BETTII is a balloon-borne far infrared (FIR: 30-100 μm) interferometer that also uses a near-infrared (NIR: 1-2.5 μm) channel for fine pointing sensing using stars. We have developed an inductive grid dichroic to divide the incoming beam into two components, by reflecting FIR light and transmitting NIR light. The dichroic is fabricated using focused electron beam technology to produce a 1 μm period, 100 nm width metal grid on a sapphire substrate in order to have high reflectance for FIR wavelengths. Here we discuss the design and the detailed manufacturing process for such a dichroic. The transmission and reflectance characteristics are also presented. We discuss them in context of the BETTII requirements.
The Balloon Experimental Twin Telescope for Infrared Interferometry (BETTII) is an 8-meter far-infrared (30-100 μm) double-Fourier Michelson interferometer designed to fly on a high altitude scientific balloon. The project began in 2011, and the payload was declared ready for flight in September 2016. Due to bad weather, the first flight was postponed until June 2017; BETTII was successfully launched on June 8, 2017 for an engineering flight. Over the course of the one night flight, BETTII acquired a large amount of technical data that we are using to characterize the payload. Unfortunately, the flight ended with an anomaly that resulted in destruction of the payload. In this paper, we will discuss the path to BETTII flight, the results of the first flight, and some of the plans for the future.
The Balloon Experimental Twin Telescope for Infrared Interferometry (BETTII) is an 8-m baseline far-infrared (FIR: 30−90 μm) interferometer providing spatially resolved spectroscopy. The initial scientific focus of BETTII is on clustered star formation, but this capability likely has a much broader scientific application. One critical step in developing an interferometer, such as BETTII, is the optical alignment of the system. We discuss how we determine alignment sensitivities of different optical elements on the interferogram outputs. Accordingly, an alignment plan is executed that makes use of a laser tracker and theodolites for precise optical metrology of both the large external optics and the small optics inside the cryostat. We test our alignment on the ground by pointing BETTII to bright near-infrared sources and obtaining their images in the tracking detectors.
The Balloon Experimental Twin Telescope for Infrared Interferometry (BETTII) is a balloon-borne, far-infrared direct detection interferometer with a baseline of 8 m and two collectors of 50 cm. It is designed to study galactic clustered star formation by providing spatially-resolved spectroscopy of nearby star clusters. It is being assembled and tested at NASA Goddard Space Flight Center for a first flight in Fall 2016. We report on recent progress concerning the pointing control system and discuss the overall status of the project as it gets ready for its commissioning flight.
We present the optics of Balloon Experimental Twin Telescope for Infrared Interferometry (BETTII) as it gets ready for launch. BETTII is an 8-meter baseline far-infrared (30-90 μm) interferometer mission with capabilities of spatially resolved spectroscopy aimed at studying star formation and galaxy evolution. The instrument collects light from its two arms, makes them interfere, divides them into two science channels (30-50 μm and 60-90 μm), and focuses them onto the detectors. It also separates out the NIR light (1-2.5 μm) and uses it for tip-tilt corrections of the telescope pointing. Currently, all the optical elements have been fabricated, heat treated, coated appropriately and are mounted on their respective assemblies. We are presenting the optical design challenges for such a balloon borne spatio- spectral interferometer, and discuss how they have been mitigated. The warm and cold delay lines are an important part of this optics train. The warm delay line corrects for path length differences between the left and the right arm due to balloon pendulation, while the cold delay line is aimed at introducing a systematic path length difference, thereby generating our interferograms from where we can derive information about the spectra. The details of their design and the results of the testing of these opto-mechanical parts are also discussed. The sensitivities of different optical elements on the interferograms produced have been determined with the help of simulations using FRED software package. Accordingly, an alignment plan is drawn up which makes use of a laser tracker, a CMM, theodolites and a LUPI interferometer.
We propose an architecture for the control system of BETTII,1 a far-infrared, balloon-borne interferometer with a baseline of 8 meters. This system involves multiple synchronized control loops for real-time pointing control and precise attitude knowledge. This will enable accurate phase estimation and control, a necessity for successful interferometry. We present the overall control strategy and describe our flight hardware in detail. We also show our current test setup and the first results of our coarse pointing loop.
Here we present the optical and limited cryogenic design for The Balloon Experimental Twin Telescope for Infrared Interferometry (BETTII), an 8-meter far-infrared interferometer designed to fly on a high-altitude scientific balloon. The optical design is separated into warm and cold optics with the cold optics further separated into the far-infrared (FIR) (30-90 microns) and near-infrared (NIR) (1-3 microns). The warm optics are comprised of the twin siderostats, twin telescopes, K-mirror, and warm delay line. The cold optics are comprised of the cold delay line and the transfer optics to the FIR science detector array and the NIR steering array. The field of view of the interferometer is 2’, with a wavelength range of 30-90 microns, 0.5” spectral resolution at 40 microns, R~200 spectral resolution, and 1.5” pointing stability. We also present the design of the cryogenic system necessary for operation of the NIR and FIR detectors. The cryogenic system consists of a ‘Buffered He-7’ type cryogenic cooler providing a cold stage base temperature of < 280mK and 10 micro-Watts of heat lift and a custom in-house designed dewar that nominally provides sufficient hold time for the duration of the BETTII flight (24 hours).
The Balloon Experimental Twin Telescope for Infrared Interferometry (BETTII) is an 8-meter baseline far-infrared interferometer designed to fly on a high altitude balloon. BETTII uses a double-Fourier Michelson interferometer to simultaneously obtain spatial and spectral information on science targets; the long baseline permits subarcsecond angular resolution, a capability unmatched by other far-infrared facilities. This program started in 2011, and is now in the process of building and testing components of the mission, aiming for first flight in fall of 2015. This paper will provide an overview of the BETTII experiment, with a discussion of current progress and of future plans.
Quad photoreceivers, namely a 2 x 2 array of p-i-n photodiodes followed by a transimpedance amplifier (TIA) per diode,
are required as the front-end photonic sensors in several applications relying on free-space propagation with position and
direction sensing capability, such as long baseline interferometry, free-space optical communication, and biomedical
imaging. It is desirable to increase the active area of quad photoreceivers (and photodiodes) to enhance the link gain,
and therefore sensitivity, of the system. However, the resulting increase in the photodiode capacitance reduces the
photoreceiver's bandwidth and adds to the excess system noise. As a result, the noise performance of the front-end quad
photoreceiver has a direct impact on the sensitivity of the overall system. One such particularly challenging application
is the space-based detection of gravitational waves by measuring distance at 1064 nm wavelength with ~ 10 pm/√Hz
accuracy over a baseline of millions of kilometers.
We present a 1 mm diameter quad photoreceiver having an equivalent input current noise density of < 1.7 pA/√Hz per
quadrant in 2 MHz to 20 MHz frequency range. This performance is primarily enabled by a rad-hard-by-design dualdepletion
region InGaAs quad photodiode having 2.5 pF capacitance per quadrant. Moreover, the quad photoreceiver
demonstrates a crosstalk of < -45 dB between the neighboring quadrants, which ensures an uncorrected direction sensing
resolution of < 50 nrad. The sources of this primarily capacitive crosstalk are presented.
KEYWORDS: Fourier transforms, Detection and tracking algorithms, Visibility, Actuators, Interferometry, Stars, Mirrors, Interferometers, Sensors, Signal to noise ratio
We present the design of a fringe tracking system for the Balloon Experimental Twin Telescope for Infrared Interferometry (BETTII). BETTII is a balloon- borne, far-infrared, 8 m-baseline interferometer with two 50 cm siderostats. Beams from the two arms are combined in the pupil plane to enable double-Fourier, spatio-spectral interferometry. To maintain the phase stability of the system, we need to actively correct of the optical path difference (OPD) between the two arms. The fringe-tracking system will work in the near-infrared and will use a reference star within the field of view to achieve two goals: overlap the beams coming from the two siderostats, and track the location of the central fringe packet, which is a measure of the OPD. The fringe tracker will share most of the optical train with the science instrument. This system is part of the overall control architecture that feeds fast steering tip/ tilt mirrors and a warm delay line to ensure proper beam combination and OPD control for the science instrument. This paper investigates the different sources of perturbations that are expected at float altitude, and derives the sensitivity of the fringe-tracking system. We show progress on validating our design using a visible light, broadband Mach-Zehnder interferometer that was developed at NASA/GSFC. This system demonstrates the viability of our OPD determination approach and provides a means of testing and characterizing several OPD determination and control algorithms.
The Balloon Experimental Twin Telescope for Infrared Interferometry (BETTII) is an 8-meter baseline far-infrared
interferometer designed to fly on a high altitude balloon. BETTII uses a double-Fourier Michelson interferometer to
simultaneously obtain spatial and spectral information on science targets; the long baseline permits subarcsecond angular
resolution, a capability unmatched by other far-infrared facilities. Here, we present key aspects of the overall design of
the mission and provide an overview of the current status of the project. We also discuss briefly the implications of this
experiment for future space-based far-infrared interferometers.
We have developed a high throughput infrared spectrometer for zodiacal light Fraunhofer lines measurements. The instrument is based on a cryogenic dual silicon Fabry-Perot etalon which is designed to achieve high signal to noise Franuhofer line profile measurements. Very large aperture silicon Fabry-Perot etalons wand fast camera optics make these measurements possible. The results of the absorption line profile measurements will provide a model free measure of the zodiacal light intensity in the near infrared. The knowledge of the zodiacal light brightness is crucial for accurate subtraction of zodiacal light foreground for accurate measure of the extragalactic background light after the subtraction of zodiacal light foreground. We present the final design of the instrument and the first results of its performance.
We are developing a near infrared spectrometer for measuring solar absorption lines in the zodiacal light in the
near infrared region. it has been recently demonstrated1 that observing single Fraunhofer line can be a powerful
tool for extracting zodiacal light parameters based on the measurements of the profile of the Mg I line at 5184 A.
We are extending this technique to the near infrared with the primary goal of measuring the absolute intensity of
the zodiacal light. This measurement will provide the crucial information needed to accurately subtract zodiacal
emission from the DIRBE (Diffuse Infrared Background Experiment) diffuse sky measurements to determine
the intensity of the extragalactic infrared background. The instrument design is based on a dual Fabry-Perot
interferometer with a narrow band filter. Its double etalon design allows to achieve high spectral contrast to
reject the bright out of band atmospheric hydroxyl emission. High spectral contrast is absolutely necessary to
achieve detection limits needed to accurately measure the intensity of the absorption line. We present the design,
the estimated performance of the instrument, and the expected results of the observing program.
Correct interpretation of a vast array of astronomical data relies heavily on understanding the properties of silicate dust
as a function of wavelength, temperature, and crystallinity. We introduce the OPASI-T (Optical Properties of
Astronomical Silicates with Infrared Techniques) project to address the need for high fidelity optical characterization
data on the various forms of astronomical dust. We use two spectrometers to provide extinction data for silicate samples
across a wide wavelength range (from the near infrared to the millimeter). New experiments are in development that will
provide complementary information on the emissivity of our samples, allowing us to complete the optical
characterization of these dust materials. In this paper, we present initial results from several materials including
amorphous iron silicate, magnesium silicate and silica smokes, over a wide range of temperatures, and discuss the design
and operation of our new experiments.
KEYWORDS: James Webb Space Telescope, Spectrographs, Electronics, Galactic astronomy, Near infrared, Astronomical imaging, Camera shutters, Silicon, Space telescopes, Imaging spectroscopy
One of the James Webb Space Telescope's (JWST) primary science goals is to characterize the epoch of galaxy formation in
the universe and observe the first galaxies and clusters of galaxies. This goal requires multi-band imaging and spectroscopic
data in the near infrared portion of the spectrum for large numbers of very faint galaxies. Because such objects are
sparse on the sky at the JWST resolution, a multi-object spectrograph is necessary to efficiently carry out the required
observations. We have developed a fully programmable array of microshutters that will be used as the field selector
for the multi-object Near Infrared Spectrograph (NIRSpec) on JWST. This device allows apertures to be opened at the
locations of selected galaxies in the field of view while blocking other unwanted light from the sky background and bright
sources. In practice, greater than 100 objects within the field of view can be observed simultaneously. This field selection
capability greatly improves the sensitivity and efficiency of NIRSpec. In this paper, we describe the microshutter arrays,
their development, characteristics, fabrication, testing, and progress toward delivery of a flight-qualified field selection
subsystem to the NIRSpec instrument team.
We have developed microshutter array systems at NASA Goddard Space Flight Center for use as multi-object
aperture arrays for a Near-Infrared Spectrometer (NIRSpec) instrument. The instrument will be carried on the
James Webb Space Telescope (JWST), the next generation of space telescope, after the Hubble Space
Telescope retires. The microshutter arrays (MSAs) are designed for the selective transmission of light from
objected galaxies in space with high efficiency and high contrast. Arrays are close-packed silicon nitride
membranes with a pixel size close to 100x200 μm. Individual shutters are patterned with a torsion flexure
permitting shutters to open 90 degrees with minimized stress concentration. In order to enhance optical
contrast, light shields are made on each shutter to prevent light leak. Shutters are actuated magnetically,
latched and addressed electrostatically. The shutter arrays are fabricated using MEMS bulk-micromachining
and packaged utilizing a novel single-sided indium flip-chip bonding technology. The MSA flight system
consists of a mosaic of 2 x 2 format of four fully addressable 365 x 171 arrays. The system will be placed in
the JWST optical path at the focal plane of NIRSpec detectors. MSAs that we fabricated passed a series of
qualification tests for flight capabilities. We are in the process of making final flight-qualified MSA systems
for the JWST mission.
KEYWORDS: Camera shutters, Electrodes, Magnetism, Indium, Silicon, James Webb Space Telescope, Optical fabrication, Metals, Microelectromechanical systems, Space telescopes
A complex MEMS device, microshutter array system, is being developed at NASA Goddard Space Flight
Center for use as an aperture array for a Near-Infrared Spectrometer (NirSpec). The instrument will be
carried on the James Webb Space Telescope (JWST), the next generation of space telescope after Hubble
Space Telescope retires. The microshutter arrays (MSAs) are designed for the selective transmission of light
with high efficiency and high contrast. Arrays are close-packed silicon nitride membranes with a pixel size
close to 100x200 &mgr;m. Individual shutters are patterned with a torsion flexure permitting shutters to open 90
degrees with a minimized mechanical stress concentration. Light shields are made on to each shutter for light
leak prevention so to enhance optical contrast. Shutters are actuated magnetically, latched and addressed
electrostatically. The shutter arrays are fabricated using MEMS bulk-micromachining technologies and
packaged using single-sided indium flip-chip bonding technology. The MSA flight concept consists of a
mosaic of 2 x 2 format of four fully addressable 365 x 171 arrays placed in the JWST optical path at the focal
plane.
KEYWORDS: Camera shutters, Electrodes, Indium, Magnetism, Silicon, James Webb Space Telescope, Optical fabrication, Metals, Microelectromechanical systems, Space telescopes
MEMS microshutter arrays (MSAs) are being developed at NASA Goddard Space Flight Center for use as an aperture
array for the Near-Infrared Spectrometer (NirSpec). The instruments will be carried on the James Webb Space
Telescope (JWST), the next generation of space telescope after Hubble Space Telescope retires. The microshutter arrays
are designed for the selective transmission of light with high efficiency and high contrast. Arrays are close-packed
silicon nitride membranes with a pixel size of 105x204 μm. Individual shutters are patterned with a torsion flexure
permitting shutters to open 90 degrees with a minimized mechanical stress concentration. Light shields are made on each
shutter for light leak prevention to enhance optical contrast. Shutters are actuated magnetically, latched and addressed
electrostatically. The shutter arrays are fabricated using MEMS technologies. Single-side indium flip chip bonding is
performed to attach microshutter arrays to substrates.
We report results of a recently-completed pre-Formulation Phase study of SPIRIT, a candidate NASA Origins Probe mission. SPIRIT is a spatial and spectral interferometer with an operating wavelength range 25 - 400 μm. SPIRIT will provide sub-arcsecond resolution images and spectra with resolution R = 3000 in a 1 arcmin field of view to accomplish three primary scientific objectives: (1) Learn how planetary systems form from protostellar disks, and how they acquire their chemical organization; (2) Characterize the family of extrasolar planetary systems by imaging the structure in debris disks to understand how and where planets form, and why some planets are ice giants and others are rocky; and (3) Learn how high-redshift galaxies formed and merged to form the present-day population of galaxies. Observations with SPIRIT will be complementary to those of the James Webb Space Telescope and the ground-based Atacama Large Millimeter Array. All three observatories could be operational contemporaneously.
The Universe appears to be expanding at an accelerating rate, driven by a mechanism called Dark Energy. The nature of Dark Energy is largely unknown and needs to be derived from observation of its effects. JEDI (Joint Efficient Dark-energy Investigation) is a candidate implementation of the NASA-DOE Joint Dark Energy Mission (JDEM). It will probe the effects of Dark Energy in three independent ways: (1) using Type Ia supernovae as cosmological standard candles over a range of distances, (2) using baryon acoustic oscillations as a cosmological standard ruler over a range of cosmic epochs, and (3) mapping the weak gravitational lensing distortion by foreground galaxies of the images of background galaxies at different distances. JEDI provides crucial systematic error checks by simultaneously applying these three independent observational methods to derive the Dark Energy parameters. The concordance of the results from these methods will not only provide an unprecedented understanding of Dark Energy, but also indicate the reliability of such an understanding. JEDI will unravel the nature of Dark Energy by obtaining observations only possible from a vantage point in space, coupled with a unique instrument design and observational strategy. Using a 2 meter-class space telescope with simultaneous wide-field imaging (~ 1 deg2, 0.8 to 4.2 μm in five bands) and multi-slit spectroscopy (minimum wavelength coverage 1 to 2 μm), JEDI will efficiently execute the surveys needed to solve the mystery of Dark Energy.
The Submillimeter Probe of the Evolution of Cosmic Structure (SPECS) is a space-based imaging and spectral ("double Fourier") interferometer with kilometer maximum baseline lengths for imaging. This NASA "vision mission" will provide spatial resolution in the far-IR and submillimeter spectral range comparable to that of the Hubble Space Telescope, enabling astrophysicists to extend the legacy of current and planned far-IR observatories. The astrophysical information uniquely available with SPECS and its pathfinder mission SPIRIT will be briefly described, but that is more the focus of a companion paper in the Proceedings of the Optical, Infrared, and Millimeter Space Telescopes conference. Here we present an updated design concept for SPECS and for the pathfinder interferometer SPIRIT (Space Infrared Interferometric Telescope) and focus on the engineering and technology requirements for far-IR double Fourier interferometry. We compare the SPECS optical system requirements with those of existing ground-based and other planned space-based interferometers, such as SIM and TPF-I/Darwin.
KEYWORDS: Camera shutters, James Webb Space Telescope, Electrodes, Silicon, Spectrographs, Magnetism, Prototyping, Failure analysis, Near infrared, Galactic astronomy
The Near Infrared Spectrograph (NIRSpec) for the James Webb Space Telescope (JWST) is a multi-object spectrograph operating in the 0.6-5.0 μm spectral range. One of the primary scientific objectives of this instrument is to measure the number and density evolution of galaxies following the epoch of initial formation. NIRSpec is designed to allow simultaneous observation of a large number of sources, vastly increasing the capability of JWST to carry out its objectives. A critical element of the instrument is the programmable field selector, the Microshutter Array. The system consists of four 175 x 384 close packed arrays of individually operable shutters, each element subtending 0.2” x 0.4”on the sky. This device allows simultaneous selection of over 200 candidates for study over the 3.6’ x 3.6’ field of the NIRSpec, dramatically increasing its efficiency for a wide range of investigations. Here, we describe the development, production, and test of this critical element of the NIRSpec.
Ultimately, after the Single Aperture Far-IR (SAFIR) telescope, astrophysicists will need a far-IR observatory that provides angular resolution comparable to that of the Hubble Space Telescope. At such resolution galaxies at high redshift, protostars, and nascent planetary systems will be resolved, and theoretical models for galaxy, star, and planet formation and evolution can be subjected to important observational tests. This paper updates information provided in a 2000 SPIE paper on the scientific motivation and design concepts for interferometric missions SPIRIT (the Space Infrared Interferometric Telescope) and SPECS (the Submillimeter Probe of the Evolution of Cosmic Structure). SPECS is a kilometer baseline far-IR/submillimeter imaging and spectral interferometer that depends on formation flying, and SPIRIT is a highly-capable pathfinder interferometer on a boom with a maximum baseline in the 30 - 50 m range. We describe recent community planning activities, remind readers of the scientific rationale for space-based far-infrared imaging interferometry, present updated design concepts for the SPIRIT and SPECS missions, and describe the main issues currently under study. The engineering and technology requirements for SPIRIT and SPECS, additional design details, recent technology developments, and technology roadmaps are given in a companion paper in the Proceedings of the conference on New Frontiers in Stellar Interferometry.
We describe the development, construction, and testing of two 384 element arrays of ion-implanted semiconducting cryogenic bolometers designed for use in far-infrared and submillimeter cameras. These two dimensional arrays are assembled from a number of 32 element linear arrays of monolithic Pop-Up bolometer Detectors (PUD) developed at NASA/Goddard Space Flight Center. PUD technology allows the construction of large, high filling factor, arrays that make efficient use of available focal plane area in far-infrared and submillimeter astronomical instruments. Such arrays can be used to provide a significant increase in mapping speed over smaller arrays. A prototype array has been delivered and integrated into a ground-based camera, the Submillimeter High Angular Resolution Camera (SHARC II), a facility instrument at the Caltech Submillimeter Observatory (CSO). A second array has recently been delivered for integration into the High-resolution Airborne Widebandwidth Camera (HAWC), a far-infrared imaging camera for the Stratospheric Observatory for Infrared Astronomy (SOFIA). HAWC is scheduled for commissioning in 2005.
The SPEED camera is being developed to study the spectral energy distributions of high redshift galaxies, Sunyaev-Zel'dovich effect in X-ray clusters and other cold objects in the universe. Its initial runs will be done on the 10 m Heinrich Hertz Submillimeter Telescope (HHSMT), with later runs using the Large Millimeter Telescope (LMT). SPEED requires a 2x2 pixel cryogenic detector array of Frequency Selective Bolometers (FSB). Each of the pixels will have four frequency bands in the ~150-350 GHz range. Here we describe the development of the detector array of these high efficiency FSBs. The FSB design provides the multi-pixel multi-spectral band capability required for SPEED in a compact, light weight, stackable array. The SPEED FSB bolometers will use proximity effect superconducting transition edge sensors (TES) as their temperature-sensing element permitting significantly higher levels of electronic multiplexing in future applications where larger numbers of detectors may be required.
HAWC (High-resolution Airborne Wideband Camera) is a facility science instrument for SOFIA (Stratospheric Observatory for Infrared Astronomy). It is a far-infrared camera designed for diffraction-limited imaging in four spectral passbands centered at wavelengths of 53, 89, 155, and 216 μm. Its detector is a 12x32 array of bolometers cooled to 0.2 K by an adiabatic demagnetization refrigerator. In this paper, we report on the development and testing of the instrument and its subsystems.
EDGE is a Long Duration Balloon (LDB) borne instrument designed to measure the large-scale anisotropy of the Cosmic Infrared Background (CIB). The goal is to use this signal as a new observational tool to measure the character of the spatial distribution of galaxies at the largest spatial scales. With a 6\arcmin\ beam mapping more than 400 square degrees of sky at 8 frequency bands between 250GHz and 1.5 THz the experiment can determine the variation of galaxy density on
spatial scales ranging from >200h-1 Mpc, where dark matter
variations are determined directly from Cosmic Microwave Background Radiation (CMBR) anisotropy, to <5h-1 Mpc where the distribution of dark matter and galaxies is determined from galaxy redshift surveys and the underlying dynamics of structure growth is non-linear. The instrument consists of a 1-meter class off-axis telescope and a Frequency Selective Bolometer (FSB) array radiometer. The FSB design provides the compact, multi-chromatic, high sensitivity focal plane needed for this measurement.
The TopHat instrument was designed to operate on the top of a high altitude balloon. From this location, the experiment could efficiently observe using a clean beam with extremely low contamination from the far side lobes of the instrument beam. The experiment was designed to scan a large portion of the sky directly above it and to map the anisotropy of the Cosmic Microwave Background (CMB) and thermal emission from galactic dust. The instrument used a one-meter class telescope with a five-band single pixel radiometer spanning the frequency range from 150-600 GHz. The radiometer used bolometric detectors operating at ~250mK. Here, we report on the flight of the TopHat experiment over Antarctica in January, 2001 and describe the scientific goals, the operation, and in-flight performance.
The Frequency Selective Bolometer (FSB) is a bolometer with a patterned frequency selective absorber, coupled with a band-reflecting backshort. The resulting unit absorbs in-band radiation, and passes out-of-band radiation. Thus a series of FSBs tuned to different bands packed in series in a light pipe forms a compact multi-band photometer. The compact form factor makes it an attractive detector for a mm-wave array camera.
We have built and characterized prototypes that demonstrate this technology. We are now developing a set of FSBs for SPEED (the SPEctral Energy Distribution camera), an FSB array camera which will observe 4 pixels in 4 mm-wave spectral bands, to be used on the Heinrich Hertz Telescope and the Large Millimeter Telescope. These FSBs are fabricated on a free-standing SiN film with TES thermometers. We will discuss the design and performance of these detectors.
SHARC II is a background-limited 350 μm and 450 μm facility camera for the Caltech Submillimeter Observatory undergoing commissioning in 2002. The key component of SHARC II is a 12 × 32 array of doped silicon 'pop-up' bolometers developed at NASA/Goddard. Each 1 mm × 1 mm pixel is coated with a 400 Ω/square bismuth film and located λ/4 above a reflective backshort to achieve >75% absorption efficiency. The pixels cover the focal plane with >90% filling factor. At 350 μm, the SHARC II pixels are separated by 0.65 λ/D. In contrast to the silicon bolometers in the predecessor of SHARC II, each doped thermistor occupies nearly the full area of the pixel, which lowers the 1/f knee of the detector noise to <0.03 Hz, under load, at the bath temperature of 0.36 K. The bolometers are AC-biased and read in 'total power' mode to take advantage of the improved stability. Each bolometer is biased through a custom ~130 MΩ CrSi load resistor at 7 K and read with a commercial JFET at 120 K. The JFETs and load resistors are integrated with the detectors into a single assembly to minimize microphonic noise. Electrical connection across the 0.36 K to 4 K and 4 K to 120 K temperature interfaces is accomplished with lithographed metal wires on dielectric substrates. In the best 25% of winter nights on Mauna Kea, SHARC II is expected to have an NEFD at 350 μm of 1 Jy Hz-1/2 or better. The new camera should be at least 4 times faster at detecting known point sources and 30 times faster at mapping large areas compared to the prior instrument.
Advances in bolometer device and readout technologies make it possible to build photon-noise limited bolometric cameras for ground-based observations at mm-wave frequencies. However, today's bolometer cameras are limited not by photon-noise of the telescope and atmosphere but by fluctuations in the atmosphere signal. To realize the full potential of bolometer cameras on large aperture ground-based telescopes, one must find a way to defeat this foreground.
The SPEctral Energy Distribution Camera - or SPEED - is a four pixel, four frequency camera planned for eventual use on the Large Millimeter Telescope (LMT). A prototype version of this camera is currently being built for initial operation on the Heinrich Hertz Telescope (HHT). SPEED incorporates Frequency Selective Bolometers to sample the sky with a frequency-independent beam simultaneously at four frequencies (from 150 to 375 GHz) in each pixel. SPEED's ability to separate the temporally varying atmospheric signal from the true sky signal will potentially result in a per-detector sensitivity between 2 and 5 times greater than that achieved with contemporary bolometer cameras. We describe the basic design and motivation for SPEED, the expected sensitivity of the camera on the LMT, and give examples of some of the science programs we will undertake.
Magnetically actuated MEMS microshutter arrays are being developed at the NASA Goddard Space Flight Center for use in a multi-object spectrometer on the James Webb Space Telescope (JWST), formerly Next Generation Space Telescope (NGST). The microshutter arrays are designed for the selective transmission of light with high efficiency and high contrast. The JWST environment requires cryogenic operation at 45K. Microshutter arrays are fabricated out of silicon-on-insulator (SOI) wafers. Arrays consist of close-packed shutters made on silicon nitride (nitride) membranes with a pixel size of 100 × 100 m. Individual shutters are patterned with a torsion flexure permitting shutters to open 90°, with a minimized mechanical stress concentration. Shutters operated this way have survived fatigue life test. The mechanical shutter arrays are fabricated using MEMS technologies. The processing includes a multi-layer metal deposition, patterning of shutter electrodes and magnetic pads, reactive ion etching (RIE) of the front side to form shutters in a nitride film, an anisotropic back-etch for wafer thinning, and a deep RIE (DRIE) back-etch, down to the nitride shutter layer, to form support frames and relieve shutters from the silicon substrate. An additional metal deposition and patterning has recently been developed to form electrodes on the vertical walls of the frame. Shutters are actuated using a magnetic force, and latched electrostatically. One-dimensional addressing has been demonstrated.
Far infrared interferometers in space would enable extraordinary measurements of the early universe, the formation of galaxies, stars, and planets, and would have great discovery potential. Since half the luminosity of the universe and 98% of the photons released since the Big Bang are now observable at far IR wavelengths (40 - 500 micrometers ), and the Earth's atmosphere prevents sensitive observations from the ground, this is one of the last unexplored frontiers of space astronomy. We present the engineering and technology requirements that stem from a set of compelling scientific goals and discuss possible configurations for two proposed NASA missions, the Space Infrared Interferometric Telescope and the Submillimeter Probe of the Evolution of Cosmic Structure.
The Cosmic Background Explorer (COBE) satellite was launched on November 18, 1989 from Vandenberg Air Force base on a Delta rocket. It carried two superfluid liquid-helium-cooled (LHe) infrared (IR) instruments in a 600 liter dewar, and three microwave radiometers mounted on the outside of the dewar. One of the LHe-cooled instruments is a ten-band photometer covering the spectral range from 1.2 to 240 micrometers - the Diffuse Infrared Background Experiment (DIRBE). A goal of the DIRBE program is to obtain full-sky infrared observations that can be used to model accurately the IR contributions arising from the interplanetary dust (IPD) and the Galaxy. Using such models, the foreground can be removed to expose and underlying extragalactic IR component produced early in formation of the universe. The nature of the IPD IR foreground detected by the DIRBE is found to be quite complex, but amenable to modelling.
The Diffuse Infrared Background Experiment (DIRBE) on board NASA's Cosmic Background Explorer (COBE) satellite has surveyed the entire sky in 10 broad photometric bands covering the wavelength region from 1 to 240 micrometers , at an angular resolution of 0.7 degree(s) (Boggess et al. 1992). the extensive spectral coverage of the DIRBE observations offers an unprecedented opportunity to undertake comprehensive large-scale studies of the content, structure, and energetics of the stellar and interstellar components of the Galaxy. Understanding the Galactic emission is not only a task of scientific value in its own right, but also a necessary step in the accurate extraction of faint cosmological emission from the DIRBE data.
The Diffuse InfraRed Background Experiment (DIRBE) onboard the cosmic Background Explorer (COBE) was designed to conduct a search for a cosmic infrared background (CIB), which is expected to be the fossil radiation from the first luminous objects in the universe. The instrument, a ten-band cryogenic absolute photometer and three-band polarimeter with a 0.7 degree(s) beam and a wavelength range from 1 - 240 micrometers , scans the sky redundantly and samples half the sky each day. During the ten month lifetime of the cryogen, the instrument achieved a nominal sensitivity on the sky of 10-9 W/m2/sr at most wavelengths, or approximately 1% of the natural background at wavelengths where the sky is very luminous. The short wavelength bands from 1 - 5 micrometers continue to operate after exhaustion of the cryogen, although at reduced sensitivity. In this paper, we review the design, testing, and in-flight performance of the DIRBE.
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