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We report studies on the mitigation of optical effects of bright low-Earth-orbit (LEO) satellites on Vera C. Rubin Observatory and its Legacy Survey of Space and Time (LSST). These include options for pointing the telescope to avoid satellites, laboratory investigations of bright trails on the Rubin Observatory LSST camera sensors, algorithms for correcting image artifacts caused by bright trails, experiments on darkening SpaceX Starlink satellites, and ground-based follow-up observations. The original Starlink v0.9 satellites are g ~ 4.5 mag, and the initial experiment "DarkSat" is g ~ 6.1 mag. Future Starlink darkening plans may reach g ~ 7 mag, a brightness level that enables nonlinear image artifact correction to well below background noise. However, the satellite trails will still exist at a signal-to-noise ratio ~ 100, generating systematic errors that may impact data analysis and limit some science. For the Rubin Observatory 8.4-m mirror and a satellite at 550 km, the full width at half maximum of the trail is about 3" as the result of an out-of-focus effect, which helps avoid saturation by decreasing the peak surface brightness of the trail. For 48,000 LEOsats of apparent magnitude 4.5, about 1% of pixels in LSST nautical twilight images would need to be masked., Originally 16 pages, 11 figures, submitted to AAS Journals. Revised to 18 pages, 12 figures, accepted for publication in AJ; v2 addresses referee comments with revised figures, text updates, and additional references. Finally 17 pages, 12 figures, published in AJ; v3 includes final proofing edits and updated metadata

The Dark Energy Spectroscopic Instrument (DESI) is a fiber-fed multi-object spectroscopic instrument under construction to measure the expansion history of the Universe using the Baryon Acoustic Oscillation technique. Management of light throughput and noise in all elements of the instrument is key to achieving the high-level DESI science requirements over the planned survey area and depth within the planned survey duration. The DESI high-level science requirements flow down to instrument performance requirements on system throughput and operational efficiency. Signal-to-noise requirements directly affect minimum required exposure time per field, which dictates the pace and duration of the entire survey. The need to maximize signal (light throughput) and to minimize noise contributions and time overhead due to reconfigurations between exposures drives the instrument subsystem requirements and technical implementation. Throughput losses, noise contributors, and interexposure reconfiguration time are budgeted, tracked, and managed as DESI Systems Engineering resources. Current best estimates of throughput losses and noise contributions from each individual element of the instrument are tracked together in a master budget to calculate overall margin on completing the survey within the allotted time. That budget is a spreadsheet accessible to the entire DESI project.

[The BigBOSS experiment is a redshift survey designed to map the large scale structure of the universe and probe the nature of dark energy. Using massively-multiplexed _ber spectroscopy over 14,000 deg2 of sky, the survey will deliver more than 20 million galaxy and quasar redshifts. The resulting three dimensional sky map will contain signatures from primordial baryon acoustic oscillations (BAO) that set a "standard ruler" distance scale. Using the BAO signature, BigBOSS will measure the cosmological distance scale to < 1% accuracy from 0.5

POLARBEAR-2 is a ground based cosmic microwave background (CMB) radiation experiment observing from Atacama, Chile. The science goals of POLARBEAR-2 are to measure the CMB polarization signals originating from the inflationary gravity-wave background and weak gravitational lensing. In order to achieve these science goals, POLARBEAR-2 employs 7588 polarization sensitive transition edge sensor bolometers at observing fre­ quencies of 95 and 150 GHz with 5.5 and 3.5 arcmin beam width, respectively. The telescope is the off-axis Gregorian, Huan Tran Telescope, on which the POLARBEAR-1 receiver is currently mounted. The polarimetry is based on modulation of the polarized signal using a rotating half-wave plate and the rotation of the sky. We present the developments of the optical and polarimeter designs including the cryogenically cooled refractive optics that achieve the overall 4 degrees field-of-view, the thermal filter design, the broadband anti-reflection coating, and the rotating half-wave plate.

BigBOSS is a proposed ground-based dark energy experiment to study baryon acoustic oscillations (BAO) and the growth of structure with a 14,000 square degree galaxy and quasi-stellar object redshift survey. It consists of a 5,000- fiber-positioner focal plane feeding the spectrographs. The optical fibers are separated into ten 500 fiber slit heads at the entrance of ten identical spectrographs in a thermally insulated room. Each of the ten spectrographs has a spectral resolution (λ/Δλ) between 1500 and 4000 over a wavelength range from 360 - 980 nm. Each spectrograph uses two dichroic beam splitters to separate the spectrograph into three arms. It uses volume phase holographic (VPH) gratings for high efficiency and compactness. Each arm uses a 4096x4096 15 μm pixel charge coupled device (CCD) for the detector. We describe the requirements and current design of the BigBOSS spectrograph. Design trades (e.g. refractive versus reflective) and manufacturability are also discussed.

BigBOSS is a proposed ground-based dark energy experiment to study baryon acoustic oscillations (BAO) and the growth of large scale structure. It consists of a fiber-fed multi-object spectrograph designed to be installed on the Mayall 4-meter telescope at Kitt Peak, Arizona. BigBOSS includes an optical corrector assembly and 5000-fiber-positioner focal plane assembly that replace the existing Mayall prime focus hardware. 40-meter long optical fiber bundles are routed from the focal plane, through the telescope declination and right ascension pivots, to spectrographs in the thermally insulated FTS Laboratory, immediately adjacent to the telescope. Each of the ten spectrographs includes three separate spectral bands. The FTS Laboratory also houses support electronics, cooling, and vacuum equipment. The prime focus assembly includes mounts for the existing Mayall f/8 secondary mirror to allow observations with Cassegrain instruments. We describe the major elements of the BigBOSS instrument, plans for integrating with the Telescope, and proposed modifications and additions to existing Mayall facilities.

BigBOSS is a Stage IV dark energy experiment based on proven techniques to study baryon acoustic oscillations and the growth of large scale structure. The 2010 Astronomy and Astrophysics Decadal Survey labeled dark energy as a key area of exploration. BigBOSS is designed to perform a 14,000 square degree survey of 20 million galaxies and quasi-stellar objects. The project involves installation of a new instrument on the Mayall 4m telescope, operated by the National Optical Astronomy Observatory. The instrument includes a new optical widefield corrector, a 5,000 fiber actuator system, and a multi-object spectrometer. Systems engineering flowdown from data set requirements to instrument requirements are discussed, along with the trade considerations and a pre-conceptual baseline design of the widefield optical corrector, spectrometer and fiber positioner systems.

Throughput of a fiber-robot-based multi-object spectrograph depends on the accuracy and precision of the fiber position system. An efficient and accurate method of quantifying the performance of an actuator is necessary during the design iteration process, final design, and for post-production characterization. A CCD camera-based optical setup was developed at the Lawrence Berkeley National Laboratory to test these parameters of fiber robot positioners. The setup is described, as well as tests used to quantify distortion and cross-check measurement by smart scope.

The BigBOSS instrument is a proposed multi-object spectrograph for the Mayall 4m telescope at Kitt Peak, which will measure the redshift of 20 million galaxies and map the expansion history of the universe over the past 8 billion years, surveying 10-20 times the volume of existing studies. For each 20 minute observation, 5000 optical fibers are individually positioned by a close-packed array of 5000 robotic positioner mechanisms. Key mechanical constraints on the positioners are: o12mm hardware envelope, o14mm overlapping patrol zones, open-loop targeting accuracy ≤ 40μm, and step resolution ≤ 5μm, among other requirements on envelope, power, stability, and speed. This paper describes the design and performance of a newly-developed fiber positioner with R-θ polar kinematics, in which a flexure-based linear R-axis is stacked on a rotational θ-axis. Benefits over the usual eccentric parallel axis θ-φ kinematic approach include faster repositioning, simplified anti-collision schemes, and inherent anti-backlash preload. Performance results are given for complete positioner assemblies as well as sub-component hardware characterization.

BigBOSS is a proposed DOE-NSF Stage IV ground-based dark energy experiment designed to study baryon acoustic oscillations (BAO) and the growth of large scale structure with a 14,000 square degree survey of the redshifts of galaxies and quasi-stellar objects. The project involves modification of existing facilities operated by the National Optical Astronomy Observatory (NOAO). Design and systems engineering of a preliminary 3 degree field of view refractive corrector, atmospheric dispersion corrector (ADC), and 5000 actuator fiber positioning system are presented.

This report is based on the BigBOSS proposal submission to NOAO in October 2010, and reflects the project status at that time with minor updates; BigBOSS is a Stage IV ground-based dark energy experiment to study baryon acoustic oscillations (BAO) and the growth of structure with a wide-area galaxy and quasar redshift survey over 14,000 square degrees. It has been conditionally accepted by NOAO in response to a call for major new instrumentation and a high-impact science program for the 4-m Mayall telescope at Kitt Peak. The BigBOSS instrument is a robotically-actuated, fiber-fed spectrograph capable of taking 5000 simultaneous spectra over a wavelength range from 340 nm to 1060 nm, with a resolution R = 3000-4800. Using data from imaging surveys that are already underway, spectroscopic targets are selected that trace the underlying dark matter distribution. In particular, targets include luminous red galaxies (LRGs) up to z = 1.0, extending the BOSS LRG survey in both redshift and survey area. To probe the universe out to even higher redshift, BigBOSS will target bright [OII] emission line galaxies (ELGs) up to z = 1.7. In total, 20 million galaxy redshifts are obtained to measure the BAO feature, trace the matter power spectrum at smaller scales, and detect redshift space distortions. BigBOSS will provide additional constraints on early dark energy and on the curvature of the universe by measuring the Ly-alpha forest in the spectra of over 600,000 2.2 < z < 3.5 quasars. BigBOSS galaxy BAO measurements combined with an analysis of the broadband power, including the Ly-alpha forest in BigBOSS quasar spectra, achieves a FOM of 395 with Planck plus Stage III priors. This FOM is based on conservative assumptions for the analysis of broad band power (kmax = 0.15), and could grow to over 600 if current work allows us to push the analysis to higher wave numbers (kmax = 0.3). BigBOSS will also place constraints on theories of modified gravity and inflation, and will measure the sum of neutrino masses to 0.024 eV accuracy.

Many astrophysicists consider the mystery of accelerated expansion of the universe by a field called dark energy as the greatest challenge to solve in cosmology. Gravitational weak lensing has been identfied as one of the best methods to provide constraints on dark energy model parameters. Weak lensing introduces image shear which can be measured statistically from a large sample of galaxies by determining the ellipticity parameters. Several papers have suggested that a goal in the ability to measure shape biases should be

It is well known that a telescope with an unobstructed circular pupil delivers a smaller diffraction pattern than one centrally obstructed by its secondary mirror. Spaceborne dark energy investigations require measuring targets over a wide range of redshifts, with the most distant galaxies being the reddest, faintest, and smallest. For any given signal-to-noise (SNR) requirement, these highest redshift targets are the most demanding in terms of mission cost (time, aperture, etc), not only because they are faint but also because the diffraction pattern is largest at the longest wavelengths being observed. At the same time, a telescope's field of view must be large -- the order of a square degree -- to survey the entire extragalactic sky in reasonable time. The large field of view imposes a minimum requirement on the size of the secondary mirror baffle. For a centrally obstructed telescope, an enlarged secondary mirror baffle further enlarges the diffraction pattern. Previously published JDEM telescopes were centrally obstructed. Here, we explore unobstructed telescope designs because these can have a nearly ideal Airy diffraction pattern, avoiding both the central obstruction and the supporting spider legs, limited only by optical manufacturing and alignment errors. They therefore can deliver the best possible SNR for a given aperture. Simulations show that a 1.1m unobstructed aperture can deliver about the same cosmological constraints as a 1.4m aperture that has a 50% linear central obstruction.

Proper propellant usage is vital to ensure successful completion of initial spacecraft mission goals and to create viable mission extension options. Opportunities exist for spacecraft operators to contribute in this area through the methods that they use to estimate fuel mass, manage its distribution throughout the propellant tanks, and thermally condition it for desiredV performance. In this paper, on-orbit propellant estimation, management, and conditioning are described for the THEMIS spacecraft constellation, which investigates magnetospheric phenomena leading to the aurora borealis. THEMIS is a NASA Medium- class Explorer mission comprised of five spacecraft currently making use of fuel reserves left over from its primary mission to execute an ambitious two part mission extension— THEMIS-Low and ARTEMIS. THEMIS-Low comprises the continuation of the original THEMIS mission with the inner three spacecraft flying in a closer formation, while ARTEMIS includes low energy transfers of two of the spacecraft into lunar orbit via two Earth-Moon libration points. Current results are provided from an ongoing nonlinear regression analysis to estimate the fuel mass through the fuel's thermal response to tank heaters. Additionally, propellant management operations for all spacecraft are summarized and the authors evaluate the potential for improvedV performance through propellant thermal conditioning. Finally, lessons learned from nominal and extended THEMIS mission operations are highlighted with an emphasis on improving on-orbit propellant estimation, management, and conditioning in future spacecraft operations.

The BigBOSS experiment is a proposed DOE-NSF Stage IV ground-based dark energy experiment to study baryon acoustic oscillations (BAO) and the growth of structure with an all-sky galaxy redshift survey. The project is designed to unlock the mystery of dark energy using existing ground-based facilities operated by NOAO. A new 4000-fiber R=5000 spectrograph covering a 3-degree diameter field will measure BAO and redshift space distortions in the distribution of galaxies and hydrogen gas spanning redshifts from 0.2< z< 3.5. The Dark Energy Task Force figure of merit (DETF FoM) for this experiment is expected to be equal to that of a JDEM mission for BAO with the lower risk and cost typical of a ground-based experiment.

The Cosmic Hot Interstellar Plasma Spectrometer (CHIPS) observatory launched on 12 January 2003, and was the first and only successful GSFC UNEX (NASA Goddard Spaceflight Center University Explorer class) mission. The UNEX program was conceived by the National Aeronautics and Space Administration (NASA) as a new class of Explorer mission charged with demonstrating that significant science and/or technology experiments can be performed by small satellites with constrained budgets and a limited schedule. The purpose of the observatory was to examine details of the local bubble thermal pressure, spatial distribution and ionization history. The observatory was also used to observe solar spectra, both scattered from the Lunar surface and via a fortuitous 2nd order scattering path. CHIPS confirmed that spectral features within the 90-260a band were much dimmer than was predicted by contemporary theories, and operated four years beyond its design lifetime. The observatory was placed in an extended safe-hold mode in April of 2008 for budgetary purposes. The spectrometer consisted of six spectrograph channels which delivered >λ/100 resolution spectra to a single detector. Cost constraints of UNEX led to a design based on a traditional aluminum structure, and an instrument with a large field of view (5° x 26°). All optical and optomechanical systems on the spectrometer performed flawlessly on orbit. We discuss the challenges, difficulties and lessons learned during the design, fabrication and execution stages of the mission.

We investigate practical implementations of the Korsch 1 (1977) annular field three mirror anastigmat (AFTMA) telescope. This TMA offers a wide, diffraction-limited field of view (FOV) suitable for space-based survey missions. The advantages of this configuration include superior stray light baffling (distinct exit pupil) and the wide range of available focal lengths available from geometrically similar configurations. Key in the design of an AFTMA is the location of the Cassegrain focus (CF) and the exit pupil (EP). We investigate the effects of telescope geometry and optimization constraints on the location of the CF and EP, and suggest a family of geometries suitable for practical and vignetting-free AFTMA designs. The space of practical designs is generalized in terms of mirror diameter, telescope focal ratio and field of view for an annular, detector-tiled focal plane. Focal ratios between roughly f/10 and f/25 produce a geometric blur of less than 0.1arcsec. For this range of f/numbers, the focal plane should be less than ½ the primary mirror (PM) diameter, and the cross-axis should be located roughly 0.6 PM diameters behind the PM vertex.

We compare and contrast the Korsch (1972) full-field three-mirror anastigmat telescope (TMA) to the Korsch (1977) annular-field TMA. Both TMAs offer flat fields with comparably good aberration correction and comparably good telephoto advantage. Both offer good accessibility of the focal plane. The advantages of the FFTMA are its extremely uniform focal length over its field, its nearly telecentric final focus, and the fact that there is no hole in the center of its field. The advantages of the AFTMA are its complete accessible cold stop (essential if a warm telescope is to be used to image the sky at near-IR wavelengths) and its low sensitivity to mirror location error. Either alternative can deliver diffraction-limited visible-wavelength images over a one degree diameter field with a two meter aperture.

SNAP is a proposed space-based experiment designed to quantify dark energy by measuring the redshift-magnitude diagram of supernovae and to quantify the growth of structure in the universe by measuring weak gravitational lensing over cosmological distances. The baseline SNAP telescope is an ambient temperature three-mirror anastigmat (TMA). The goal of the stray light design is to ensure that stray light in the 0.4 to 1.7 micron wavelength range does not exceed a small fraction of Zodiacal radiation within the mission's target field near the North ecliptic pole. At visible wavelengths, we expect the primary source of stray light will be starlight scattered by the primary mirror. In our longest wavelength NIR band we expect thermal emission from the mirrors and structure will dominate. Scattered stray light is mitigated by an internal field stop, and a cold (140K) internal aperture stop. Stray light scattered by mirror roughness and particulate contamination, as well as scattering from the telescope baffles are modeled and quantified. The baseline design and analyses contained herein ensure that stray light will be less than 10% of Zodiacal in all bands.

SNAP is a proposed space-based experiment designed to study dark energy and alternate explanations of the acceleration of the universe's expansion by performing a series of complementary systematic-controlled astrophysical measurements. The principal mission activities are the construction of an accurate Type Ia supernova Hubble diagram (the supernova program) and conducting a wide-area weak gravitational lensing (WL) survey. WL measurements require highly constant point spread function (PSF) second moments (ellipticity), and the aim of this study is to expand on the 2005 Sholl, et al. preliminary work, specifically via use of the Ball Aerospace integrated modeling tool, EOSyM (End-to-end Optical System Model). This modeling environment combines thermal, structural and optical effects, including alignment errors, manufacturing residuals and diffraction, in an integrated model of the telescope. Thermo-mechanically induced motions and deformations of the mirrors are modeled as well as other disturbances, and corresponding ellipticity variations of the PSF are quantified for typical operational scenarios. In this study, the effects of seasonal variations in solar flux, transients introduced when pointing the body-fixed Ka-band antenna toward Earth, 90° roll maneuvers (planned every three months of operations) and structure dimensional changes associated with composites desorption are quantified and introduced into the optical system. Uncertainty in the telescope ellipticity distribution may be reduced by examination of foreground stars within the field of view. Reference is made to ongoing work on the use of foreground stars in quantifying the PSF.

THEMIS is a NASA Middle Explorer (MIDEX) program, managed in a “Principal Investigator (PI) mode” by the University of California at Berkeley (UCB). THEMIS employs five identical micro-satellites (otherwise called “probes”) launched by a single, Delta II/7925 rocket into a highly eccentric, 14.6 Earth radii (RE), orbit and equipped with hydrazine Reaction Control Systems (RCS) to attain their final orbits ranging from 10RE to 30RE. ATK was contracted by UCB to build the five Probes and the Probe Carrier. Aerojet-General Corporation, located in Redmond, WA, was selected by ATK Aerospace to build the RCS systems for the probes. The objective of the THEMIS mission is to place the five probes in one, two and four-day period orbits so that once every four days the probes align near their apogees along the Sun-Earth line, and capture correlated measurements of energy flow in Earth’s magnetosphere in order to pin-point the origin of magnetospheric sub-storms – a fundamental space weather process. The probes were successfully launched on 17 February 2007 out of Cape Canaveral and have been placed in their initial coast-phase positions in anticipation of their final placements in winter of 2007. This paper describes some of the challenges presented to the program by the I&T process, safety issues, and mission operations.

SNAP is a proposed space-based experiment designed to study dark energy and alternate explanations of the acceleration of the universe's expansion by performing a series of complementary systematic-controlled astrophysical measurements. The principal mission activities are the construction of an accurate Type Ia supernova Hubble diagram (the supernova program), and conducting a wide-area weak gravitational lensing (WL) survey. WL measurements benefit from a highly constant point spread function (PSF). The goal of this study is to quantify the anticipated variations in PSF arising from on-orbit thermal variations and and shrinkage associated with dryout of the composite telescope metering structure. A combined thermo-mechanical-optical analysis tool was developed, and WL metrics whisker and effective anisotropy quantified for thermal and composite structure dryout effects. Stability limits necessary for WL are defined, and compared to stability tolerances defined for the supernova program. The mission is designed for operations at at the Earth-Sun L2 Lagrange point, where thermal disturbances from Earth are minimal. In this study, the effects of seasonal variations in solar flux, transients introduced when pointing the body-fixed Ka-band antenna toward Earth and 90° roll maneuvers (planned every three months of operations) are quantified, and introduced into the optical system. Whisker and effective anisotropy were computed, and found to be well below the WL requirement for stability. The effects of composite structure shrinkage due to on-orbit H 2 O desorption are discussed, and estimated to be below WL limits for daily observations, at the beginning of the WL phase of the mission.

The Supernova/Acceleration Probe (SNAP) is a proposed space-based experiment designed to study the dark energy and alternative explanations of the acceleration of the Universe's expansion by performing a series of complementary systematics-controlled astrophysical measurements. We here describe a self-consistent reference mission design that can accomplish this goal with the two leading measurement approaches being the Type Ia supernova Hubble diagram and a wide-area weak gravitational lensing survey. This design has been optimized to first order and is now under study for further modification and optimization. A 2-m three-mirror anastigmat wide-field telescope feeds a focal plane consisting of a 0.7 square-degree imager tiled with equal areas of optical CCDs and near infrared sensors, and a high-efficiency low-resolution integral field spectrograph. The instrumentation suite provides simultaneous discovery and light-curve measurements of supernovae and then can target individual objects for detailed spectral characterization. The SNAP mission will discover thousands of Type Ia supernovae out to z = 3 and will obtain high-signal-to-noise calibrated light-curves and spectra for a subset of > 2000 supernovae at redshifts between z = 0.1 and 1.7 in a northern field and in a southern field. A wide-field survey covering one thousand square degrees in both northern and southern fields resolves {approx} 100 galaxies per square arcminute, or a total of more than 300 million galaxies. With the PSF stability afforded by a space observatory, SNAP will provide precise and accurate measurements of gravitational lensing. The high-quality data available in space, combined with the large sample of supernovae, will enable stringent control of systematic uncertainties. The resulting data set will be used to determine the energy density of dark energy and parameters that describe its dynamical behavior. The data also provide a direct test of theoretical models for the dark energy, including discrimination of vacuum energy due to the cosmological constant and various classes of dynamical scalar fields. If we assume we live in a cosmological-constant-dominated Universe, the matter density, dark energy density, and flatness of space can all be measured with SNAP supernova and weak-lensing measurements to a systematics-limited accuracy of 1%. For a flat universe, the density-to-pressure ratio of dark energy or equation of state w(z) can be similarly measured to 5% for the present value w{sub 0} and {approx} 0.1 for the time variation w' {triple_bond} dw/d ln a|{sub z=1}. For a fiducial SUGRA-inspired universe, w{sub 0} and w' can be measured to an even tighter uncertainty of 0.03 and 0.06 respectively. Note that no external priors are needed. As more accurate theoretical predictions for the small-scale weak-lensing shear develop, the conservative estimates adopted here for space-based systematics should improve, allowing even tighter constraints. While the survey strategy is tailored for supernova and weak gravitational lensing observations, the large survey area, depth, spatial resolution, time-sampling, and nine-band optical to NIR photometry will support additional independent and/or complementary dark-energy measurement approaches as well as a broad range of auxiliary science programs.

The Supernova Acceleration Probe (SNAP) will use Type Ia supernovae (SNe Ia) as distance indicators to measure the effect of dark energy on the expansion history of the Universe. (SNAP's weak-lensing program is described in a companion White Paper.) The experiment exploits supernova distance measurements up to their fundamental systematic limit; strict requirements on the monitoring of each supernova's properties lead to the need for a space-based mission. Results from pre-SNAP experiments, which characterize fundamental SN Ia properties, will be used to optimize the SNAP observing strategy to yield data, which minimize both systematic and statistical uncertainties. SNAP has achieved technological readiness and the collaboration is poised to begin construction., Comment: 12 pages, 4 figures; White paper to the Dark Energy Task Force

For true insight into the nature of dark energy, measurements of the precision and accuracy of the Supernova/Acceleration Probe (SNAP) are required. Precursor or scaled-down experiments are unavoidably limited, even for distinguishing the cosmological constant. They can pave the way for, but should not delay, SNAP by developing calibration, refinement, and systematics control (and they will also provide important, exciting astrophysics).

SNAP is a candidate for the Joint Dark Energy Mission (JDEM) that seeks to place constraints on the dark energy using two distinct methods. The first, Type Ia SN, is discussed in a companion white paper. The second method is weak gravitational lensing, which relies on the coherent distortions in the shapes of background galaxies by foreground mass structures. The excellent spatial resolution and photometric accuracy afforded by a 2-meter space-based observatory are crucial for achieving the high surface density of resolved galaxies, the tight control of systematic errors in the telescope's Point Spread Function (PSF), and the exquisite redshift accuracy and depth required by this project. These are achieved by the elimination of atmospheric distortion and much of the thermal and gravity loads on the telescope. The SN and WL methods for probing dark energy are highly complementary and the error contours from the two methods are largely orthogonal., 17 pages, 6 figures; White paper to the Dark Energy Task Force

We present the baseline telescope design for the telescope for the SuperNova/Acceleration Probe (SNAP) space mission. SNAP’s purpose is to determine expansion history of the Universe by measuring the redshifts, magnitudes, and spectral classifications of thousands of supernovae with unprecedented accuracy. Discovering and measuring these supernovae demand both a wide optical field and a high sensitivity throughout the visible and near IR wavebands. We have adopted the annular-field three-mirror anastigmat (TMA) telescope configuration, whose classical aberrations (including chromatic) are zero. We show a preliminary optmechanical design that includes important features for stray light control and on-orbit adjustment and alignment of the optics. We briefly discuss stray light and tolerance issues, and present a preliminary wavefront error budget for the SNAP Telescope. We conclude by describing some of the design tasks being carried out during the current SNAP research and development phase.

A wide field space-based imaging telescope is necessary to fully exploit the technique of observing dark matter via weak gravitational lensing. This first paper in a three part series outlines the survey strategies and relevant instrumental parameters for such a mission. As a concrete example of hardware design, we consider the proposed Supernova/Acceleration Probe (SNAP). Using SNAP engineering models, we quantify the major contributions to this telescope's Point Spread Function (PSF). These PSF contributions are relevant to any similar wide field space telescope. We further show that the PSF of SNAP or a similar telescope will be smaller than current ground-based PSFs, and more isotropic and stable over time than the PSF of the Hubble Space Telescope. We outline survey strategies for two different regimes - a ``wide'' 300 square degree survey and a ``deep'' 15 square degree survey that will accomplish various weak lensing goals including statistical studies and dark matter mapping., 25 pages, 8 figures, 1 table, replaced with Published Version

The Cosmic Hot interstellar Plasma Spectrometer (CHIPS), successfully launched on 2003 January 12, provides astronomers with an observatory dedicated to observation of the hot interstellar medium in the extreme ultraviolet. We describe here the otpical and photometric performance of the spectrograph based on calibrations of the individual components, end-to-end vacuum tests, and in-orbit observations of the Moon.

We describe the design and development of the CHIPS microchannel plate detector. The Cosmic Hot Interstellar Plasma Spectrometer will study the diffuse radiation of the interstellar medium in the extreme ultraviolet band pass of 90a to 260a. Astronomical fluxes are expected to be low, so high efficiency in the band pass, good out-of-band rejection, low intrinsic background, and minimal image non-linearities are crucial detector properties. The detector utilizes three 75mm diameter microchannel plates (MCPs) in an abutted Z stack configuration. A NaBr photocathode material deposited on the MCP top surface enhances the quantum detection efficiency. The charge pulses from the MCPs are centroided in two dimensions by a crossed-delayline (XDL) anode. A four panel thin-film filter array is affixed above the MCPs to reduce sensitivity to airglow and scattered radiation, composed of aluminum, polyimide/boron, and zirconium filter panes. The detector is housed in a flight vacuum chamber to preserve the hygroscopic photocathode, the pressure sensitive thin-film filters, and to permit application of high voltage during ground test.

The CHIPS observatory was launched on 12 January 2003, and is the first UNEX (NASA Goddard Spaceflight Center University Explorer class) mission. It is currently on-orbit and performing diffuse spectroscopy in the 90-260a wavelength band. The instrument is integrated with a custom 3-axis stabilized mini-satellite, designed for roughly one year of operation. The purpose of the observatory is examination of details of the local bubble thermal pressure, spatial distribution and ionization history. The spectrometer consists of six spectrograph channels which deliver >lambda/100 resolution spectra to a single detector. Cost constraints of UNEX led to a design based on a traditional aluminum structure, and an instrument with a large field of view (5° x 26°) for the dual purpose of increasing sensitivity in the photon-starved 90-260a band, and to reduce requirements on spacecraft pointing. All optomechanical systems on the spectrometer, including coalignment, thermal, front cover and vacuum door release are performing well on orbit. We discuss design, test and operational performance of these systems, as well as launch loads and thermal system considerations.

In the age of "Faster, Better, Cheaper", NASA's Goddard Space Flight Center has been looking for a way to implement university based world class science missions for significantly less money. The University Explorer (UNEX) program is the result. UNEX missions are designed for rapid turnaround with fixed budgets in the $10 million US dollar range. The CHIPS project was selected in 1998. The CHIPS mission has passed the Confirmation Review in August 2000 and held a Design Verification Review in April 2001. Many lessons have already been learned from the CHIPS UNEX project. This paper will discuss the early issues surrounding the use of commercial satellite constellations as the bus and the politics of small satellites using foreign launchers. The difficulties of finding a spacecraft in the UNEX price range will be highlighted. The advantages of utilizing Internet technologies from the earliest phases of the project through to communications with the spacecraft on orbit will be discussed. The current state of the program will be summarized and the project's plans for the future will be charted.

The SuperNova/Acceleration Probe (SNAP) will measure precisely the cosmological expansion history over both the acceleration and deceleration epochs and thereby constrain the nature of the dark energy that dominates our universe today. The SNAP focal plane contains equal areas of optical CCDs and NIR sensors and an integral field spectrograph. Having over 150 million pixels and a field-of-view of 0.34 square degrees, the SNAP NIR system will be the largest yet constructed. With sensitivity in the range 0.9-1.7 μm, it will detect Type Ia supernovae between z = 1 and 1.7 and will provide follow-up precision photometry for all supernovae. HgCdTe technology, with a cut-off tuned to 1.7 μm, will permit passive cooling at 140 K while maintaining noise below zodiacal levels. By dithering to remove the effects of intrapixel variations and by careful attention to other instrumental effects, we expect to control relative photometric accuracy below a few hundredths of a magnitude. Because SNAP continuously revisits the same fields we will be able to achieve outstanding statistical precision on the photometry of reference stars in these fields, allowing precise monitoring of our detectors. The capabilities of the NIR system for broadening the science reach of SNAP are discussed.

A well-adapted spectrograph concept has been developed for the SNAP (SuperNova/Acceleration Probe) experiment. The goal is to ensure proper identification of Type Ia supernovae and to standardize the magnitude of each candidate by determining explosion parameters. An instrument based on an integral field method with the powerful concept of imager slicing has been designed and is presented in this paper. The spectrograph concept is optimized to have very high efficiency and low spectral resolution (R {approx} 100), constant through the wavelength range (0.35-1.7{micro}m), adapted to the scientific goals of the mission.

CHIPS is a NASA UNEX mission designed for diffuse background spectroscopy in the EUV bandpass from 90-260a. The spectrometer is optimized for peak resolution near 170 a, in order to study diffuse emissions from cooling million degree plasma. Details of local bubble thermal pressure, spatial distribution, and ionization history are the goals of CHIPS observations. We discuss the opto-mechanical design adopted to meet the throughput, signal to noise, and spectral resolution requirements within the mass, volume, and budgetary constraints of a UNEX Delta-II secondary payload. Mechanical tolerance requirements for the six spectrometer channels are discussed, along with details of the lightweight mounting scheme for CHIPS diffraction gratings, front cover slit mechanisms and thermal design. Finally, visible light and vacuum alignment techniques are discussed, as well as with methods employed to minimize stray light.

We present a status report on CHIPS, the Cosmic Hot Interstellar Plasma Spectrometer. CHIPS is the first NASA University-Class Explorer (UNEX) project. CHIPS was selected in 1998 and is now scheduled for launch in December of 2002. The grazing incidence CHIPS spectrograph will survey the sky and record spectra of diffuse emission in the comparatively unexplored wavelength band between 90 and 260 a. These data will provide important new constraints on the temperature, ionization state, and emission measure of hot plasma in the "local bubble" of the interstellar medium.

The SuperNova/Acceleration Probe (SNAP) mission will require a two-meter class telescope delivering diffraction limited images spanning a one degree field in the visible and near infrared wavelength regime. This requirement, equivalent to nearly one billion pixel resolution, places stringent demands on its optical system in terms of field flatness, image quality, and freedom from chromatic aberration. We discuss the advantages of annular-field three-mirror anastigmat (TMA) telescopes for applications such as SNAP, and describe the features of the specific optical configuration that we have baselined for the SNAP mission. We discuss the mechanical design and choice of materials for the telescope. Then we present detailed ray traces and diffraction calculations for our baseline optical design. We briefly discuss stray light and tolerance issues, and present a preliminary wavefront error budget for the SNAP Telescope. We conclude by describing some of tasks to be carried out during the upcoming SNAP research and development phase.

The SuperNova / Acceleration Probe (SNAP) is a space-based experiment to measure the expansion history of the Universe and study both its dark energy and the dark matter. The experiment is motivated by the startling discovery that the expansion of the Universe is accelerating. A 0.7 square-degree imager comprised of 36 large format fully-depleted n-type CCD's sharing a focal plane with 36 HgCdTe detectors forms the heart of SNAP, allowing discovery and lightcurve measurements simultaneously for many supernovae. The imager and a high-efficiency low-resolution integral field spectrograph are coupled to a 2-m three mirror anastigmat wide-field telescope, which will be placed in a high-earth orbit. The SNAP mission can obtain high-signal-to-noise calibrated light-curves and spectra for over 2000 Type Ia supernovae at redshifts between z=0.1 and 1.7. The resulting data set can not only determine the amount of dark energy with high precision, but test the nature of the dark energy by examining its equation of state. In particular, dark energy due to a cosmological constant can be differentiated from alternatives such as "quintessence", by measuring the dark energy's equation of state to an accuracy of +/-0.05, and by studying its time dependence., This paper will be published in SPIE Proceedings Vol 4835 and is made available as an electronic preprint with permission of SPIE

The Supernova/Acceleration Probe (SNAP) is a proposed space-borne observatory that will survey the sky with a wide-field optical/near-infrared (NIR) imager. The images produced by SNAP will have an unprecedented combination of depth, solid-angle, angular resolution, and temporal sampling. For 16 months each, two 7.5 square-degree fields will be observed every four days to a magnitude depth of AB=27.7 in each of the SNAP filters, spanning 3500-17000\AA. Co-adding images over all epochs will give AB=30.3 per filter. In addition, a 300 square-degree field will be surveyed to AB=28 per filter, with no repeated temporal sampling. Although the survey strategy is tailored for supernova and weak gravitational lensing observations, the resulting data will support a broad range of auxiliary science programs., Comment: This paper will be published in SPIE Proceedings Vol. 4836 and is made available as an electronic preprint with permission of SPIE. One print or electronic copy may be made for personal use only. Systematic or multiple reproduction, distribution to multiple locations via electronic or other means, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited

The proposed SuperNova/Acceleration Probe (SNAP) mission will have a two-meter class telescope delivering diffraction-limited images to an instrumented 0.7 square degree field in the visible and near-infrared wavelength regime. The requirements for the instrument suite and the present configuration of the focal plane concept are presented. A two year R&D phase, largely supported by the Department of Energy, is just beginning. We describe the development activities that are taking place to advance our preparedness for mission proposal in the areas of detectors and electronics.