The LRCTF (Laser Ranging Characterization and Test Facility) is a unique facility built at NASA GSFC to provide thermal-optical testing of the next generation GPS LRA (Global Positioning Satellite’s Laser Retroreflector Array) laser ranging target. The 400mm diameter target is an array consisting of 48 total internal reflection retroreflectors and has an optical cross section requirement of 100 MSM (million square meters). To verify that the array meets this requirement during on-orbit conditions, the LRCTF is equipped with a 400mm test beam, a data product output consisting of full aperture FFDPs (Far Field Diffraction Patterns) and a thermal chamber. The FFDPs are used to calculate the OCS. This paper will describe the facility design, alignment approach, and verification process.
KEYWORDS: James Webb Space Telescope, Optical components, Space telescopes, Optical testing, Sensors, Calibration, Data modeling, Human-machine interfaces, Error analysis, Analytical research
NASA’s James Webb Space Telescope (JWST) is a 6.5m diameter, segmented, deployable telescope for cryogenic IR space astronomy. The JWST Observatory includes the Optical Telescope Element (OTE) and the Integrated Science Instrument Module (ISIM), that contains four science instruments (SI) and the Fine Guidance Sensor (FGS). The SIs are mounted to a composite metering structure. The SIs and FGS were integrated to the ISIM structure and optically tested at NASA's Goddard Space Flight Center using the Optical Telescope Element SIMulator (OSIM). OSIM is a full-field, cryogenic JWST telescope simulator. SI performance, including alignment and wavefront error, was evaluated using OSIM. We describe test and analysis methods for optical performance verification of the ISIM Element, with an emphasis on the processes used to plan and execute the test. The complexity of ISIM and OSIM drove us to develop a software tool for test planning that allows for configuration control of observations, implementation of associated scripts, and management of hardware and software limits and constraints, as well as tools for rapid data evaluation, and flexible re-planning in response to the unexpected. As examples of our test and analysis approach, we discuss how factors such as the ground test thermal environment are compensated in alignment. We describe how these innovative methods for test planning and execution and post-test analysis were instrumental in the verification program for the ISIM element, with enough information to allow the reader to consider these innovations and lessons learned in this successful effort in their future testing for other programs.
KEYWORDS: Sensors, Electron multiplying charge coupled devices, Signal to noise ratio, Photon counting, Cameras, Imaging spectroscopy, Charge-coupled devices, Exoplanets, Analog electronics, Signal detection
We present the progress of characterization of a low-noise, photon counting Electron Multiplying Charged Coupled Device (EMCCD) operating in optical wavelengths and demonstrate possible solutions to the problems of Clock-Induced Charge (CIC) and other trapped charge through sub-bandgap illumination. Such a detector will be vital to the feasibility of future space-based direct imaging and spectroscopy missions for exoplanet characterization, and is scheduled to y on-board the AFTA-WFIRST mission. The 512×512 EMCCD is an e2v detector housed and clocked by a Nüvü Cameras controller. Through a multiplication gain register, this detector produces as many as 5000 electrons for a single, incident-photon-induced photoelectron produced in the detector, enabling single photon counting operation with read noise and dark current orders of magnitude below that of standard CCDs. With the extremely high contrasts (Earth-to-Sun flux ratio is ~ 10-10) and extremely faint targets (an Earth analog would measure 28th - 30th magnitude or fainter), a photon-counting EMCCD is absolutely necessary to measure the signatures of habitability on an Earth-like exoplanet within the timescale of a mission's lifetime, and we discuss the concept of operations for an EMCCD making such measurements.
KEYWORDS: Principal component analysis, Modulation, Sensors, James Webb Space Telescope, Temperature metrology, Staring arrays, Detector arrays, Control systems, Space telescopes, Near infrared
We present principal component analysis (PCA) of a flight-representative James Webb Space Telescope Near Infrared Spectrograph (NIRSpec) Detector Subsystem. Although our results are specific to NIRSpec and its T ~ 40 K SIDECAR ASICs and 5 μm cutoff H2RG detector arrays, the underlying technical approach is more general. We describe how we measured the system’s response to small environmental perturbations by modulating a set of bias voltages and temperature. We used this information to compute the system’s principal noise components. Together with information from the astronomical scene, we show how the zeroth principal component can be used to calibrate out the effects of small thermal and electrical instabilities to produce cosmetically cleaner images with significantly less correlated noise. Alternatively, if one were designing a new instrument, one could use PCA to determine a set of environmental requirements (temperature stability, electrical stability, etc.) that enabled the planned instrument to meet performance requirements.
The Near-Infrared Spectrograph (NIRSpec) is one of the four science instruments of the James Webb Space Telescope. Its focal plane consists of two HAWAII-2RG sensors operating in the wavelength range of 0.6 to 5.0 µm and, as part of characterizing NIRSpec, the noise properties of these detectors under dark and illuminated conditions were studied. Under dark conditions, and as already known, 1/f noise in the detector system causes somewhat higher noise levels than can be accounted for by a simple model that includes white read noise and shot noise on integrated charge. More surprisingly, for high levels of accumulated charge, significantly lower total noise than expected was observed. This effect is shown to be due to pixel-to-pixel correlations introduced by signal-dependent interpixel crosstalk, with an interpixel coupling factor, α , that ranges from ∼0.01 for zero signal to ∼0.03 close to saturation.
KEYWORDS: Sensors, James Webb Space Telescope, Detector arrays, Calibration, Readout integrated circuits, Space telescopes, Hubble Space Telescope, Systems modeling, Near infrared, Iris
IRS2 is a Wiener-optimal approach to using all of the reference information that Teledyne’s HAWAII-2RG
(H2RG) detector array provides. Using a new readout pattern, IRS2 regularly interleaves reference pixels with
the normal pixels during readout. This differs from conventional clocking, in which the reference pixels are read
out infrequently, and only in a few rows and columns around the outside edges of the detector array. During
calibration, the data are processed in Fourier space, which is close to the noise’s eigenspace. Using IRS2, we have
reduced the read noise of the James Webb Space Telescope (JWST) Near Infrared Spectrograph’s (NIRSpec)
H2RGs by 15% compared to conventional readout. We are attempting to achieve further gains by calibrating out
a recently recognized non-stationary noise component that appears at the frame rate. Teledyne’s new HAWAII-
4RGs (H4RG) build in a flexible capability to interleave reference pixels. We eagerly look forward to applying
IRS2 techniques to H4RGs when the opportunity arises.
KEYWORDS: Sensors, Interference (communication), Signal detection, James Webb Space Telescope, Cadmium sulfide, Infrared sensors, Quantum efficiency, Capacitance, Spectrographs, Signal processing
NIRSpec (Near Infrared Spectrograph) is one of the four science instruments of the James Webb Space Telescope
(JWST) and its focal plane consists of two HAWAII-2RG sensors operating in the wavelength range 0.6−5.0μm.
As part of characterizing NIRSpec, we studied the noise properties of these detectors under dark and illuminated
conditions. Under dark conditions, and as already known, 1/f noise in the detector system produces somewhat
more noise than can be accounted for by a simple model that includes white read noise and shot noise on
integrated charge. More surprisingly, at high flux, we observe significantly lower total noise levels than expected.
We show this effect to be due to pixel-to-pixel correlations introduced by signal dependent inter-pixel crosstalk,
with an inter-pixel coupling factor, α, that ranges from ~ 0.01 for zero signal to ~ 0.03 close to saturation.
In a previous paper,1 we described a method for significantly reducing the read noise of HAWAII-2RG (H2RG)
and SIDECAR application specific integrated circuit (ASIC) based detector systems by making better use of
reference signals. "Improved Reference Sampling & Subtraction" (IRS2; pronounced "IRS-square") is based on:
(1) making better use of the H2RG's reference output, (2) sampling reference pixels more frequently in the time
domain, and (3) optimal subtraction of both the reference output and reference pixels in the Fourier domain.
Here we demonstrate that IRS2 works as expected using an engineering grade James Webb Space Telescope
(JWST) SIDECAR ASIC and H2RG detector array. We were able to reduce the read noise per frame from
25 e- rms using traditional JWST readout to 10 e- rms per frame using IRS2. The only aspect of the system
that we changed to make these impressive improvements was the SIDECAR ASIC readout software -we did not
change the hardware.
The Extrasolar Planet Observation Characterization and the Deep Impact Extended Investigation missions (EPOXI) are
currently observing the transits of exoplanets, a comet nucleus at short range, and Earth using the High Resolution
Instrument (HRI) - a 0.3 m f/35 telescope - on the Deep Impact flyby spacecraft. The HRI is in a permanently
defocused state with the instrument point of focus about 0.6 cm before the focal plane due to the use of a reference flat
mirror that became a powered optic due to thermal warping during ground thermal-vacuum testing. Consequently, the
point spread function (PSF) covers approximately nine pixels FWHM and is characterized by a patch with three-fold
symmetry due to the three-point support structures of the primary and secondary mirrors. The PSF is also strongly color
dependent varying in shape and size with change in filtration and target color. While defocus is highly desirable for
exoplanet transit observations to limit sensitivity to intra-pixel variation, it is suboptimal for observations of spatially
resolved targets. Consequently, all images used in our analysis of such objects were deconvolved with an instrument
PSF. The instrument PSF is also being used to optimize transit analysis. We discuss development and usage of an
instrument PSF for these observations.
KEYWORDS: Sensors, James Webb Space Telescope, Interference (communication), Clocks, Detector arrays, Signal detection, Field effect transistors, Infrared sensors, Amplifiers, Mendelevium
We present a process for characterizing the correlation properties of the noise in large two-dimensional detector
arrays, and describe an efficient process for its removal. In the case of the 2k × 2k HAWAII-2RG detectors (H2RG)
detectors from Teledyne which are being used on the Near Infrared Spectrograph (NIRSpec) on the James Webb
Space Telescope (JWST), we find that we can reduce the read noise by thirty percent. Noise on large spatial
scales is dramatically reduced. With this relatively simple process, we provide a performance improvement that
is equivalent to a significant increase in telescope collecting area for high resolution spectroscopy with NIRSpec.
KEYWORDS: Sensors, James Webb Space Telescope, Electronics, Near infrared, Calibration, Infrared sensors, Space telescopes, Infrared telescopes, Infrared radiation, Astronomy
The James Webb Space Telescope, an infrared-optimized space telescope being developed by NASA for launch in 2014,
will utilize cutting-edge detector technology in its investigation of fundamental questions in astrophysics. JWST's near
infrared spectrograph, NIRSpec utilizes two 2048 × 2048 HdCdTe arrays with Sidecar ASIC readout electronics
developed by Teledyne to provide spectral coverage from 0.6 microns to 5 microns. We present recent test and
calibration results for the "pathfinder NIRSpec detector subsystem" as well as data processing routines for noise
reduction and cosmic ray rejection.
The James Webb Space Telescope (JWST) Near Infrared Spectrograph (NIRSpec) incorporates two 5 μm cutoff
(λco =5 μm) 2048×2048 pixel Teledyne HgCdTe HAWAII-2RG sensor chip assemblies. These detector arrays,
and the two Teledyne SIDECAR application specific integrated circuits that control them, are operated in space
at Τ ~ 37 K. This article focuses on the measured performance of the first flight-candidate, and near-flight
candidate, detector arrays. These are the first flight-packaged detector arrays that meet NIRSpec's challenging
6 e- rms total noise requirement. The current version of this paper has had a correction made to it at the request of the author. Please see the linked Errata for further details.
We present interim results from the characterization test development for the Detector Subsystem of the Near-Infrared
Spectrograph (NIRSpec). NIRSpec will be the primary near-infrared spectrograph on the James Webb Space Telescope
(JWST). The Detector Subsystem consists of a Focal Plane Assembly containing two Teledyne HAWAII-2RG arrays,
two Teledyne SIDECAR cryogenic application specific integrated circuits, and a warm Focal Plane Electronics box. The
Detector Characterization Laboratory at NASA's Goddard Space Flight Center will perform the Detector Subsystem
characterization tests. In this paper, we update the initial test results obtained with engineering grade components.
There are good reasons for extending the spectral range of observation to shorter wavelengths than currently envisaged
for terrestrial planet-finding missions utilizing a 4-m, diffraction-limited, optical telescope. The angular resolution at
shorter wavelengths is higher, so that the image of an exoplanet is better separated from that of the much brighter star.
Due to the higher resolution, the exozodiacal background per resolution element is smaller, so exposure times are
reduced for the same incident flux. Most importantly, the sensitivity to the presence of life on habitable exoplanets is
increased over a hundred-fold by access to the ozone biomarker in the mid-ultraviolet. These benefits must be weighed
against challenges arising from the faintness of exoplanets in the mid-UV. Here, we describe the benefits, technical
challenges and some proposed solutions for detecting ozone in the atmospheres of Earth-like exoplanets.
The Terrestrial Planet Finder-Occulter (TPF-O) is a proposed mission to find and characterize planets around nearby
stars. It uses a telescope and an external occulter to suppress the starlight so that the planets close to the star can be
observed. We have constructed Design Reference Missions (DRMs) that show that the TPF-O architecture can achieve
the science requirements. A 4.0 meter telescope and occulter system should be able to find Earth-like planets in the
equivalent search space of 42.7 continuous habitable zones (CHZ) and characterize the planets including detection of
water (at 1000 ppm) and oxygen (at 21%) in the planet's atmosphere. With a smaller telescope (2.4 meter) and occulter,
we can still probe 21.9 CHZs and detect water and oxygen in many of the planets detected.
The Space Telescope European Co-ordinating Facility (ST-ECF) and National Institute of Standards and Technology (NIST) are collaborating to study hollow cathode calibration lamps as used onboard the Hubble Space Telescope (HST). As part of the STIS Calibration Enhancement (STIS-CE) Project we are trying to improve our understanding of the performance of hollow cathode lamps and the physical processes involved in their long term operation. The original flight lamps from the Faint Object Spectrograph (FOS) and the Goddard High Resolution Spectrograph (GHRS) are the only lamps that have ever been returned to Earth after extended operation in space. We have taken spectra of all four lamps using NIST's 10.7-m normal-incidence spectrograph and Fourier transform spectrometer (FTS) optimized for use in the ultraviolet (UV). These spectra, together with spectra archived from six years of on-orbit operations and pre-launch spectra, provide a unique data set--covering a period of about 20 years--for studying aging effects in these lamps. Our findings represent important lessons for the choice and design of calibration sources and their operation in future UV and optical spectrographs in space. Our results will be directly used for planning science operations of the Cosmic Origins Spectrograph
(COS) which is going to be installed on the HST during the next servicing mission.
KEYWORDS: Computing systems, Control systems, Space telescopes, Human-machine interfaces, Data archive systems, Safety, MATLAB, Deformable mirrors, Wavefronts, Software development
The Wavefront Control Testbed (WCT) is used to demonstrate the wavefront sensing and control algorithms and procedures that will be used on the Next Generation Space Telescope (NGST). The Segmented Telescope Control Software, written in MATLAB®, is the primary development and operational tool used. The software has an extensive graphical user interface that allows the user to interact with the hardware and algorithms.
A variety of additional software programs support the Segmented Telescope Control Software (STCS). Various hardware control software interacts with MATLAB via TCP/IP connections. When access to the hardware is unnecessary or undesirable, we can access the model server that simulates the system. A stand-alone safety monitoring LabVIEW program alerts technicians if a hardware failure occurs. A C program gives the operator a graphical way of monitoring the network connections to the various systems. An Interactive Data Language (IDL) data archiving routine creates a database to monitor and maintain the testbed data and executes the MATLAB to Flexible Image Transport System (FITS) translator. Additionally we have implemented a web-based bug tracking and plan to add experiment scheduling and a document archive.
Due to the nature of the testbed, these software programs are constantly evolving, causing a variety of challenges over the years. This poster will describe these software elements and the issues that have arisen trying to use them together.
The NGST Wavefront Control Testbed (WCT) is a joint technology program managed by the Goddard Space Flight Center (GSFC) and the Jet Propulsion Laboratory (JPL) for the purpose of developing technologies relevant to the NGST optical system. The WCT provides a flexible testing environment that supports the development of wavefront sensing and control algorithms that may be used to align and control a segmented optical system. WCT is a modular system consisting of a Source Module (SM), Telescope Simulator Module (TSM) and an Aft-Optics (AO) bench. The SM incorporates multiple sources, neutral density filters and bandpass filters to provide a customized point source for the TSM. The telescope simulator module contains a flip-in mirror that selects between a small deformable mirror and three actively controlled spherical mirror segments. The TSM is capable of delivering a wide range of aberrated, unaberrated, continuous and segmented wavefronts to the AO optical bench for analysis. The AO bench consists of a series of reflective and transmissive optics that images the exit pupil of the TSM onto a 349 actuator deformable mirror that is used for wavefront correction. A Fast Steering Mirror (FSM) may be inserted into the system (AO bench) to investigate image stability and to compensate for systematic jitter when operated in a closed loop mode. We will describe the optical design and performance of the WCT hardware and discuss the impact of environmental factors on system performance.
The off-axis location of the Advanced Camera for Surveys causes strong geometric distortion in all detectors -- the Wide Field Camera (WFC), High Resolution Camera (HRC), and Solar Blind Camera (SBC). Dithered observations of rich star cluster fields are used to calibrate the distortion. We describe the observations obtained, the algorithms used to perform the calibrations and the accuracy achieved. We present our best current calibration of the geometric distortion of each of the detectors.
The Far Ultraviolet Spectroscopic Explorer (FUSE) satellite was launched into orbit on June 24, 1999. FUSE is now making high resolution ((lambda) /(Delta) (lambda) equals 20,000 - 25,000) observations of solar system, galactic, and extragalactic targets in the far ultraviolet wavelength region (905 - 1187 angstroms). Its high effective area, low background, and planned three year life allow observations of objects which have been too faint for previous high resolution instruments in this wavelength range. In this paper, we describe the on- orbit performance of the FUSE satellite during its first nine months of operation, including measurements of sensitivity and resolution.
The Space Telescope Imaging Spectrograph (STIS) is a versatile HST instrument covering the 115 - 1000 nm wavelength range in a variety of spectroscopic and imaging modes. Coverage of the ultraviolet range (115 - 310 nm) is provided by two Multi- Anode Microchannel Array (MAMA) detectors built by Ball Aerospace. The FUV MAMA covers the 115 - 170 nm range using an opaque CsI photocathode on the microchannel plate; the NUV MAMA covers the 165 - 310 nm range using a semi-transparent Cs2Te photocathode on the detector window. Both MAMAS utilize a 1024 X 1024 anode format, but detected photon events are positioned to half the spacing of the anode lines, leading to a 2048 X 2048 format for the final readout. The active area of each detector is 25.6 X 25.6 mm. Since the installation of STIS onto the Hubble Space Telescope (HST) in February 1997, the MAMAs have carried out a varied program of astronomical observing and in-flight calibration. The detectors have performed extremely well. In this report, we briefly describe the design of the STIS MAMA detectors, provide illustrative examples of their scientific use on HST, and summarize their technical performance in orbit, in such areas as sensitivity, resolution, flat-field uniformity and stability, signal-to-noise capability, dynamic range, and background.
The Space Telescope Imaging Spectrograph (STIS), installed into the Hubble Space Telescope (HST) during the second servicing mission (SSM) in February 1197, has undergone the required servicing mission orbital verification (SMOV). The overall sensitivity of STIS is summarized for direct imagery in the visible with the charge coupled device (CCD), the near UV multi-anode microchannel-array (NUV MAMA) and the far UV MAMA (FUV MAMA) detectors and likewise for the spectroscopic modes. The FUV MAMA has exceedingly low background. The NUV MAMA has a higher, temperature-dependent background due to window phosphorescence. The principle gains of the CCD over WFPCs for limiting imaging sensitivity are: high quantum efficiency, wide bandpass, low dark current and low readout-noise. The CCD, like the WFPC2 CCDs, must ge annealed periodically to heat the hot pixels generated by radiation hits. Throughput of all modes has been stable at the 1 percent level or better except for the far UV, where sensitivity is dropping slowly across the order, but more rapidly below the Lyman alpha, and beyond 150 nm. This loss in sensitivity may be due to contamination similar to that which affected the first generation HST instruments. The thermal environment for STIS is warmer than specified in the HST Interface Control Document with the result that the back end of the STIS optical bench is not under positive thermal control. Temperature swings occur due to the spacecraft solar orientation and also due to power cycling of the MAMA low voltage power supplies that are turned off during orbits that encounter the South Atlantic Anomaly. Some motion of spectral and direct image formats occurs on the detector that is correlatable with changing aft bulkhead temperature and changes in external heatloads. The MAMA detectors are capable of time-tagging photon events within 125 microsecond resolution. The Crab Pulsar was used as a time standard and demonstrates the desired performance.
The Space Telescope Imaging Spectrograph (STIS) is a second- generation instrument for the Hubble Space Telescope (HST), designed to cover the 115-1000 nm wavelength range in a versatile array of spectroscopic and imaging modes that take advantage of the angular resolution, unobstructed wavelength coverage, and dark sky offered by the HST. STIS was successfully installed into HST in 1997 February and has since completed a year of orbital checkout, capabilities that it brings to HST, illustrate those capabilities with examples drawn from the first year of STIS observing, and describe at a top level the on-orbit performance of the STIS hardware. We also point the reader to related papers that describe particular aspects of the STIS design, performance, or scientific usage in more detail.
The space telescope imaging spectrograph (STIS) was designed as a versatile spectrograph capable of maintaining or exceeding the spectroscopic capabilities of both the Goddard High Resolution Spectrograph and the Faint Object Spectrograph (FOS) over the broad bandpass extending from the UV through the visible. STIS achieves performance gains over the aforementioned first generation Hubble Space Telescope instruments primarily through the use of large a real detectors in both the UV and visible regions of the spectrum. Simultaneous spatial and spectral coverage is provided through long slit or slitless spectroscopy. This paper will review the detector design and in-flight performance. Attention will be focussed on the key issue of S/N performance. Spectra obtained during the first few months of operation, illustrate that high signal-to-noise spectra can be obtained while exploiting STIS's multiplexing advantage. From analysis of a single spectrum of GD153, with counting statistics of approximately 165, a S/N of approximately 130 is achieved per spectral resolution element in the FUV. In the NUV a single spectrum of GRW + 70D5824, with counting statistics of approximately 200, yields a S/N of approximately 150 per spectral resolution element. An even higher S/N capability is illustrated through the use of the fixed pattern split slits in the medium resolution echelle modes where observations of BD28D42 yield a signal-to-noise of approximately 250 and approximately 350 per spectral resolution element in the FUV and NUV respectively.
The Space Telescope Imaging Spectrograph (STIS) operates from the UV to near IR providing a general purpose, imaging spectroscopic capability. An internal, two mirror relay system corrects the spherical aberration and astigmatism present at the STIS field position. Low and medium resolution imaging spectroscopy is possible throughout the spectral range and over the 25 arcsecond UV and 52 arcsecond visible fields. High resolution echelle spectroscopy capability is also provided in the UV. Target acquisition is accomplished using the STIS cameras, either UV or visible; these cameras may also be used to provide broad band imaging over the complete spectral range or with the small selection of available bandpass filters. A wide selection of slits and apertures permit various combinations of spectral resolution and field size in all modes. On board calibration lamps provide wavelength calibration and flat fielding capability. We report here on the optical performance of STIS as determined during orbital verification.
The STIS instrument was installed into HST in February 1997 during the Servicing Mission 2. It has almost completed checkout and is beginning its science program, and is working well. Several scientific demonstration observations were taken to illustrate some of the range of scientific uses and modes of observation of STIS.
The second servicing mission for the Hubble Space Telescope (HST), scheduled for early 1997, will be the first change in the spectroscopic capabilities of HST since its initial deployment. The Space Telescope Imaging Spectrograph (STIS) is a multipurpose instrument covering the far ultraviolet (FUV) through near infrared spectral range. It acquires spectra at several spectral resolutions, which facilitates observations at many distances and brightnesses. STIS will replace both of the first generation spectrographs, the Goddard High Resolution Spectrograph and the Faint Object Spectrograph. This will allow the addition of a Near- Infrared Camera. STIS required the development and testing of many high quality diffraction gratings, including several very difficult echelles for the FUV. The methods and results of this grating development program are presented. The results serve as a snapshot of industry capabilities for producing high quality spaceborne diffraction gratings.
The Space Telescope Imaging Spectrograph is a second generation ultraviolet and visible light spectrograph being developed by NASA for installation on the Hubble Space Telescope during the planned 1997 servicing mission. The current schedule calls for the instrument to be completed in the spring of 1996, with an extensive period of functional, environmental, and calibration tests during the summer. A calibration plan has been developed to ensure that the basic performance characteristics of this versatile instrument will be verified and documented before launch, and that the necessary operational and data reduction databases will be adequatley populated. Our strategies to measure the radiometric sensitivity, flat-field response, dispersion relations, resolving power, scattered light, slit functions, and other properties are described. As the execution of the calibration program is still over a year away (at the time of the conference at which this paper was presented), discussion and suggestions from the engineering and scientific communities will be welcomed.
We present the results of an investigation of the in-orbit performance of the Digicon detectors in the Faint Object Spectrograph (FOS), conducted as part of the commissioning phase of the Hubble Space Telescope. This paper includes orbital results on detector background noise, sensor image stability, and photometric stability along with several typical FOS observations. This information should be of general interest to designers of future spacecraft detectors and to astronomers observing with the FOS instrument.
The Goddard High Resolution Spectrograph (GHRS) has completed Orbital Verification and is well into the Science Verification phase of its mission. The instrument performance has been flawless, and many significant early science observations have been completed. The GHRS digicon detectors are well calibrated including the determination of operating parameters, detector geometry, and noise sensitivity. Tests using calibration lamps and standard UV stars have confirmed the instrument sensitivity and spectral resolving powers of Lambda/Delta-Lambda = 2000, 20,000, and 90,000. The sensitivity has not changed since the 1984 baseline ground based calibration. The GHRS flight software has been thoroughly tested, and is controlling all instrument observing as expected. Basic target acquisition testing and GHRS alignment calibrations have been successfully completed, and targets are routinely being located within 2-3 arcsecs of the initial pointing. Observations have been successfully performed using both the 2.0 x 2.0 arcsec aperture, and the smaller 0.25 x 0.25 arcsec aperture. The extended point spread function caused by the spherical aberration of the HST primary mirror has been well measured, and observing methods to deal with it have been developed. The aberrated image allows approximately 70 percent of the total energy into the large science aperture, and 15 percent of the total energy into the smaller aperture. Numerous science assessment observations of interesting astronomical targets have been completed, and indicate the extreme usefulness of the GHRS to the scientific community.
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