Establishing improved spectrophotometric standards is important for a broad range of missions and is relevant to many astrophysical problems. ACCESS, “Absolute Color Calibration Experiment for Standard Stars”, is a series of rocket-borne sub-orbital missions and ground-based experiments designed to enable improvements in the precision of the astrophysical flux scale through the transfer of absolute laboratory detector standards from the National Institute of Standards and Technology (NIST) to a network of stellar standards with a calibration accuracy of 1% and a spectral resolving power of 500 across the 0.35 − 1.7μm bandpass. This paper describes the sub-system testing, payload integration, avionics operations, and data transfer for the ACCESS instrument.
Establishing improved spectrophotometric standards is important for a broad range of missions and is relevant to many astrophysical problems. ACCESS, Absolute Color Calibration Experiment for Standard Stars", is a series of rocket-borne sub-orbital missions and ground-based experiments designed to enable improvements in the precision of the astrophysical flux scale through the transfer of absolute laboratory detector standards from the National Institute of Standards and Technology (NIST) to a network of stellar standards with a calibration accuracy of 1% and a spectral resolving power of 500 across the 0.35 - 1.7μm bandpass. This paper describes the payload status, sub-system testing, and data transfer for the ACCESS instrument.
Establishing improved spectrophotometric standards is important for a broad range of missions and is relevant
to many astrophysical problems. ACCESS, “Absolute Color Calibration Experiment for Standard Stars”, is a
series of rocket-borne sub-orbital missions and ground-based experiments designed to enable improvements in
the precision of the astrophysical flux scale through the transfer of absolute laboratory detector standards from
the National Institute of Standards and Technology (NIST) to a network of stellar standards with a calibration
accuracy of 1% and a spectral resolving power of 500 across the 0.35−1.7μm bandpass. Achieving a calibration
accuracy of 1% not only requires an accurate calibration transfer from the detector standards to the instrument,
but it also requires characterization and stability of the detector as well as a thermal background that contributes
less than 1% to the flux per resolution element in the near-infrared (1.7μm) spectral region of the ACCESS
bandpass. This paper describes the thermal mechanical design for achieving a low thermal background across
the ACCESS spectral bandpass.
Establishing improved spectrophotometric standards is important for a broad range of missions and is relevant
to many astrophysical problems. ACCESS, “Absolute Color Calibration Experiment for Standard Stars”, is a
series of rocket-borne sub-orbital missions and ground-based experiments designed to enable improvements in
the precision of the astrophysical flux scale through the transfer of absolute laboratory detector standards from
the National Institute of Standards and Technology (NIST) to a network of stellar standards with a calibration
accuracy of 1% and a spectral resolving power of 500 across the 0.35-1.7µm bandpass.
ACCESS, Absolute Color Calibration Experiment for Standard Stars, is a series of rocket-borne sub-orbital
missions and ground-based experiments designed to enable improvements in the precision of the astrophysical
flux scale through the transfer of absolute laboratory detector standards from the National Institute of Standards
and Technology (NIST) to a network of stellar standards with a calibration accuracy of 1% and a spectral resolving
power of 500 across the 0.35.1.7μm bandpass.
Establishing improved spectrophotometric standards is important for a broad range of missions and is relevant
to many astrophysical problems. Systematic errors associated with problems such as dark energy now compete
with the statistical errors and thus limit our ability to answer fundamental questions in astrophysics.
The ACCESS design, calibration strategy, and an updated preliminary performance estimate are discussed.
The On-board Calibration Monitor (OCM) is being developed as an integral component of the ACCESS instrument
and as a pathfinder for other missions. It provides stable full-aperture illumination of the telescope for
tracking the instrument sensitivity as a function of time. The light source is composed of an ensemble of LED
pairs with central wavelengths that span the spectral range of the instrument and illuminate a diffuser that is
observed by the full optical path. Feedback stabilization of the light source intensity is achieved by photodiode
monitoring of each LED. This stable source will provide real time knowledge of the ACCESS sensitivity throughout
the 5-year duration of the program. The present status of the design and laboratory evaluation of the OCM
system will be presented.
ACCESS is a recently approved rocket-borne payload with a ground-based calibration and performance monitoring
program that is designed to transfer the National Institute of Standards and Technology (NIST) absolute
laboratory standards to the stars with a precision and calibration accuracy of 1% across the 0.35-1.7μm bandpass.
This represents a significant improvement in the absolute and relative astrophysical flux calibration system,
particularly at near-infrared (NIR) wavelengths.
Establishing improved spectrophotometric standards is important for a broad range of missions and is relevant
to many astrophysical problems. In particular, the precise calibration of the flux scale across the bandpass
extending from 0.35-1.7 μm is fundamental to the precise determination of dark energy measurements based
on SNeIa photometry.
Since its launch in 1999, the Far Ultraviolet Spectroscopic Explorer (FUSE) has had a profound impact on many areas of astrophysics. Although the prime scientific instrument continues to perform well, numerous hardware failures on the attitude control system, particularly those of gyroscopes and reaction wheels, have made science operations a challenge. As each new obstacle has appeared, it has been overcome, although sometimes with changes in sky coverage capability or modifications to pointing performance. The CalFUSE data pipeline has also undergone major changes to correct for a variety of instrumental effects, and to prepare for the final archiving of the data. We describe the current state of the FUSE satellite and the challenges of operating it with only one reaction wheel and discuss the current performance of the mission and the quality of the science data.
The far ultraviolet spectral region (roughly 900 - 1200 Å) is densely packed with strong atomic, ionic and molecular transitions of astrophysical importance. Many of these transitions provide unique access to the associated species. This spectral region is also technically challenging: optical reflectivities are limited, contamination control requirements are severe and detectors must be windowless. The Far Ultraviolet Spectroscopic Explorer (FUSE) covers the spectral region 905 -1187 Å with a resolution ~ 15 km s-1. The mission, launched in June 1999 and now in its fifth year of science operations, has reaped a rich scientific harvest from this spectral region. This paper will examine the lessons learned from the FUSE mission with the perspective of looking ahead to possible future missions. In order to build on the scientific advances of the FUSE mission, such a mission would require both increased sensitivity and higher spectral resolution. We conclude that achieving these requirements will necessitate, among other advances, new approaches to far ultraviolet mirror coating technology. We also examine the possibility of accessing the far ultraviolet using an ultraviolet observatory designed for longer wavelength ultraviolet radiation.
Shull et al. have asserted that the contribution of stars, relative to quasars, to the metagalactic
background radiation that ionizes most of the baryons in the universe
remains almost completely unknown at all epochs. The potential to
directly quantify this contribution at low redshift has recently become
possible with the identification by GALEX of large numbers of
sparsely distributed faint ultraviolet galaxies. Neither STIS nor
FUSE nor GALEX have the ability to efficiently survey these sparse
fields and directly measure the Lyman continuum radiation that may leak
into the low redshift (z < 0.4) intergalactic medium. We present
here a design for a new type of far ultraviolet spectrograph, one that
is more sensitive, covers wider fields, and can provide spectra and
images of a large number of objects simultaneously, called the
Far-ultraviolet Off Rowland-circle Telescope for Imaging and
Spectroscopy (FORTIS). We intend to use a sounding rocket flight to
validate the new instrument with a simple long-slit observation of the
starburst populations in the galaxy M83. If however, the long-slit
were replaced with microshutter array, this design could isolate the
chains of blue galaxies found by GALEX over an ~30' diameter
field-of-view and directly address the Lyman continuum problem in a
long duration orbital mission. Thus, our development of the sounding
rocket instrument is a pathfinder to a new wide field spectroscopic
technology for enabling the potential discovery of the long
hypothesized but elusive Lyman continuum radiation that is thought to leak from low redshift galaxies and contribute to the ionization of the universe.
The Far Ultraviolet Spectroscopic Explorer is a NASA Origins mission
launched in June 1999 to obtain high-resolution spectra of astronomical sources at far-ultraviolet wavelengths. The science objectives require the satellite to provide inertial pointing at arbitrary positions on the sky with sub-arcsecond accuracy and stability. The requirements were met using a combination of ring-laser gyroscopes, three-axis magnetometers, and a fine error sensor for attitude knowledge, and reaction wheels for attitude control. Magnetic torquer bars are used for momentum management of the reaction wheels, and coarse sun sensors for safe mode pointing.
The gyroscopes are packaged as two coaligned inertial reference units of three orthogonal gyroscopes each. There are four reaction wheels: three oriented along orthogonal axes, the fourth skewed at equal angles (54.7°) with respect to the others. Early in the mission the gyroscopes began showing signs of aging more rapidly than expected, and one failed after two years of operation. In addition, two of the orthogonal wheels failed in late 2001. The flight software has been modified to employ the torquer bars in
conjunction with the two remaining wheels to provide fine pointing control. Additional new flight software is under development to provide attitude control if both gyroscopes fail on one or more axes.
Simulations indicate that the pointing requirements will still be met, though with some decrease in observing efficiency. We will describe the new attitude control system, compare performance characteristics before and after the reaction wheel failures, and
present predicted performance without gyroscopes.
KEYWORDS: Satellites, Space operations, Observatories, Data archive systems, Astronomy, Control systems, Gyroscopes, Space telescopes, Telescopes, Stars
The Far Ultraviolet Spectroscopic Explorer satellite (FUSE)
is a NASA Origins mission launched on 1999 June 24 and
operated from the Johns Hopkins University Homewood campus in
Baltimore, MD. FUSE consists of four aligned telescopes feeding
twin far-ultraviolet spectrographs that achieve a spectral
resolution of R=20,000 over the 905-1187 Å spectral region.
This makes FUSE complementary to the Hubble Space Telescope
and of broad general interest to the astronomical community.
FUSE is operated as a general-purpose observatory with
proposals evaluated and selected by NASA.
The FUSE mission concept evolved dramatically over time. The
version of FUSE that was built and flown was born out of the
"faster, better, cheaper" era, which drove not only the
mission development but also plans for operations. Fixed price
contracts, a commercial spacecraft, and operations in the
University environment were all parts of the low cost strategy.
The satellite performs most functions autonomously, with ground
contacts limited typically to seven 12-minute contacts per
day through a dedicated ground station. All support functions
are managed by a staff of 40 scientists and engineers located at
Johns Hopkins. In this configuration, we have been able to achieve
close to 30% average on-target science efficiency. In short, FUSE is a successful example of the "faster, better, cheaper" philosophy.
The Far Ultraviolet Spectroscopic Explorer is a NASA astrophysics satellite which produces high-resolution spectra in the far-ultraviolet (90.5 - 118.7 nm bandpass) using a high effective area and low background detectors. The observatory was launched on its three-year mission from Cape Canaveral Air Station on 24 June 1999. The instrument contains four co- aligned, normal incidence, off-axis parabolic mirrors which illuminate separate Rowland circle spectrograph channels equipped with holographically ruled diffraction gratings and delay line microchannel plate detectors. The telescope mirrors have a 352 X 387 mm aperture and 2245 mm focal length and are attached to actuator assemblies, which provide on-orbit, tip, tilt, and focus control. Two mirrors are coated with silicon carbide (SiC) and two are coated with lithium fluoride over aluminum (Al:LiF). We describe mirror assembly in-flight optical and mechanical performance. On-orbit measurements of the far-ultraviolet point spread function associated with each mirror are compared to expectations based on pre-flight laboratory measurements and modeling using the Optical Surface Analysis Code and surface metrology data. On-orbit imaging data indicate that the mirrors meet their instrument-level requirement of 50% and 95% slit transmission for the high- and mid-resolution spectrograph entrance slits, respectively. The degradation of mirror reflectivity during satellite integration and test is also discussed. The FUV reflectivity of the SiC- and Al:LiF-coated mirrors decreased about 6% and 3%, respectively, between coating and launch. Each mirror is equipped with three actuators, which consist of a stepper motor driving a ball screw via a two-stage planetary gear train. We also discuss the mechanical performance of the mirror assemblies, including actuator performance and thermal effects.
The Far Ultraviolet Spectroscopic Explorer (FUSE) is a NASA astrophysics satellite designed to produce high resolution spectra in the far-ultraviolet (90.5-118.7 nm bandpass) with a high effective area (20-70 cm2) and low background detector. It was launched on a three-year mission in June 1999 aboard a Boeing Delta II rocket. The satellite has been performing routine science observations since December 1999. FUSE contains four co-aligned, normal incidence, off-axis parabolic primary mirrors which illuminate separate Rowland circle spectrograph channels equipped with holographically ruled diffraction gratings and microchannel plate detectors. Fine error sensors (slit jaw cameras) operating in the visible on two of the channels are used for target acquisition and guiding. The FUSE mission was first proposed in the late 1980s, and experienced several major conceptual changes prior to fabrication, assembly, and testing, which lasted from 1996 through 1999. During the program, we realized both positive and negative aspects to our design and processes that may apply to other space missions using telescopes and spectrographs. The specific topics we address are requirements, design, component specification, integration, and verification. We also discuss on-orbit alignment and focus. These activities were complicated by unexpected levels of motion between the optical elements, and the logistical problems associated with limited ground contact passes in low Earth orbit. We have developed methods to characterize the motions and mitigate their resultant effects on the science data through a combination of observing techniques and modifications to the data reduction software.
The Far Ultraviolet Spectroscopic Explorer (FUSE) satellite was launched on June 24, 1999. FUSE is designed to make 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 angstrom). 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. FUSE has now been in orbit for one year. We discuss the accomplishments of the FUSE mission during this time, and look ahead to the future now that normal operations are under way.
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 FUSE is an astrophysics satellite designed to provide high resolution spectra with large effective area over the interval 90.5-118.7 nm. The FUSE instrument consists of four co-aligned, normal incidence, off-axis parabolic primary mirrors which illuminate separate Rowland circle spectrograph channels equipped with holographic gratings and delay line microchannel plate detectors.
H2 and Pt-Ne gal map spectra acquired during the pre- flight optical testing of the FUSE have been studied in order to refine the science data processing procedure. Using automated spectral line identification software and raytrace models of instrument point spread functions, a pre-flight optical performance assessment was completed. The method by which the raw 2D FUSE detector data are converted into calibrated, 1D spectra is described and illustrated using spectrograph integration and test data (I and T). A pre- flight estimate of the resolving power of FUSE is presented along with a thorough description of the associated approximations used to convert measured values of (lambda) /(Delta) (lambda) to resolving power. The PSF of data from spectrograph I and T and optical-end-to-end testing are presented, and the test factors causing their different appearances is discussed.
The FUSE is an astrophysics satellite designed to make observations at high spectral resolving power in the 90.5- 118.7 nm bandpass. This NASA Origins mission will address many important astrophysical problems, including the variations in the deuterium/hydrogen ratio in the Milky Way and in extragalactic clouds, the kinematics and distribution of O5+ and other hot gas species in the Galactic disk and halo, the properties of molecular hydrogen in interstellar clouds having a wide variety of temperatures and densities, and the properties of stellar and planetary atmospheres. Between August 1997 and January 1999 an extensive series of vacuum optical test was conducted, first with the spectrograph alone and then with the full satellite in flight-like conditions. Numerous UV spectra were obtained and found to be consistent with performance requirements. We also obtained visible light images with the fine error sensor camera, whose performance will be critical for meeting the demanding pointing requirements of FUSE. In this paper we present estimates of the performance of the instrument, including spectral resolution, line shapes, and effective area. We also present data on the visible light performance of the FES.
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 Far UV Spectroscopic Explorer (FUSE) satellite will make high spectral resolving power measurements of astrophysical objects in the 905-1187 angstrom wavelength region from low- earth orbit. Its high effective area and low background will permit observations of solar system, galactic, and extragalactic targets that have been too faint for previous instruments at this high resolution. Integration and test of the FUSE instrument is currently underway in preparation for launch in late 1998. We describe the current status of the FUSE satellite, including details of the optical and mechanical measurements made during component and subsystem- level testing. In addition, we make an estimate of instrument on-orbit performance from data obtained during instrument integration and test.
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 Far Ultraviolet Spectroscopic Explorer (FUSE) satellite will obtain high spectral resolving power ((lambda) /(Delta) (lambda) equals 30,000) measurements of astrophysical objects in the 905 - 1195 angstroms wavelength region from low-earth orbit. The instrument's high effective area (30 - 100 cm2) and low detector background will permit observations of solar system, galactic, and extragalactic targets that have been too faint for previous instruments at this high resolution. The instrument design achieves both high resolution and high throughput by using four nearly identical optical channels. The optics consist of four normal incidence mirrors, four high density holographically-ruled diffraction gratings, and a pair of large format double delay line detectors. These components are supported by a graphite-composite structure. A commercially-procured spacecraft provides pointing stability of 0.5 arcseconds (1 (omega) ), by using data from a Fine Error Sensor included in the instrument. In early 1995 the FUSE mission was reconstructed to be a lower-cost, PI-class mission. The construction phase began in December, 1995, and launch is scheduled for late 1998. We present a description of the FUSE instrument, including details of the optical and mechanical design, along with an estimate of its on-orbit performance.
A recently constructed calibration facility utilizing a Manson soft x-ray line source in the wavelength range of 8 - 114 angstroms and a Penning ionization discharge (PID) in the 100 - 350 angstroms range, has been used to map the reflectivity across a curved layered synthetic microstructure (LSM) coated surface. This calibrated mirror was also used to image the Al III emission ((lambda) equals 170 - 175 angstroms) from the PID.
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