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The Far Ultraviolet Spectroscopic Explorer: 1 year in…
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The Far Ultraviolet Spectroscopic Explorer: 1 year in orbit
David J. Sahnow*a, H. Warren Moosa, Scott D. Friedmana, William P. Blaira, Steven J. Conarda,
Jeffrey W. Kruka, Edward M. Murphya, William R. Oegerlea, Thomas B. Akea,b
a
Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD 21218
b
Computer Sciences Corporation, Laurel, MD 20707
ABSTRACT
The Far Ultraviolet Spectroscopic Explorer (FUSE) satellite was launched on June 24, 1999. FUSE is designed to make
high resolution (O/'O = 20,000 - 25,000) observations of solar system, galactic, and extragalactic targets in the far
ultraviolet wavelength region (905 - 1187 Å). 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.
Keywords: FUSE, satellites, ultraviolet, spectroscopy
1. INTRODUCTION
The Far Ultraviolet Spectroscopic Explorer (FUSE) was designed to measure the ultraviolet spectra of astronomical
objects at moderate to high resolution.1 The last long-term mission to explore the far ultraviolet (FUV) wavelength
region was Copernicus, in the 1970s. Since that time, there have been significant advances in fields such as optical
coatings and detector technology; this allowed an instrument to be designed that was a significant advance over what was
available at that time.
FUSE was built under the auspices of the NASA Origins program, and has served as one of the pathfinders for the PI-
class concept, where the Principal Investigator has ultimate responsibility for all phases of the mission, including design
and construction of the entire satellite, along with mission and science operations. The FUSE development team was led
by Dr. Warren Moos at the Johns Hopkins University, and included the University of Colorado and the University of
California, Berkeley, along with many industrial partners in the U.S., and the governments of France and Canada.
After numerous redesigns and reconfigurations of its design during the early 1990s - without any changes to its primary
scientific goals - construction began in the mid-1990s, leading to its launch into orbit on June 24, 1999. The FUV
detectors were turned on during August 1999, and a period of characterization and in-orbit checkout began. The first
science observations were taken in October 1999, during this checkout period. A transition to normal science operations
occurred in December 1999, and normal operations have continued since then. The mission was designed for a nominal
three year lifetime in order to have adequate time to make major contributions to several scientific projects including
determining the abundance ratio of deuterium to hydrogen (D/H), and studying hot gas in the Milky Way and beyond by
measuring OVI transitions at 1032 and 1037 Å. In addition, a significant fraction of observing time is available to the
Guest Investigator (GI) community through a competitive peer review process. The fraction of time available to GIs
increases each year as the PI team projects are completed. In any extended mission, all time except that set aside for
calibration will be available to Guest Investigators.
*
Correspondence: Email: sahnow@pha.jhu.edu; Telephone: 410 516 8503; Fax: 410 516 5494
2. SATELLITE DESIGN
2.1. Optical design
The FUSE instrument (Fig 1) consists of four coaligned optical channels. Each channel includes an off-axis parabolic
primary mirror, a focal plane assembly (FPA) containing three primary spectrograph entrance apertures (1.25"+20",
4"+20", and 30"+30"); a large, holographically-ruled, aberration-corrected spherical grating with a high groove density;
and half of a double delay line microchannel plate detector. The gratings and mirrors of two of the channels are coated
with silicon carbide (SiC), which provides an approximately constant reflectivity across the entire FUV, while the
remaining two are coated with lithium fluoride Al+LiF Coated
(LiF) over aluminum, which has a much higher Grating #2
reflectivity above ~1000 Å. Each of the LiF SiC Coated Al+LiF Coated
channels also has a visible-light Fine Error Sensor Grating #2 Grating #1
(FES) which is used for guiding. Other papers in
this volume describe the design and performance of
the telescope mirrors,2 detectors,3 and FESs,4 so
only a brief overview of the optical system is Rowland Circles
presented here. Details of the overall design have
also been given elsewhere,5 as have initial
performance results6,7.
Focal Plane
Since the reflectivity of most materials in the FUV Assemblies (4)
is poor, the optical design was developed with the Detectors (2)
goal of reducing the number of reflections while
maximizing the collecting area and providing
redundant wavelength coverage over most of the z
bandpass. In addition, constraints of the Delta II
launch vehicle limited the envelope of the satellite. y x
Lessons learned in the design, construction, and use
of the optical system is covered in an
accompanying report.8 Al+LiF Coated
Mirror #2
The mirrors and FPAs are adjustable on orbit in
order to maintain coalignment and focus of the Al+LiF Coated
channels. Once the four channels are coaligned, the Mirror #1
satellite pointing is adjusted in order to select a SiC Coated
particular aperture. Mirror #2
The spectra from the four channels fall on the SiC Coated
detectors as shown in Figure 2. Each of the four Mirror #1
detector segments contains both a SiC and LiF Figure 1 A schematic view of the FUSE optical system
spectrum from the source, in addition to background
spectra from the earth's geocorona. The design
ensures that the failure of a single detector segment does not have a major impact on the wavelength coverage.
Differences in detector performance, grating parameters, and alignment lead to a varying two-dimensional point spread
functions across each of the detectors, however, even at the same wavelength. This makes it difficult to combine data
from multiple channels at full resolution.
2.2. Data handling and processing
The location of each photon that reaches one of the detectors is sent via the science data bus to the Instrument Data
System (IDS). The IDS is responsible for storing this data as individual photon events (time tag or TTAG mode) with
periodic time markers (typically once per second), or assembling the data into a two-dimensional histogram (spectral
image or HIST mode). Targets with relatively low count rates (-2500 counts per second [cps]) are taken in time tag
mode in order to preserve information on the arrival time of each event. At higher rates (up to ~32,000 cps), the onboard
X
(16383,0)
Segment 2B (0,0)
0 (16383,0)
Segment 2A (0,0)
0
Y
1104 MDRS 1017 1005 MDRS 917
HIRS
LWRS
SiC HIRS
LWRS
MDRS MDRS
979
HIRS
LWRS
1074 LiF 1087
HIRS
LWRS
1179
(16383,1023)
) (0,1023) (16383,1023)
) ) (0,1023)
)
)
(0,1023) (16383,1023) (0,1023)
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)
988 MDRS 1082 1095 MDRS 1187
HIRS
LWRS
LiF HIRS
LWRS
MDRS MDRS
Y
1090
HIRS
LWRS
1004
SiC 992
HIRS
LWRS
905
(0,0)
0 (16383,0) (16383,0)
Segment 1A Segment 1B
X
Not to scale
Figure 2 Location of the spectra on the detectors. Pixel coordinates and wavelength coverage is labeled on each segment.
memory would be quickly exhausted, so the data from only the relevant part of the detector is binned (typically by 8) in
the y direction and stored as a two-dimensional image. Since the arrival times of the individual photons are lost in this
process, Doppler corrections for the orbital velocity can only be made on the image as a whole; as a result, each
observation is divided into multiple exposures during each orbit. The y binning has only a small effect on the resolution
of the spectra.
2.3. Satellite pointing
The FUSE spacecraft bus, built by Orbital Sciences Corporation and located below the instrument section, contains the
power, attitude control, and communications systems for the satellite. The instrument and spacecraft together make up
the FUSE satellite. The spacecraft fine pointing is accomplished with the assistance of error signals supplied by the FES.
The performance of the pointing and guiding system is described elsewhere in these proceedings.9
3. SCIENCE OPERATIONS
Science and mission operations are co-located in the FUSE Satellite Control Center (SCC) at the Johns Hopkins
University in Baltimore. Planning and scheduling of observations, generation of command loads, communication with
the satellite through an autonomous ground station, receipt and processing of science and engineering data, monitoring of
the health and safety of the satellite and instrument, computation of the FUSE orbital elements, off-line analysis of
science and engineering data, and maintenance of flight software all occur at this location.
The FUSE low earth orbit (768 km, 100 minute period, 25º inclination) adds a great deal of complexity to operations
compared with a satellite in a higher orbit. Communications with FUSE is primarily through a dedicated 5 meter ground
station antenna located at the University of Puerto Rico in Mayaguez (UPRM). Typically 7 passes per day, ranging in
length from 6 to 12 minutes, are available for communicating with the satellite. This is supplemented by passes from a
commercial facility in Hawaii, run by Universal Space Network. During the commissioning phase of the satellite, up to 7
Hawaii passes per day (using a 3 meter antenna) were used in order to supplement the UPRM passes. More recently, a 13
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Figure 3 Outline of the CALFUSE pipeline
meter antenna has become available in Hawaii, and approximately 12 to 14 passes per week are typically scheduled in
order to improve observing efficiency by providing the ability to dump data during the daily UPRM blackout period.
Since FUSE is out of touch with the SCC for most of the day, a high degree of autonomy has been built into the satellite
to allow unaided target and guide star acquisitions, instrument alignment, and health and safety checks of the instrument
by flight software. Observations proceed autonomously with no intervention from the ground. Scripts controlling each
observation are generated from the ground and uploaded several times per week. (Details of scripted operations have
been described by Artis et al.10) Science data are transmitted to the ground station at 1 megabit/sec and are stored locally
on disk at the ground station. The data are then transmitted to the SCC via an ISDN line after the ground station contact
is over. Real-time engineering telemetry is available in the SCC during each pass.
The SCC is staffed 24 hours per day by members of the Mission Operations Team who handle real time communications
and monitor satellite health and safety.
3.1. Mission planning and operational efficiency
Overall observing efficiency is limited by the low earth orbit, which results in most targets being occulted for part of
every orbit. In addition, observations cannot be made during SAA passages, slewing and peakups add overhead, and the
scattered light properties of the FES4 limit the ability to acquire targets in daylight. Unplanned mirror motions due to
thermal effects8 also decrease the efficiency of observations, particularly in the narrower slits. Early in the mission,
detector SEUs3 also had a major negative impact on the efficiency. Because of these effects, and a slower than expected
spectrograph outgassing rate, it took longer to begin normal science operations than originally anticipated.
Since that time, operational constraints, such as limitations on the pointing angles, have been implemented. In addition,
more peakup time was allocated in order to maintain coalignment. Despite these complications, the on-orbit efficiency
has now reached the level predicted before launch, and is slowly increasing as the number of calibration observations
and other special tests decreases with time. For the previous one month period, the weekly efficiency (calculated by
dividing the total time making science observations by the total number of seconds in 7 days) has been between 26% and
33%. Even higher numbers would be obtained if calibration observations, many of which can also be used for scientific
purposes, were included in this total.
3.2. Science Data Processing
Science data sent to the ground is first processed in the SCC to remove the downlink packet structure, and is then passed
through the OPUS pipeline,11 where the telemetry packet structure is removed, the raw data is converted to FITS format,
and the header keywords are populated. At that point they are passed through the CALFUSE pipeline, which is the set of
calibration routines used to correct the data for instrumental effects and generate one-dimensional, calibrated spectra for
scientific analysis. Figure 3 shows an outline of the CALFUSE pipeline, and the processing steps required for
calibration. Details are given in the FUSE Data Handbook.12 Later this year, the CALFUSE pipeline will be made
publicly available so that observers can reprocess data at their home institutions, using the latest calibration and
characterization data.
Both the raw and calibrated data are archived in the Multimission Archive at the Space Telescope Science Institute
(MAST),13 where it is available to the original proposer. After a six month proprietary period (for both PI-team and GI
data; calibration data has no proprietary period), the data become public and freely available to anyone. The short
proprietary period was designed to encourage quick turnaround on the data analysis, to allow astronomers to see the
early data before proposals have to be prepared for later cycles. The first PI-team data became public in April, 2000.
4. CURRENT STATUS AND FUTURE PLANS
As of late July 2000, all subsystems are operating nominally. The focus of the spectrograph has been optimized in the
largest spectrograph aperture, and measured resolution is in the range of 18,500 to 24,000 depending on wavelength.
This is somewhat lower than had been expected, but it is adequate for most of the scientific problems FUSE is designed
to address. The narrowest slit shows ~10% higher resolution.
A normal calibration plan is being interleaved with scientific observations. Much of this is identical to prelaunch plans,
including monitoring of standard stars for flux calibrations, and periodic quasi-flat fields with the onboard stimulation
lamps. Some modifications have been necessary in order to obtain high signal-to-noise ratio observations of bright stars
for flat field determination. These tests, and other optimizations, are ongoing as normal science observations continue.
Since launch through late July 2000, roughly 8.1 million seconds worth of exposures have been made. This number
includes PI, GI, calibration, and checkout observations. It includes 129 distinct programs, and 607 separate targets. Since
routine operations have now truly become routine, the Mission Operations and Science Data Processing groups are now
ahead looking towards more complex observation scenarios, such as observations of moving targets. In addition,
attempts will be made to improve efficiency even further.
The FUSE PI-team observing program is concentrated on several large scientific programs. These include determining
the deuterium abundance in a wide variety of environments, and measuring the properties of OVI in the Milky Way and
Magellenic Clouds. In addition to these, however, a large number of small and medium size projects are under
investigation. The first scientific results from the PI-team observations were published in late July 2000.14
Guest Investigator observations have been underway since early this year. These observations are being made along with
the PI team observations as targets reach a favorable viewing geometry. The cycle 2 proposals deadline has passed, and
the selection process will soon begin; approximately 3.6 million seconds will be available to Guest Investigators.
FUSE contains no expendable items, such as cryogens, and also is designed with redundancy in most satellite systems.
As a result, we are looking forward to a long and successful life, and many years of successful scientific investigations.
More information on the FUSE mission is available at http://fuse.pha.jhu.edu/.
ACKNOWLEDGEMENTS
We gratefully acknowledge the contributions of the many people who have contributed to the success of the FUSE
mission. This work is based on data obtained by the NASA-CNES-CSA FUSE mission operated by the Johns Hopkins
University. Financial support to U. S. participants has been provided by NASA contract NAS5-32985.
REFERENCES
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9
T. B. Ake, H. L. Fischer, J. W. Kruk, P. K. Murphy, W. R. Oegerle, "FUSE attitude control: target recognition and fine
guidance performance," Proc. SPIE 4139, 2000.
10
D. A. Artis, L. J. Frank, B. K. Heggestad, "Scripted Operations in the Far Ultraviolet Spectroscopic Explorer Flight
Software," 51st International Astronautical Congress, International Astronautical Federation, 2000.
11
J. F. Rose, C. Heller-Bowyer, M. A. Rose, M. Swam, W. Miller, G. A. Kriss and W. R. Oegerle, "OPUS: The FUSE
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12
W. Oegerle, E. Murphy, G. Kriss, "The FUSE Data Handbook," http://fuse.pha.jhu.edu/analysis/dhbook.html
13
Access to the FUSE archive is at http://archive.stsci.edu/fuse/.
14
Astrophysical Journal Letters, 538, pp. L1-L98, 2000.