic optical and
single-mode fiber beam combiners. This information is the basis for estimates of potential gains that could be afforded
by the implementation of adaptive optics. In addition to the very important partial compensation for higher order
atmosphere-induced wavefront errors, the benefits include reduction of static and quasi-static aberrations, reduction of
residual tilt error, compensation for differential atmospheric refraction, and reduction of diffractive beam propagation
losses, each leading to improved flux throughput and instrumental visibility, and to associated gains in operability and
scientific productivity.
To date, about 17 hot Jupiters have been directly detected by photometric and/or spectroscopic observations.
Only 2 of them, however, are non-transiting hot Jupiters and the rest are all transiting ones. Since non-transiting
hot Jupiter systems are analogs of high contrast binaries, optical/infrared long baseline interferometers can resolve
them and detect the planets if highly stable and precise closure phase measurements are obtained. Thus, this is
a good opportunity for optical/infrared interferometers to contribute to the field of exoplanet characterization.
To reach this goal, detailed calibration studies are essential. In this paper, we report the first results of our
closure phase calibration studies. Specifically, we find strong closure phase drifts that are highly correlated with
target positions, i.e., altitude and azimuth angle. The correlation is stronger with altitude. Our experiments
indicate that the major cause of the drifts is probably longitudinal dispersion. We are able to find a strategy with
multiple approaches to reduce this effect, and are able to model the closure phase drift with a quadratic function
of both altitude and azimuth. We then use this model to calibrate the drifts, and test this new calibration scheme
with the high contrast binary ε Per. Although we can find a better orbital solution with this new method, we
have also found difficulties to interpret the orbit of ε Per, which may stem from possible mis-calibrations or the
influence of the third component in the system. More investigations are definitely necessary to address this issue
and to further confirm our calibration strategy.
This paper presents the current status of the VEGA (Visible spEctroGraph and polArimeter) instrument installed
at the coherent focus of the CHARA Array, Mount Wilson CA. Installed in september 2007, the first science
programs have started during summer 2008 and first science results are now published. Dedicated to high angular (0.3mas) and high spectral (R=30000) astrophysical studies, VEGA main objectives are the study of circumstellar environments of hot active stars or interactive binary systems and a large palette of new programs dedicated to fundamental stellar parameters. We will present successively the main characteristics of the instrument and its current performances in the CHARA environment, a short summary of two science programs and finally we will develop some studies showing the potential and difficulties of the 3 telescopes mode of VEGA/CHARA.
The Sydney University Stellar Interferometer (SUSI) has been enhanced by installation of the PAVO beam combiner,
which uses an electron-multiplying CCD detector giving a fast, low-noise 2D readout. This allows PAVO to provide
wide-band wavelength dispersed beam combination, which improves sensitivity and scientific productivity. PAVO also
provides pupil segmentation which improves the instrumental fringe visibility. A remote operations facility has been
established, which allows SUSI to be operated from Sydney or elsewhere. A new control system for the longitudinal
dispersion corrector and siderostats is under development. Installation has commenced of a high precision differential
astrometry system (MUSCA) which aims to detect planets in binary star systems.
The CHARA Array is a six-telescope optical/IR interferometer operated by the Center for High Angular Resolution
Astronomy of Georgia State University and is located at Mount Wilson Observatory just to the north of Los Angeles
California. The CHARA Array has the largest operational baselines in the world and has been in regular use for scientific
observations since 2004. In this paper we give an update of instrumentation improvements, primarily focused on the
beam combiner activity. The CHARA Array supports seven beam combiners: CHARA CLASSIC, a two-way high
sensitivity K/H/J band system; CLIMB, a three-way K/H/J open air combiner, FLUOR, a two-way K band high
precision system; MIRC, a four/six-way H/K band imaging system; CHAMP, a six way K band fringe tracker; VEGA, a
four way visible light high spectral resolution system; and PAVO, a three-way visible light high sensitivity system. The
paper will conclude with a review of science results obtained over the last few years, including our most recent imaging results.
This paper presents the first empirical measurement of the K1-band effective wavelength and bandwidth of the
CHARA Classic beam combiner on the CHARA Array. Prior to this work, the accepted effective wavelength
value used for CHARA Classic data (2.1501μm) came from a model of the system; it was not derived from
measurements done on the system directly. We employ two data collection methods for our observations: using
the Optical Path Length Equalizer (OPLE) cart to scan through the interference fringes and using the dither
mirror to scan through the fringes. The two observational methods yield similar effective wavelength measurements
(2.141±0.003μm with the OPLE cart and 2.136±0.002μm with the dither mirror). Both of these results are lower than the previously adopted effective wavelength value, but by less than 0.7%. The bandwidth values
measured by the two methods differ from each other by almost 5% (0.334 ± 0.002μm with the OPLE cart and
0.351±0.003μm with the dither mirror). Our results establish the first estimate of the uncertainty in the effective
wavelength and bandwidth.
Based on the success of four-telescope imaging with the Michigan Infrared Combiner (MIRC) on the CHARA
Array, our Michigan-based group will now upgrade our system to combine all six CHARA telescope simultaneously.
In order to make this observationally efficient, we have had to improve a number of subsystems and
commission new ones, including the new CHAMP fringe tracker, the introduction of photometric channels, the
upgrading of the realtime operating systems, and the obvious hardware and software upgrades of the control
system and the data pipeline. Here we will discuss the advantages of six-telescope operation, outline our upgrade
plans and discuss our current progress.
Two identical three-way beam combiners have been installed at the CHARA Array. The new setup is an extension of the
two-beam pupil plain combiner, which has been in use thus far. Using the new beam combiners we can now obtain
phase closure data in H, K or J band on two sets of three telescopes. A new optical design has been implemented to
image the six outputs of the combiners onto six separate pixels in the infrared detector array. The new optical
arrangement provides reduced background and spatial filtering. The magnitude limit of this beam combiner has reached
7.8 in K magnitude mainly as a result of better image quality by the new infrared camera optics.
We present the procedure used to optically align the CHARA telescopes. We show that the beam quality,
delivered by the CHARA telescopes E1, E2 and W2, is significantly better now than in 2008. RMS wavefront
error is about 200 nm. The astigmatism observed in W1 is more likely due to a combination of a mechanical
problem in the mounting and misalignment. We present wavefront quality results from four telescopes. Further
beam quality improvements can be expected when the second part of the alignment procedure (tuning) will be
carried out later this year.
The VEGA spectrograph and polarimeter has been recently integrated on the visible beams of the CHARA
Array. With a spectral resolution up to 35000 and thanks to operation at visible wavelengths, VEGA brings
unique capabilities in terms of spatial and spectral resolution to the CHARA Array. We will present the main
characteristics of VEGA on CHARA, some results concerning the performance and a preliminary analysis of the
first science run.
The Sydney University Stellar Interferometer (SUSI) has now been operating with a significanly more sensitive beam
combiner system (the Red Table) for several years. A number of novel results from the realms of single, binary and
pulsating star astrophysics are presented. Simultaneous dual beam-combiner operation with the Red Table and an
upgraded Blue Table has been demonstrated, enabling a high spectral dispersion oberving mode for stellar studies. Plans
for a major upgrade are briefly described. One component of this will be remote operation of the array for routine
observational data taking, with first steps on this path reported here with the inception of the Sydney Remote Operations
Center. The backbone of future plans with SUSI center around the installation of the southern of the twin PAVO
instruments. PAVO is also discussed in more detail in Ireland et al, this proceedings.
The CHARA Michigan Phase-tracker (CHAMP) is a real-time fringe tracker for the CHARA Array, a six-telescope
long baseline optical interferometer on Mount Wilson, California. CHAMP has been optimized for
tracking sensitivity at J, H, or K bands and is not meant as a science instrument itself. This ultimately results
in maximum sensitivity for all the science beam combiners that benefit from stabilized fringes. CHAMP was
designed, built, and tested in the laboratory at the University of Michigan and will be delivered to the CHARA
Array in 2008. We present the final design of CHAMP, highlighting some its key characteristics, including a novel
post-combination transport and imaging system. We also discuss testing and validation studies and present first
closed-loop operation in the laboratory.
Although direct direction of light from hot Jupiters has recently been achieved by measurements from the
Spitzer Space Telescope and the Hubble Space Telescope, information on those hot Jupiters are still not enough
to break all the model degeneracies and provide detailed conclusions. More detections that can measure the
astrometric orbits and flux variations of hot Jupiters, especially in the near-IR, are necessary. One promising
way to reach this goal is to use precision closure phase measurements obtained with ground-based long baseline
optical interferometers. Here we present our preliminary closure phase studies on the nearby hot Jupiter system
υ And b using CHARA-MIRC. Our data analysis shows our closure phase precisions are at ~ 0.4σ and ~ 0.6σ
level of the required signal for detections for the short and long triangles of CHARA respectively. In order to
make real detections, we have several improvements in the future to increase the signal-to-noise of the data. Once
these improvements are realized, our goal of directly detecting light from υ And b will be feasible to achieve.
The Precision Astronomical Visible Observations (PAVO) beam combiner is a new concept in visible beam
combination, recently commissioned at the CHARA array. By creating spatially-modulated fringes in a pupil
plane and then dispersing with an integral field unit, PAVO utilizes the full multi-r0 aperture of the CHARA
array over a standard 50% (630-950nm) bandwidth. In addition, minimal optimized spatial filtering ensures
calibration that is in principle as good as using single-mode fibers. We describe the design of and initial results
from the PAVO instrument.
The CHARA Array is a six telescope optical/IR interferometer run by the Center for High Angular Resolution
Astronomy of Georgia State University (GSU) and is located at Mount Wilson Observatory just to the north of Los
Angeles, California. The CHARA Array has the largest operational baselines in the world and has been in regular use for
scientific observations since 2004. In this paper we give an update of instrumentation improvements, primarily focused
on the beam combiner activity. The CHARA Array supports seven beam combiners: CHARA CLASSIC, a two way
high sensitivity K/H band system; CLIMB, an upgrade to CLASSIC that includes closure phase measurements; FLUOR,
a two way K band high precision system; MIRC, a six way H/K band imaging system; CHAMP, a six way K band fringe
tracker; VEGA, a 4 way visible light high spectral resolution system; and PAVO, a 3 way visible light high sensitivity
system. The paper will conclude with a brief review of some science results obtained over the last few years.
Using the sub-milli-arcsecond resolution of the CHARA interferometer array and
combining light with the 2-telescope combiner CHARA Classic, we have detected
strong near-infrared (NIR) emission interior to the dust-sublimation radius of
Herbig Ae stars MWC275 and AB Aur. The large contribution of this emission
component, which we argue to be hot gas, to the total NIR spectral energy distribution
(SED) is not predicted by current models of the dust evaporation front,
indicating that the NIR disk is more complicated than expected. Furthermore,
we demonstrate that the structure of the evaporation front in MWC275 is time
variable, making single epoch, large uv coverage observations critical to decoding
front geometry. With the commissioning of CHARA Michigan Phase Tracker
in the summer of 2008, the Michigan Infrared Combiner (a 6 telescope combiner
at CHARA) will become an ideal instrument for studying the evaporation
front, achieving the required sensitivities to begin the first "true" interferometric
imaging of the gas-dust transition region in young stellar objects (YSOs).
Here, we summarize results on the evaporation front structure obtained with
CHARA Classic and describe future prospects with CHARA MIRC in elucidating
morphology of the gas-dust transition region.
The efficiency of the CHARA Array has proven satisfactory for the scientific programs enabled by the first-generation
beam combination and detector systems. With multi-beam combination and more ambitious scientific goals,
improvements in throughput and efficiency will be highly leveraged. Engineering data from several years of nightly
operations are used to infer atmospheric characteristics and raw instrumental visibility in both classic optical and single-
mode fiber beam combiners. This information is the basis for estimates of potential gains that could be afforded by the
implementation of adaptive optics. This includes reduction of static and quasi-static aberrations, reduction of residual
tilt error, compensation for differential atmospheric refraction, reduction of diffractive beam propagation losses, each
leading to improved flux throughput and instrumental visibility, and to associated gains in operability and scientific
productivity.
We report the first scientific results from the Michigan Infrared Combiner (MIRC), including the first resolved
image of a main-sequence star besides the Sun. Using the CHARA Array, MIRC was able to clearly resolve the
well-known elongation of Altair's photosphere due to centrifugal distortion, and was also able to unambiguously
image the effect of gravity darkening. In this report, we also show preliminary images of the interacting binary
β Lyr and give an update of MIRC performance.
A new CCD based tip/tilt detection system was installed in the CHARA array on August 21, 2005. The new system can serve six telescopes simultaneously and is sensitive to a wavelength as long as 1 μm. The tip/tilt camera is based on an E2V CCD39-01, a small (80×80) back illuminated frame transfer device with a pixel size of 24×24 μm2. The measured read-out noise and conversion gain of the camera is 6.4 e- at 384 kpx s-1 and 1.1 e-/ADU, respectively at a temperature of -30 C°. Nine quad-pixel channels have been created on the CCD in a 10×10 pixel sub-array close to one of the read out amplifiers. Vignetting on the quad-pixel channels is negligible. Crosstalk between adjacent channels has been eliminated. The image scale on the CCD is 3.46 arcsecs/pixel. The limiting magnitude is expected to be V=12 at 20 ms integration time under good seeing conditions.
We describe a project for the installation of a visible focal instrument at the CHARA Array, named VEGA for Visible spEctroGraph and polArimeter. This new instrument will further open the visible domain and offer both spectral and polarimetric capabilities at the CHARA Array. It will create a new and unique scientific niche for the CHARA Array, especially in the context of international competition. The combination of the visible domain and high spectral resolution mode combined with a good sensitivity will allow VEGA/CHARA to carve out a new piece of observational phase space and compliment many existing or planned near-infrared interferometers. VEGA will help make CHARA the interferometer with the largest spectral and spatial resolution worldwide.
Extrasolar planetary systems are assumed as a sample to exhibit random orbital inclinations. The chance exists that a few of the 152 extrasolar planetary systems known to date may have face-on orbits for which the sin i factor will make a stellar-mass companion mimic a planetary-mass object. Such systems may thus harbor a late spectral type stellar companion instead of planets. Using Georgia State University's CHARA Array, we are undertaking an observing program on accessible extrasolar planetary systems that is expected to be completed in 2007. This effort will assist in culling the exoplanet list of some very low-inclination stellar interlopers that may be present. We will also determine the diameters of the central stars in an effort to refine our knowledge of the evolutionary status of the host stars.
KEYWORDS: Adaptive optics, Calibration, Point spread functions, Modulation transfer functions, Visibility, Telescopes, Interferometry, Signal to noise ratio, Deconvolution, Binary data
Aperture masking interferometry and Adaptive Optics (AO) are two of the competing technologies attempting
to recover diffraction-limited performance from ground-based telescopes. However, there are good arguments
that these techniques should be viewed as complementary, not competitive. Masking has been shown to deliver
superior PSF calibration, rejection of atmospheric noise and robust recovery of phase information through the use
of closure phases. However, this comes at the penalty of loss of flux at the mask, restricting the technique to bright
targets. Adaptive optics, on the other hand, can reach a fainter class of objects but suffers from the difficulty
of calibration of the PSF which can vary with observational parameters such as seeing, airmass and source
brightness. Here we present results from a fusion of these two techniques: placing an aperture mask downstream
of an AO system. The precision characterization of the PSF enabled by sparse-aperture interferometry can now
be applied to deconvolution of AO images, recovering structure from the traditionally-difficult regime within the
core of the AO-corrected transfer function. Results of this program from the Palomar and Keck adaptive optical
systems are presented.
In 2002, the Fiber Linked Unit for Optical Recombination (FLUOR) has been moved from the Infrared Optical Telescope Array (IOTA) to the CHARA Array. We present here the main upgrades that followed the installation, the new features installed, including spectral dispersion, and the current capabilities of the instrument.
We have obtained high resolution orbital data with the CHARA Array for the bright star 12 Persei, a resolved double-lined spectroscopic
binary, an example of a Separated Fringe Packet Binary. We describe the data reduction process involved. By using a technique we have developed of 'side-lobe verniering', we can obtain an improved precision in separation of up to 25 micro-arcsec along a given baseline. For this object we find a semi-major axis 0.3 of Barlow, Scarfe, and Fekel (1998) [BSF], but with an increased inclination angle. The revised masses are therefore almost 6% greater than those of BSF. The overall accuracy in the masses is about 1.3%, now primarily limited by the spectroscopically determined radial velocities. The precision of the masses due to the interferometrically derived "visual" orbit alone is only about 0.2%. We expect that improved RVs and improved absolute calibration can bring down the mass errors to below 1%.
The Michigan Infrared Combiner (MIRC) has been designed for two primary goals: 1) imaging with all six CHARA telescopes simultaneously in the near-infrared, 2) direct detection of "hot Jupiter" exoplanets using precision closure phases. In September 2005, MIRC was commissioned on-sky at the CHARA Array on Mt. Wilson, CA, successfully combining light from 4 telescopes simultaneously. After a brief overview of MIRC features and design philosophy, we provide detailed description of key components and present results of laboratory tests. Lastly, we present first results from the commissioning run, focusing on engineering performance. We also present remarkable on-sky closure phase results from the first night of recorded data with the best-ever demonstrated closure phase stability and precision (ΔΦ = 0.03 degrees).
We present the design for a near-infrared (JHK) fringe tracker to be used at the CHARA Array, a long baseline optical interferometer located at Mount Wilson Observatory. The CHARA Michigan Phase-tracker (CHAMP) is being fabricated and tested at the University of Michigan and will be transported to the CHARA Array for general use. CHAMP is separate from the science combiners and can therefore be optimized for fringe tracking. It will modulate around fringe center by 1-2λ at up to 500 Hz and calculate phase offsets in real-time using a modified 'ABCD' method . Six pair-wise Mach-Zehnder combiners will phase the entire Array. We give an overview of the optical layout and discuss our design strategy. Components such as the path-length modulators, low-OH fiber transport system, 1024x1024 HAWAII-1 detector, and control computer are discussed.
Georgia State University's Center for High Angular Resolution Astronomy (CHARA) operates a multi-telescope, long-baseline, optical/infrared interferometric array on Mt. Wilson, California. We present a brief update on the status of this facility along with summaries of the first scientific results from the Array.
Observational modes in which simultaneous high spatial and spectral information are recovered, without the complexity and expense of a dispersed detection system, have been discussed for some time. Sometimes called Double Fourier/Spatio-Spectral Interferometry (DFSSI), these methods fuse the concepts of Fourier Transform Spectrometry with high spatial resolution interferometry. The basic underlying principle comes from the idea that different spectral components, yielding different fringe frequencies, can be separated out in the fringe spectrum for individual study. However in practice, seeing fluctuations have the effect of shifting and blurring together the fringe frequencies making it difficult to isolate discrete spectral components. DFSSI has not been widely exploited in astronomical interferometry, due in part to such considerations. Here we propose a closely-related, although distinct technique which is the analog of DFSSI implemented in the spatial (delay) space rather than the time (frequency) domain. We propose the name Double-Fourier Spatio-Spectral Decoding to distinguish it from the latter. The technique relies on careful calibration of the fringe envelope shape, which is a function of the shape of the overall bandpass of the interferometer. We show that for astrophysical systems with interesting variations in spatial structure for neighboring spectral regions (such as stars with emission-line winds) that it is possible to untangle separate spatial and spectral components without a multi-channel dispersed fringe detector. The principle has been demonstrated successfully with observations of the prototype emission-line object P Cygni
at the CHARA array.
Using the FLUOR beam-combiner installed at the CHARA Array (Mt. Wilson, CA), we have obtained highprecision visibility measurements of Vega, one of the prototypic debris-disk stars, known to be surrounded by a large amount of cold dust in a ring-like structure at 80-100 AU. The combination of short and long baselines has allowed us to separately resolve the stellar photosphere and the close environment of the star (less than 8 AU). Our observations show a significant deficit in square visibility at short baselines with respect to the expected visibility of a simple UD stellar model (ΔV2 equal or equivalent to 2%), suggesting the presence of an extended source of emission around Vega. The sparse (u, v) plane coverage does not allow the discrimination between a point source and an extended circumstellar emission as the source of the extended emission. However, we show that the presence of a point-like source within the FLUOR field-of-view (1" in radius, i.e., 7.8 AU at the distance of Vega) is highly unlikely. The excess emission is most likely due to the presence of hot circumstellar dust in the inner part of Vega's debris disk, with a flux ratio of 1.29 plus or minus 0.19% between the integrated dust emission and the stellar photosphere. Complementing this result with archival photometric data in the near- and mid-infrared and taking into account a realistic photospheric model for the rapidly rotating Vega, we derive the expected physical properties of the circumstellar dust by modelling its Spectral Energy Distribution. The inferred properties suggest that the Vega system could be currently undergoing major dynamical perturbations.
We present the design of the Michigan Infra-Red Combiner (MIRC). MIRC
is planned for deployment at the Georgia State University CHARA array
to simultaneously combine all six telescope beams in an image-plane
combiner. The novel design incorporates spatial-filtering with
single-mode fiber optics, a synthetic (densified) pupil, and a
low-resolution spectrometer to allow good calibration and efficient
aperture synthesis imaging in the near-infrared. In addition, the
focalization and spectrometer optics can accommodate an integrated
optics component with minimal re-alignment. The MIRC concept can be
scaled-up for interferometer arrays with more telescopes.
Georgia State University's Center for High Angular Resolution Astronomy (CHARA) operates a multi-telescope, long-baseline, optical/infrared interferometric array on Mt. Wilson, California. We present an update on the status of this facility along with a sample of preliminary results from current scientific programs.
A remote operations center for Georgia State University's Center for High Angular Resolution Astronomy (CHARA) Array is in the final stages of implementation on the university campus in Atlanta, GA. Several technological considerations were incorporated into the overall design including a secure network infrastructure with an acceptable end-to-end latency, a control room replete with appropriate computing and projection systems, an efficient client-server model, and a data archival system. Although independent of the local weather, remote operations have practical considerations, such as routine preparations requiring on-site personnel and the observation of astronomical targets with celestial coordinates appropriate to the Local Sidereal Time (LST) and U-V plane coverage of the array.
The CHARA Array consists of six 1-meter telescopes. The telescopes are at fixed positions laid out in a Y-shaped pattern, where the longest available baseline is 330 meters. The resolving power of this interferometric array operating at visible and short infrared wavelengths is better than one milli-arcsecond. The current infrared beam combination system is capable of combining the light from any two of the six telescopes in the array. With the existing infrared beam combination and detection system, we routinely observe in K and H band, where our magnitude limit is 6.
The CHARA Array is a six element optical and near infrared interferometer built by Georgia State University on Mount Wilson in California. It is currently operating in the K and H bands and has the largest baseline (330 m) in operation of any similar instrument in the world. We expect to begin I band operations in 2002. We will present an update of the status of the instrumentation in the Array and set out our plans for the near term expansion of the system.
Individually resolved packets produced by scans from the CHARA Interferometer Array for binary stars can be analyzed in terms of the astrometry of the binary without using visibilities. We considered various methods for finding the locations of the packets, including autocorrelation and Shift-and-Add, but our best results were obtained from a method of direct packet fitting.
This method was put to use in analyzing two data sets each for the stars 12 Persei and Beta Arietis respectively. These data were taken between Nov 6 and 15, 2001 with the CHARA Array 330 m E1-S1 baseline. Some 460 to 830 scans were taken in both directions with the auxiliary PZT, and seeing conditions were fair to poor for these runs (r0 ≈ 7 cm).
This procedure yielded a projected separation for each data set, with an intrinsic accuracy of 0.15 - 0.3 mas. This represents an order of magnitude improvement over speckle interferometry techniques. The orbits were refined by a maximum likelihood technique. In the case of 12 Per the semimajor axis obtained was α = 53.53 mas, compared with the previous orbit of 53.38 mas, a small increase of 0.27%, which implies a mass increase of 0.8%, an insignificant change for this well-established orbit. For Beta Arietis, we find that α = 35.62 versus the previous orbit's value of 36.00 mas. This is a 1.0% decrease, resulting in a mass decrease of 3.0% for this system.
The Center for High Angular Resolution Astronomy (CHARA) has constructed an array of six alt-az telescopes at Mount Wilson Observatory in southern California. Together with the central beam combining facility, the telescopes operate as an optical/near-infrared interferometer with a maximum baseline of 330 meters. Due to practicality and cost constraints, some of the long path delay required for path length compensation occurs out of vacuum. A
consequence is a spectrally dispersed beam along the optical axis which decreases fringe contrast. To combat this visibility loss, wedges of glass are placed in the beam to chromatically equalize path lengths. Each set of glass wedges is called a Longitudinal Dispersion Compensator (LDC).
The design and fabrication phases for the LDC systems are described. Beginning with the material selection process, a glass with similar dispersive qualities to air within the observing bandwidths was selected. Next was the optomechanical design which included custom engineered optical mounts for the glass wedges, high precision translation stages for automated thickness variation and calibration adjustments. Following this, the hardware driver, software controls, and the user interface were written. Finally, the LDC components were assembled, integrated into the Beam Synthesis Facility, and
tested. The quantified results are presented and demonstrate an improvement to the interferometric measurements.
The fibered beam combiner FLUOR, which has provided high accuracy
visibility measurements on the IOTA interferometer, is being moved to
the CHARA array which provides five 1m telescopes on baselines ranging from 35 to 330m. The combination CHARA/FLUOR makes it possible for the first time to achieve sub-milliarcsecond resolution in the K band, with a dynamic range of 100 or more.
We explore the scientific potential of CHARA/FLUOR, most notably in the domains of high contrast binaries and the characterization of Cepheid pulsations, and present some of the anticipated developements.
During the 2001 observing season, the CHARA Array was in regular operation for a combined program of science, technical development, test, and commissioning. Interferometric science operations were carried out on baselines up to 330 meters -- the maximum available in the six-telescope array. This poster gives sample results obtained with the approximately north-south telescope pair designated S1-E1. At operating wavelengths in the K band, the 330 m baseline is well suited to diameter determinations for angular diameters in the range 0.6 - 1.2 milliarcseconds. This is a good
range for study of a wide range of hot stars. In this poster, angular
diameters for a set of A,B and F stars are compared to results derived from other sources. These confirm CHARA performance in the range 3-10% in visibility. The normal stars follow a normal spectral type - surface brightness relation, and a classical Be star deviates from the norm by an amount consistent with its apparent colors.
Georgia State University's Center for High Angular Resolution Astronomy (CHARA) operates a multi-telescope, long-baseline, optical/infrared interferometric array on Mt. Wilson, California. Since its inception, one of the primary scientific goals for the CHARA Array has been the resolution of spectroscopic binary stars, which offer tremendous potential for the determination of fundamental parameters for stars (masses, luminosities, radii and effective temperatures). A new bibliographic catalog of spectroscopic binary orbits, including a calculated estimate of the anticipated angular separation of the components, has been produced as an input catalog in planning observations with the Array. We briefly describe that catalog, which will be made available to the community on the Internet, prior to discussing observations obtained with our 330-m baseline during the fall of 2001 of the double-lined spectroscopic systems β Aur and β Tri. We also describe the initial results of an inspection of the extrasolar planetary system υ And.
Atmospheric fluctuations cause a jitter in the fringe position of ground-based stellar interferometers. To efficiently use the few photons available, an active fringe tracking system is necessary to stabilize the fringe position. This paper briefly describes the different methods of fringe tracking being implemented at the CHARA array, as well as the fringe tracking systems themselves.
In this paper we describe the telescope optics, manufacturing tolerances and the geometric alignment procedure of the CHARA telescopes. We also report on our efforts to test and refine the alignment of the telescopes by implementing the curvature sensing method. The results of the first experiments on telescope W1 show that we can get consistent results with this method. We also found a slight distortion caused by the lateral support of the primary mirror.
The CHARA Array at Mt. Wilson uses a PICNIC array camera for fringe detection, connected to a realtime fringe tracking computer running RTLinux. This paper describes the PC- and RTLinux-based camera controller and software that is used to allow high-speed, deterministic, low-latency readout of frames from the camera, as well as a camera simulator that mimics the behavior of the camera. This camera controller is built from commercial off-the-shelf (COTS) PC hardware and uses software running on the free RTLinux operating system, resulting in a very inexpensive camera controller system. The hardware costs for the system, including the PC (although excluding the costs of analog signal interfaces and power supplies), are less than $2000. The controller is capable of reading out arbitrary subimages from the camera, can quickly switch between different readout patterns, and is capable of controlling either CCD cameras or infrared array cameras. Detailed camera timing can be supplied by and/or tuned by the end user, as desired. In addition, a camera simulator unit has been developed. This camera simulator allows the development of camera interface hardware without the risk of damage to the expensive camera. The camera controller described connects to the Niro camera supplied to CHARA by Mark Shure, and the camera simulator mimics the behavior of this camera.
We present results of our survey of faint companions to O-stars using the adaptive optics (AO) system on the 3.63-meter Advanced Electro-Optical System (AEOS) telescope at the summit of Haleakala, on the island of Maui. The AEOS telescope is part of the United States Air Force's Maui Space Surveillance Site.
We have surveyed most of the O-stars brighter than V magnitude 8.0 in the declination range of -25 to +65 degrees for faint companions. We are using the I-band (800 nm central wavelength, 150 nm approximate FWHM) for the survey. This is done for two reasons: 1) the distinctly red filter will de-emphasize the O-star primary and enhance the faint (presumably redder) secondary, increasing the dynamic range; and 2) using I-band allows all of the shorter wavelength light to be sent to the AO system, increasing its performance for fainter stars. We describe the scientific results of our survey as well as the reduction process we used to generate relative photometric results from a 12-bit frame transfer camera with no native ability to generate a bias frame.
The CHARA Array at Mt. Wilson consists of six telescopes spread over hundreds of meters of rugged territory. Making efficient use of such a large physical instrument requires automation and tele-operation of the distributed resources. One system which is key to making daily operations routine is the enclosure control system, which is used to open and close the walls of the enclosure in order to enable quick equilibration of the telescope with its environment in order to minimize ground seeing effects on observations. This paper describes this enclosure control system, which is a distributed hardware/software system consisting of software running on a central control station in the operations room, together with software and hardware installed on six remote computers. The system must be robust in the presence of absent or intermittent nodes or network connections, must provide for both manual or remote control of the enclosures, and must provide for hardware and personnel safety. Remote operation of the system from Atlanta, Georgia has been demonstrated, and the system has proven extremely robust in regular use to date.
Georgia State University's Center for High Angular Resolution Astronomy (CHARA) is building an interferometric array of telescopes for high resolution imaging at optical and infrared wavelengths. The `CHARA Array' consists of six 1-m diameter telescopes arranged in a Y-shaped configuration with a maximum baseline of approximately 350 m. Construction of the facility will be completed during 2000, and the project will enter a phase in which beam combination subsystems will be brought on line concurrently with initial scientific investigations. This paper provides an update on recent progress, including our reaching the significant milestone of `first fringes' in November 1999. An extensive collection of project technical reports and images are available at our website.
The CHARA Array employs vacuum light pipes between the telescopes and the beam combination area. The complex terrain of the Mt. Wilson site poses interesting problems, with light pipes both underground and suspended up to 10 meters above ground. Telescope to beam-combination distances are up to about 180 meters. The support scheme and alignment strategy will be described.
The CHARA array achieved first fringes late last year and is currently being expanded on Mount Wilson CA. This presentation is a follow on from the overview given by Hal McAlister and will give more technical detail on the optical systems, with a focus on the telescopes, the delay lines, the control system, and the beam combining scheme. Combining more than three beams is not a simple problem with no obvious best solution, and we have by no means locked ourselves into a particular design. Preliminary designs will be shown, the first beam combiner will also be discussed along with our plans for future development.
During 1996 and 1997 more than 20 nights observing time have been used by, or allocated to, the CHARPA group at Georgia State University using the Mount Wilson Institute Adaptive Optics mounted on the Hooker 100 inch telescope on Mt. Wilson. Several scientific programs are being pursued including: differential photometry of binary stars; a search for faint companions of local solar type stars; attempts to image dust shells around YSOs; and experiments involving the combination of non-redundant aperture masking interferometry and adaptive optics. We have learned, and continue to learn, a great deal about the problems associated with, and methods of calibration of, adaptive optics images, especially in the area of accurate photometric measurements. So far, more than 30 binary systems have been measured in multiple filters and several previously unknown faint companions to local stars have been identified.
The telescope requirements of optical interferometry are somewhat different from conventional astronomy. The need for multiple units (in the CHARA case initially five, eventually seven) accentuates the importance of cost control, and at the same time provides opportunity for cost savings by careful procurement and production practices. Modern ideas about telescope enclosures offer significantly reduced dome seeing, but it is difficult to capture these benefits at low cost. The CHARA group has followed a series of design and bid procedures intended to optimize the costperformance of the telescope+enclosures. These have led to a compact but massive telescope design, blending modern and classical features, an unusual mirror blank selection process (directly ompeting several mirror blank technologies) , and a novel telescope enclosure concept which allows a continuous trade between wind protection and natural ventilation. This contribution will review and motivate the design decisions and show the resulting equipment and facilities.
Traditional methods of data collection in active fringe tracking Michelson stellar interferometers involve logging and analyzing the signal within the fringe tracking system for the scientific information about the object being observed. While these methods are robust and have produced excellent scientific results, they become more problematic as next-generation Michelson stellar interferometers are built with more telescopes and the aim of performing routine imaging. The Center for High Angular Resolution Astronomy (CHARA) Array is one such next-generation instrument presently under construction on Mt. Wilson, north of Los Angeles, California. The CHARA array will feature a separation of the tasks of active fringe tracking and imaging. In anticipation of the advantages afforded by the task separation, a prototype imager was developed. The prototype imager employs single-mode fiber optic strands to convey the light form simulated telescopes to a smaller, non-redundant, remapped pupil plane, which in turn feeds a low resolution prism spectrograph. The spectrograph features two cylindrical optical elements whose net effect is to focus the light to a smaller plate scale in the spectral dimension than in the orthogonal spatial dimension.
A brief description of the problems found and methods used to calculate the exact position of each mirror of the CHARA Array on Mount Wilson. The problem is highly constrained due to polarization and logistic considerations, yet many unusable solutions are still possible.
This paper introduces a new method for tracking fringes in ground-based optical stellar interferometry at low light levels under conditions of variations in OPD due to atmospheric turbulence. Like the group delay tracking (GDT), the method involves processing of short-exposure frames of channeled spectra when operating in a fringe tracking mode. But while the GDT is based on the fast Fourier transform, the proposed method is base don the formalism of optical statistical decisions and Markov chain framework. The corresponding algorithm, doubled DBA makes it possible to utilize all available information about the problem's nonlinearities and statistics such as Poisson photon arrivals, Gaussian readout noise, the statistics of atmospheric turbulence. Simulation results show that the DBA tracks stars up to two bolometric magnitudes dimmer than is possible with the GDT.
The Center for High Angular Resolution Astronomy (CHARA) at Georgia State University is building an
interferometric array of telescopes for high resolution imaging at optical and infrared wavelengths. The "CHARA Array" will initially consist offive 1-rn diameter telescopes arranged in a Y-shaped configuration with a maximum baseline of approximately 350 m. The facility is being constructed on Mt. Wilson, near Pasadena, California, a site noted for stable atmospheric conditions that often gives rise to exceptional image quality. The Array will be capable of submilliarcsecond imaging and will be devoted to a broad program of science aimed at fundamental stellar astrophysics in the visible and the astrophysics of young stellar objects in the infrared (2.2μm) spectral regions. This project is being funded in approximately 50/50% shares by Georgia State University and the National Science Foundation. The CHARA Array is expected to become operational during 1999. This paper presents a project status report. An extensive collection of project reports and images are available at our website (http://www.chara.gsu.edu).
The Center for High Angular Resolution Astronomy (CHARA) at Georgia State University is building an interferometric array of telescopes for high resolution imaging at optical and infrared wavelengths. The 'CHARA Array' will initially consist of five 1-m diameter telescopes arranged in a Y-shaped configuration with a maximum baseline of approximately 350 m. The facility will be located on Mt. Wilson, near Pasadena, California, a site noted for its stable atmoshperic conditions that often gives rise to exceptional image quality. The Array will be capable of submilliarcsecond imaging and will be devoted to a broad program of science aimed at fundamental stellar astrophysics in the visible and the astrophysics of young stellar objects in the infrared (2.2 micrometers ) spectral regions. This project has been supported by the National Sceince Foundation through Phase A feasibility and Phase B preliminary design stages, and NSF awarded 5.6 million dollars towards the construction of the facility in October 1994. Georgia State University is committed to providing an additional 5.8 million dollars in construction funds. The CHARA Array is expected to be operational late this decade. This paper will provide a summary overview of the project.
The CHARA array is an optical and IR imaging array of seven 1-m aperture telescopes with a Y-shaped configuration contained within a 400-m diameter circle. The facility will be capable of submilliarcsecond imaging and will be devoted to a broad program of science aimed at fundamental stellar astrophysics in the visible and the astrophysics of young stellar objects in the infrared spectral regions. The concept for the array has been carried through Phase A feasibility and Phase B preliminary design stages with funding provided by the National Science Foundation. This paper will provide a progress report on the status of the project.
In order to design adaptive optics systems and to study the effect of atmospheric turbulence on astronomical images it is helpful to know the temporal power spectra of the various Zernike polynomial terms. These coefficients correspond directly to standard aberration terminology like tilt, astigmatism, spherical aberration and so on. An analytical method based on work by Roddier et al. for predicting these power spectra using the current average wind speed, Frieds parameter r(Omicron ) and the aperture size is presented along with some experimental results testing their predictive power. The data are also shown to confirm the important results of Fried as well as Noll.
The problem of designing a beam combiner that involves more than two or three beams is complex and by no means solved. No such system has been built to date. The CHARA Array will require a beam combining system that can cope with up the seven beams with a number of spectral channels. The design of a beam combiner is probably driven more by the available technology than any theoretical constraints. Many tradeoffs between ease of manufacture and required integration time are involved. A design solution for CHARA will be presented that will incorporate spatial fringe encoding and forming fringes in the image plane.
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