We present characterization results from a photon counting imaging detector consisting of one microchannel plate (MCP) and an array of two readout integrated circuits (ROIC) that record photon position. The ROICs used in the position readout are the high event rate ROIC (HEROIC) devices designed to handle event rates up to 1 MHz per pixel, recently developed by the Ball Aerospace and Technologies Corporation in collaboration with the University of Colorado. An opaque cesium iodide (CsI) photocathode sensitive in the far-ultraviolet (FUV; 122-200 nm), is deposited on the upper surface of the MCP. The detector is characterized in a chamber developed by CU Boulder that is capable of illumination with vacuum-ultraviolet (VUV) monochromatic light and measurement of absolute ux with a calibrated photodiode. Testing includes investigation of the effects of adjustment of internal settings of the HEROIC devices including charge threshold, gain, and amplifier bias. The detector response to high count rates is tested. We report initial results including background, uniformity, and quantum detection efficiency (QDE) as a function of wavelength.
The next generation of astronomical photocathode / microchannel plate based UV photon counting detectors will overcome existing count rate limitations by replacing the anode arrays and external cabled electronics with anode arrays integrated into imaging Read Out Integrated Circuits (ROICs). We have fabricated a High Event Rate ROIC (HEROIC) consisting of a 32 by 32 array of 55 μm square pixels on a 60 μm pitch. The pixel sensitivity (threshold) has been designed to be globally programmable between 1 × 103 and 1 × 106 electrons. To achieve the sensitivity of 1 × 103 electrons, parasitic capacitances had to be minimized and this was achieved by fabricating the ROIC in a 65 nm CMOS process. The ROIC has been designed to support pixel counts up to 4096 events per integration period at rates up to 1 MHz per pixel. Integration time periods can be controlled via an external signal with a time resolution of less than 1 microsecond enabling temporally resolved imaging and spectroscopy of astronomical sources. An electrical injection port is provided to verify functionality and performance of each ROIC prior to vacuum integration with a photocathode and microchannel plate amplifier. Test results on the first ROICs using the electrical injection port demonstrate sensitivities between 3 × 103 and 4 × 105 electrons are achieved. A number of fixes are identified for a re-spin of this ROIC.
We present a conceptual design for a high-resolution optical spectrograph appropriate for mounting at Cassegrain on a large aperture telescope. The design is based on our work for the Gemini High Resolution Optical Spectrograph (CUGHOS) project. Our design places the spectrograph at Cassegrain focus to maximize throughput and blue wavelength coverage, delivering R=40,000 resolving power over a continuous 320–1050 nm waveband with throughputs twice those of current instruments. The optical design uses a two-arm, cross-dispersed echelle format with each arm optimized to maximize efficiency. A fixed image slicer is used to minimize optics sizes. The principal challenge for the instrument design is to minimize flexure and degradation of the optical image. To ensure image stability, our opto-mechanical design combines a cost-effective, passively stable bench employing a honeycomb aluminum structure with active flexure control. The active flexure compensation consists of hexapod mounts for each focal plane with full 6-axis range of motion capability to correct for focus and beam displacement. We verified instrument performance using an integrated model that couples the optical and mechanical design to image performance. The full end-to-end modeling of the system under gravitational, thermal, and vibrational perturbations shows that deflections of the optical beam at the focal plane are <29 μm per exposure under the worst case scenario (<10 μm for most orientations), with final correction to 5 μm or better using open-loop active control to meet the stability requirement. The design elements and high fidelity modeling process are generally applicable to instruments requiring high stability under a varying gravity vector.
Traditional mirror manufacturing, particularly for astronomical purposes, requires substantial lead time, due to the nature of the materials and the grinding/polishing process. We propose a new technique for rapid, low-cost production of large, lightweight precision optics by fusing several technologies which in combination we call frozen membrane mirror technology (FMMT). FMMT combines well-understood subsystem technologies, including electrostatic control of membrane mirrors, adaptive optics, wavefront sensing and control, and inflatable structures technology to shorten production time. The basic technique is to control the surface of a reflective coated membrane mirror with electrostatic actuation and wavefront sensor feedback and freeze the membrane shape. We discuss the details of the concept and present results of early lab testing. We focus on the optical regime, but this technology has applicability from the microwave to x-ray spectral bands. Starting with a flexible membrane mirror, one can envision techniques for deployment of large apertures in space.
Kepler is NASA’s first space mission dedicated to the study of exoplanets. The primary scientific goal is statistical—to estimate the frequency of planetary systems associated with sun-like stars, especially the detection of earth-size planets in the habitable zones. Kepler was launched into an Earth-trailing heliocentric “drift-away” orbit (period=372 days ) in March 2009. The instrument detects the faint photometric signals of transits of planets across the stellar disks of those systems with orbital planes fortuitously oriented in our line of sight. Since the probability of such alignments is small, Kepler must observe a large number of stars. In fact, Kepler is monitoring approximately 150,000 stars with a 30-min cadence. The scientific goals led to the choice of a classical Schmidt telescope, and requirements on field-of-view, throughput, spectral bandpass, image quality, scattered light, thermal and opto-mechanical stability, and in-flight adjustment authority. We review the measurement requirements, telescope design, prelaunch integration, alignment, and test program, and we describe the in-flight commissioning that optimized the performance. The stability of the flight system has enabled increasing recognition of small effects and sophistication in data processing algorithms. Astrophysical noise arising from intrinsic stellar variability is now the dominant term in the photometric error budget.
Silicon immersion gratings (SIGs) offer several advantages over the commercial echelle gratings for high
resolution infrared (IR) spectroscopy: 3.4 times the gain in dispersion or ~10 times the reduction in the
instrument volume, a multiplex gain for a large continuous wavelength coverage and low cost. We
present results from lab characterization of a large format SIG of astronomical observation quality. This
SIG, with a 54.74 degree blaze angle (R1.4), 16.1 l/mm groove density, and 50x86 mm2 grating area, was
developed for high resolution IR spectroscopy (R~70,000) in the near IR (1.1-2.5 μm). Its entrance
surface was coated with a single layer of silicon nitride antireflection (AR) coating and its grating surface
was coated with a thin layer of gold to increase its throughput at 1.1-2.5 m. The lab measurements have
shown that the SIG delivered a spectral resolution of R=114,000 at 1.55 m with a lab testing
spectrograph with a 20 mm diameter pupil. The measured peak grating efficiency is 72% at 1.55 m,
which is consistent with the measurements in the optical wavelengths from the grating surface at the air
side. This SIG is being implemented in a new generation cryogenic IR spectrograph, called the Florida IR
Silicon immersion grating spectrometer (FIRST), to offer broad-band high resolution IR spectroscopy
with R=72,000 at 1.4-1.8 um under a typical seeing condition in a single exposure with a 2kx2k H2RG IR
array at the robotically controlled Tennessee State University 2-meter Automatic Spectroscopic Telescope
(AST) at Fairborn Observatory in Arizona. FIRST is designed to provide high precision Doppler
measurements (~4 m/s) for the identification and characterization of extrasolar planets, especially rocky
planets in habitable zones, orbiting low mass M dwarf stars. It will also be used for other high resolution
IR spectroscopic observations of such as young stars, brown dwarfs, magnetic fields, star formation and
interstellar mediums. An optimally designed SIG of the similar size can be used in the Silicon Immersion
Grating Spectrometer (SIGS) to fill the need for high resolution spectroscopy at mid IR to far IR (~25-300 μm) for the NASA SOFIA airborne mission in the future.
Kepler is NASA's first space mission dedicated to the study of exoplanets. The primary scientific goal is statistical - to
estimate the frequency of planetary systems associated with sun-like stars. Kepler was launched into an Earth-trailing
heliocentric "drift-away" orbit in March 2009, and is monitoring 150,000 stars. The instrument detects the faint
photometric signals of transits of those systems whose orbital planes are oriented in our line-of-sight. An Earth-Sun
analog will produce a transit depth of 80 parts per million (ppm), lasting for at most a few tens of hours, and repeating
once per "year". The instrumentation was designed to provide photometric data with a precision of 20 parts per million
in 6.5 hours for 12th magnitude stars, resulting in a signal-to-noise ratio of 4 for an Earth-Sun transit. The stability of the
flight system enables the precision of the data that reveal subtle instrumental and astrophysical effects that in turn allow a
deeper understanding of the performance of the hardware, to enhanced operational procedures, and to novel post-processing
of the data. The data are approaching the sensitivity needed to detect transits of terrestrial planets. Intrinsic
stellar variability is now the most significant component of the photometric error budget.
Kepler is NASA's first space mission dedicated to the study of exoplanets. The primary scientific goal is statistical - to
estimate the frequency of planetary systems associated with sun-like stars, especially the detection of earth-size planets
in the Habitable Zones. Kepler was launched into an Earth-trailing heliocentric "drift-away" orbit (period = 372 days) in
March 2009. The instrument detects the faint photometric signals of transits of planets across the stellar disks of those
systems with orbital planes fortuitously oriented in our line-of-sight. Since the probability of such alignments is small
Kepler must observe a large number of stars. In fact, Kepler is monitoring approximately 150,000 stars with a 30-minute
cadence. These scientific requirements led to the choice of a classical Schmidt telescope, and requirements on field-of-view
(FOV), throughput, spectral bandpass, image quality, scattered light, thermal and opto-mechanical stability and in-flight
adjustment authority. We review the pre-launch integration, alignment and test program, and we describe the in-flight
commissioning that optimized the optical performance of the observatory. The stability of the flight system has
enabled increasing recognition of small effects and increasing sophistication in data processing algorithms. Astrophysical
noise arising from intrinsic stellar variability is now the dominant term in the photometric error budget.
We present results of a study of a deployable version of the Advanced Technology Large-Aperture Space Telescope
(ATLAST), designed to operate in a Sun-Earth L2 orbit. The primary mirror of the segmented 9.2-meter aperture has 36
hexagonal 1.315 m (flat-to-flat) glass mirrors. The architecture and folding of the telescope is similar to JWST, allowing
it to fit into the 6.5 m fairing of a modest upgrade to the Delta-IV Heavy version of the Evolved Expendable Launch
Vehicle (EELV). We discuss the overall observatory design, optical design, instruments, stray light, wavefront sensing
and control, pointing and thermal control, and in-space servicing options.
Precise testbeds are required to investigate the physics and engineering aspects of suppressing extrasolar starlight
sufficiently to discern faint companion planets. In addition, testbeds that can simultaneously produce star and planet
stimuli will be necessary ground support equipment for evaluating instruments designed for imaging and characterizing
extrasolar planets. Integral to this is the ability to represent the broad spectral bands and relative geometry of stars and
planets. We have built upon the Terrestrial Planet Finder Coronagraph (TPF-C) requirements as well as those of
programs like Extrasolar Planet Imaging Coronagraph (EPIC) and Eclipse to develop a star/planet simulator (SPS) that,
in conjunction with other testbed modules, can facilitate the pursuit of pertinent questions. The star/planet simulator
developed has a broadband visible light source that illuminates independently adjustable star and planet sources (angular
separation and orientation, relative magnitude). It is capable of providing either collimated or direct imaged light to
proposed instruments and can be configured to produce the source stimuli in a vacuum environment. We will describe
the physical set-up, measurements, and initial observations as well as the plans for combining with a coronagraphic
testbed.
The scientific achievements of the Hubble Space Telescope have motivated interest in a larger and more powerful
successor that can operate at ultraviolet and optical wavelengths. NASA recently supported a Visions Mission study
called the Modern Universe Space Telescope. The scientific goals require the angular resolution expected from a 10m
aperture in visible light and the sensitivity provided by 50m2 of collecting area. The approach developed by the MUST
study team uses a segmented primary mirror that can be assembled in space. Assembly offers advantages over
deployment with regard to mass and volume efficient stowage in the launch vehicle fairing, and simplicity of the
mechanical and structural design. If the system is designed thoughtfully from the beginning, then robotic techniques
such as those investigated for HST servicing might be used to great advantage. Alternatively, if the space operations
infrastructure included in the Vision for Space Exploration is developed, then either astronaut EVA or telerobotic
assembly techniques could be employed. In either case, in-space assembly enables a telescope that is substantially larger
than the diameter of the launch vehicle.
There is a growing consensus that a substantial fraction of the matter in the universe, especially what we think of as normal baryonic matter, exists in a tenuous, hot filamentary intergalactic medium often referred to as the Cosmic Web. Improving our understanding of the web has been a high priority scientific goal in NASA's planning and roadmapping activities. NASA recently supported an Origins Probe study that explored the observable phenomenology of the web in detail and developed concepts for the instrumentation and mission. The Baryonic Structure Probe operates in the ultraviolet spectral region, using primarily O VI (λλ 1032, 1038 angstrom) and HI Ly α (λ 1216 angstrom) as tracers of the web. A productive investigation requires both moderate resolution (R = λ/Δλ ~ 30000) absorption line spectroscopy using faint background quasars as continuum sources, and imaging of the diffuse filaments in emission lines of the same ions.
Spectroscopic sensitivity to quasars as faint as V ~ 19 will probe a large number of sight lines to derive physical
diagnostics over the redshift range 0 < z < 1. Spectral imaging with a wide field of view and sensitivity to a redshift
range 0 < z < 0.3 will map the filaments in a large volume of the universe after the web had evolved to near its modern
structure. This paper summarizes the scientific goals, identifies the measurement requirements derived from them, and
describes the instrument concepts and overall mission architecture developed by the BSP study team.
The optical telescope for a spaceborne coronagraph to detect terrestrial to Jovian-sized planets has unusually stringent phase and amplitude requirements - far exceeding a "conventional" telescope like Hubble or the James Webb Space Telescope. The key engineering requirements will be summarized based on probable mission science objectives and an engineering solution with a monolithic primary mirror on the order of 6 meters by 4 meters. We will also present an optical design for a sub-scale coronagraphic simulator as a logical and essential step in examining the system sensitivities. Testbed simulations will include F, G, and K stars and companion planets ranging in size from earth-like up to Jovian-like.
The telescope for a Terrestrial Planet Finder (TPF) coronagraph has exceedingly stringent phase and amplitude requirements, especially for the large, monolithic primary mirror (possibly as large as 4 meters by 10 meters). The pertinent derived engineering requirements will be summarized based on a described set of science objectives to simulate solar type stars and their companion earth-size planets. We will also present an optical design for a sub-scale coronagraphic testbed as an essential step in examining the system sensitivities. The major subassemblies of the testbed include: 1) a star/planet simulator that affords variation in contrast, adjustable relative separation and angular orientation and 2) a relay optical system representative of a TPF 3-mirror telescope that allows the imposition of known optical perturbations over the desired wavefront spatial frequencies. We will compare these TPF testbed mirror wavefront requirements with levels recently achieved on the Advanced Mirror System Demonstrator and planned for the James Webb Space Telescope (JWST).
We present the preliminary calibration results for the Cosmic Origins Spectrograph, a fourth generation replacement instrument for the Hubble Space Telescope due to be installed in mid-2005. The Cosmic Origins Spectrograph consists of two spectroscopic channels: a far ultraviolet channel that observes wavelengths between 1150 and 2000 Åand a near ultraviolet channel that observes between 1700 and 3200 Å. Each channel supports moderate (R≈20,000) and low (R≈2000) spectral resolution. We discuss the calibration methodology, test configurations, and preliminary end-to-end calibration results. This includes spectral resolution, system efficiency, flat fields, and wavelength scales for each channel. We also present the measured transmission of the Bright Object Aperture (BOA) and the measured spatial resolution.
Radiation induced phosphorescence of UV window materials has been identified as a source of background signal in UV detectors for as long as these detectors have flown in space, but there is little detailed knowledge of the spectrum, decay time constants or thermal dependence of the phosphorescence. We present initial results of a study undertaken to characterize this source of background signal, including spectra, decay timescale analysis, and preliminary assessment of depopulation/deexcitation techniques. The ultimate goals of this study are to identify and evaluate phosphorescence mitigation techniques and to identify the source of the phosphorescence in optical materials.
During our NASA sponsored study of candidate architectures for the Terrestrial Planet Finder mission we estimated the values of observable properties that would be accessible to an instrument intended to detect starlight reflected by a planet in the habitable zone of the system. These properties include architecture and wavelength independent geometrical properties such as angular separation between the star and planet, and timescales associated with orbital motion. Properties that do depend on the detection technique and wavelength include the brightness of the planet, its contrast relative to the star, and variability associated with diurnal and seasonal phenomena. The search space for a reflected light TPF is the range of these parameters calculated for a sample of 200 main sequence stars whose stellar properties make them potential targets. A scientific investigation such as that described by the TPF Science Working Group then leads to requirements on the sensitivity of the system, angular resolution, suppression of starlight and operational efficiency. We will describe our star sample, the search space of planetary observables and apparent system requirements.
KEYWORDS: Planets, Stars, Signal to noise ratio, Space telescopes, Hubble Space Telescope, Coronagraphy, Wavefronts, Exoplanets, Telescopes, Point spread functions
Recent advances in deformable mirror technology for correcting wavefront errors and in pupil shapes and masks for coronagraphic suppression of diffracted starlight enable a powerful approach to detecting extrasolar planets in reflected (scattered) starlight at visible wavelengths. We discuss the planet-finding performance of Hubble-like telescopes using these technical advances. A telescope of aperture of at least 4 meters could accomplish the goals of the Terrestrial Planet Finder (TPF) mission. The '4mTPF' detects an Earth around a Sun at five parsecs in about one hour of integration time. It finds molecular oxygen, ozone, water vapor, the 'red edge' of chlorophyll-containing land-plant leaves, and the total atmospheric column density -- all in forty hours or less. The 4mTPF has a strong science program of discovery and characterization of extrasolar planets and planetary systems, including other worlds like Earth. With other astronomical instruments sharing the focal plane, the 4mTPF could also continue and expand the general program of astronomical research of the Hubble Space Telescope.
The Next Generation Space Telescope will be the premier instrument for astrophysical research a decade from now. This paper describes the reference concept for the observatory being studied by a prime contractor team led by TRW and Ball Aerospace. We give an overview of the space segment of the mission, and the rationale for its heliocentric orbit at the Sun-Earth L2 Lagrangian point. At the time of this meeting many details of the engineering design are still open for trade studies. We highlight a few whose resolution will have implications for the scientific performance of the observatory, and for which preferences and recommendations from the scientific community are welcomed.
Jon Morse, James Green, Dennis Ebbets, John Andrews, Sara Heap, Claus Leitherer, J. Linsky, Blair Savage, J. Shull, Theodore Snow, S. Alan Stern, John Stocke, Erik Wilkinson
We present an overview of the expected performance and science goals of the cosmic origins spectrograph (COS), a fourth generation instrument to be installed aborad the Hubble Space Telescope (HST) during the fourth HST servicing mission scheduled for late 2002. COS is a UV spectrograph optimized for observing faint point sources with moderate spectral resolution. The instrument has two channels: a far- UV channel that is sensitive in the 1150-1775 angstrom wavelength range and a near-UV channel that operates between 1750-3200 angstrom. The COS science team program concentrates on QSO absorption line systems and the IGM, dynamics of the ISM in galaxies and galaxy halos, UV extinction in the Milky Way, horizontal-branch stars in globular clusters, and volatile gases in the atmospheres of solar system bodies.
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 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.
The scientific goals of the Goddard High Resolution Spectrograph (GHRS) require data of very high integrity, with high S/N, spectral resolution, wavelength calibration and photometric accuracy. The instrument was designed to have optical, structural, thermal and detector performance capable of delivering nearly photon limited ultraviolet spectra. Many important characteristics such as geometrical formats, sensitivity functions, resolving power and linearity were predicted from design specifications, and were tested and verified. Instrumental properties which may limit the ultimate performance, such as stray light, distortions and geometrical instabilities were anticipated but not precisely modeled. A careful test program, comprising several distinct pre- and post-launch phases identified, characterized and in several cases eliminated potentially troublesome aspects. In the spirit of 'lessons learned' this paper describes a number of optical and detector related artifacts, how they were identified and measured, and either eliminated by reworking the hardware, or calibrated and compensated for during data reduction. Builders of future instruments should be aware of the types of anomalies we encountered so they may be avoided in the design phases, or well calibrated in the testing phase. We will discuss aspects of the testing programs, both technical and programmatic, which contributed to the successful commissioning of the GHRS and its current return of high quality spectroscopic data.
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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