The Far Ultraviolet Spectroscopic Explorer Mission

Poster paper presented at the 1995 AAS meeting in Pittsburgh

N.B. Specific performance data has changed. See the FUSE Observer's Guide for up-to-date numbers


D.J. Sahnow, S.D. Friedman, W. Moos, M.E. Perry, W.R. Oegerle (Johns Hopkins University)

J.C.Green (University of Colorado)

O.H.W.Seigmund (University of California, Berkeley)


ABSTRACT

The Far Ultraviolet Spectroscopic Explorer (FUSE) satellite will make high spectral resolution (lambda/delta lambda = 30,000) measurements in the 905-1195 Å bandpass from low-earth orbit. Its high sensitivity and low background will permit observations of solar system, galactic, and extragalactic targets that have been too faint for previous instruments at this resolution. Lead by the Johns Hopkins University (JHU), FUSE was recently restructured as a Principal Investigator class mission, with a rapid development schedule and a 1998 launch. Both mission development and mission operations will be centered at JHU. A large fraction of the observation time will be available for guest investigation.

We present a description of the FUSE mission, including details of the instrument design and its estimated performance. Although constrained by the launch vehicle shroud and by grating technology, the optical design achieves a high effective area (30-100 cm2) by using four nearly identical 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 and are thermally controlled to less than one degree Celsius. The instrument and the commercially- procured spacecraft will be launched into an 800 km, 25 degree inclination orbit on a Med- Lite expendable launch vehicle.

1. INTRODUCTION

The spectral region between 900 and 1200 Å contains many of the most important transitions for studying astrophysical processes. Since the Copernicus mission, launched in 1972, there have been no long duration missions to obtain spectra in this region. Shorter missions, such as HUT, IMAPS, and ORFEUS have shown the promise of this wavelength region, but they have not had the combination of effective area, resolution and wavelength coverage necessary to fully explore this region.

FUSE, the Far Ultraviolet Spectroscopic Explorer, was originally proposed to answer a set of fundamental questions about the nature of the Universe posed by the Astronomy Survey Committee of the National Academy of Sciences in 1982 (Field et al. 1982). In 1986, a team of scientists led by Dr. Warren Moos at the Johns Hopkins University proposed an Explorer mission to help answer these questions. In 1988 the mission was chosen for a competitive Phase A study, and in 1989 funding was awarded for Phase B.

Since that time, the FUSE program has undergone a number of major redesigns. As initially conceived, it contained a grazing incidence telescope and mechanically-ruled aspheric grating, and was meant to be attached to the Explorer Platform after reaching orbit via the space shuttle. Since the beginning of Phase B, the launch has been moved to an expendable launch vehicle, and costs constraints forced the optics to be redesigned to normal incidence optics.

The most recent restructuring occurred in late 1994 and early 1995, when the FUSE budget was reduced to $100 million. At that time, FUSE became a PI-class mission, with Johns Hopkins University taking over responsibility for all aspects of the mission, including the instrument, spacecraft, and ground operations.

The FUSE program is a collaboration between the following institutions:

Johns Hopkins University - Homewood        Science Oversight, Project Management, Systems     
                                           Engineering, Instrument Structure, Mirror          
                                           Assemblies, Integration & Test                     

Johns Hopkins University - Applied         Spacecraft (commercially procured), Instrument     
Physics Laboratory                         Data System                                        

University of Colorado                     Spectrograph, Focal Plane Assemblies               

University of California, Berkeley         Detectors                                          

Canadian Space Agency                      Fine Error Sensors, Baffles                        

CNES (France)                              Diffraction Gratings                               

2. INSTRUMENT DESIGN

A schematic view of the instrument is shown in Figure 1.

Figure 1 The FUSE instrument. The total length of the instrument is approximately 4 meters.

The design includes four coaligned telescope mirrors; four Focal Plane Assemblies (FPAs), each of which contains four apertures; four spherical, aberration-corrected holographically-recorded diffraction gratings; and two microchannel plate detectors with delay line anodes. A visible light Fine Error Sensor (FES) maintains subarcsecond pointing of the entire spacecraft. A composite structure maintains the positions of the optical elements to the several micron level while the temperature is controlled to 1 degree C. Some important instrument parameters are given in Table 1.


Table 1 Instrument Parameters

Mirror Assemblies           Type                              Off Axis Parabolas               
                            Size                              387 x 352 mm clear aperture      
                            Focal Length                      2245 mm                          
                            Substrate                         Zerodur                          
                            Coating                           2 SiC, 2 Al+LiF                  
                                                                                               
Focal Plane Assemblies      Apertures                         1.5 x 20, 4 x 20, 30 x 30,       
                                                              0.2 arcsec diameter              
                                                                                               
Spectrograph                Rowland Circle Diameter           1652 mm                          
                            Grating Type                      Spherical, aberration            
                                                              corrected, holographically       
                                                              ruled                            
                            Grating Size                      260 x 259 mm clear aperture      
                            Characteristic Grating Line       5767  lines / mm (SiC) 5350      
                            Density                           lines / mm (LiF)                 
                            Grating Coatings                  2 SiC, 2 Al+LiF                  
                                                                                               
Detectors                   Type                              MCP with double delay line       
                                                              anode. Curved to match Rowland   
                                                              circle                           
                            Size                              179 x 10 mm in two 85 x 10 mm    
                                                              segments, with 9 mm gap          
                            Photocathode                      KBr                              
                                                                                               
Fine Error Sensors          Field Of View                     20 x 20 arcmin                   
                            Faint Limit                       mv = 16 star with S/N > 10 in    
                                                              a 10 second integration          


The four ultraviolet optical channels are divided into two pairs based on their wavelength coverage and coatings. The short wavelength channels, known as the silicon carbide (SiC) channels, cover from the low wavelength cutoff at 905 Å to approximately 1100 Å. Both the mirrors and gratings in these two channels are coated with SiC, which has a relatively constant reflectivity of 30%. The long wavelength channels cover from just below 1000 Å to 1195 Å. They are coated with lithium fluoride (LiF) over aluminum, and are known as the LiF channels. LiF has higher reflectivity than SiC above 1030 Å, but drops sharply shortward of this wavelength.

The four channel design, which was chosen to maximize the telescope collecting area and keep the gratings to a reasonable size while fitting within the limited constraints of the launch vehicle, is a very robust design, since nearly the entire wavelength region is covered by more than one channel, and the important 1000 - 1100 Å region is covered by all four.

Each of the four mirror assemblies contains a nearly identical off-axis parabolic mirror with a focal length of 2245 mm. Each mirror is mounted to three actuators which allow movement in tip, tilt, and focus in order to maintain their coalignment and to ensure the telescope focus is on the Rowland circle.

The light from each telescope passes through a slit in the FPA, which contains four apertures. These include a narrow (1.5 x 20 arcsec) slit which guarantees the highest spectral resolution at the cost of some effective area; a wider (4 x 20 arcsec) slit which passes essentially all the light and will be used for extended objects; a large square (30 x 30 arcsec) slit; and a pinhole ( 0.2 arcsec diameter) for bright objects which would otherwise overwhelm the detector. The FPAs can move in the focus direction and along the Rowland circle in flight. This capability allows some on orbit focusing, and allows detector flat field determination using an iterative restoration technique.

After passing through the slits, photons pass into the spectrograph cavity, where they are dispersed by the gratings. Each channel has a 260 x 259 mm grating with the same coatings as its mirror. The spectrograph is of Rowland circle design, with a diameter of 1652 mm. In order to correct for the aberrations, a holographically recorded grating with curved grooves is used. Even with this correction, the image of a point source has a vertical extent of up to 0.9 mm, and the light from a single line is curved. Figure 2 shows raytraced spots from the spectrograph at a number of different wavelengths across the band.

Figure 2 Raytraced spots , binned to the detector pixel size, for an on axis, point source at 995, 1040, and 1085 Å, showing the variation in spot shape with wavelength. They have been artificially offset in x in order to fit on the same figure.

Figure 3 is a photograph of a flight-size grating, manufactured by Jobin-Yvon, undergoing test at the Laboratoire Astronomie Spatiale (LAS) in Marseille. This grating has demonstrated a resolving power of 30,000 with a FUSE prototype detector.

Figure 3 A full size (260 x 259 mm) FUSE prototype grating under test at LAS.

Light from two channels (one SiC and one LiF) are dispersed onto separate areas on each of two detectors. The detectors are photon counting microchannel plate detectors with double delay line anodes, and they have an active area of 179 x 10 mm, divided into two 85 x 10 mm segments with a 9 mm gap between them. The front surface of the MCPs is curved to match the Rowland circle. During Phase B, a one segment prototype with flat MCPs was constructed (Figure 4), and its performance was investigated.

Figure 4 A FUSE prototype detector constructed in Phase B. The active area (dark area in center) is 60 x 10 mm. The flight detector will consist of two 85 x 10 mm segments.

Fine pointing of the satellite is made using a Fine Error Sensor (FES), which images the field around the target in visible light. Centroided images of guide stars will be used to maintain the spacecraft pointing stability to 0.5 arcseconds, 1 sigma. Two FESs are included in the design (one on each LiF channel), but only one will be operated at a time.

3. THE FUSE MISSION

The FUSE spacecraft, which is responsible for the attitude control, telemetry, etc., will be commercially procured by the Johns Hopkins University Applied Physics Laboratory. The modular design of the instrument and spacecraft should ease the integration process. A spacecraft contract will be in place later this year.

FUSE will be launched into a 25 inclination, 800 km orbit with a period of 101 minutes on the new Med-Lite expendable launch vehicle. Launch is currently scheduled for late 1998. Communications will be through dedicated ground stations. This low earth orbit should allow an observational efficiency of 35% over the three year mission.

Operations will be controlled from a Spacecraft Control Center (SCC) at the Johns Hopkins University Homewood Campus. Since the orbit limits contact to only a few minutes during each orbit, most operations will be autonomous. This includes target acquisition, peakup, and data collection. Planning of observation will also occur in the SCC, as will the pipeline processing, which will convert the raw, two dimensional detector images into one dimensional spectra for use by the community.

A large fraction of FUSE’s observing time will be made available to the scientific community through a peer reviewed Guest Investigator program. Most of the remainder will be reserved for several key projects which will require large blocks of observing time. A pre-mission workshop will be held 12-18 months before launch in order to examine the PI team science plan. It is expected that Guest Investigator proposals will be solicited from the community in 1998.

4. EXPECTED PERFORMANCE

Since high resolution is required for the scientific problems that FUSE will address, the goal is to obtain spectra with a resolution of (lambda/delta lambda) = 30,000. The spectral resolution of the instrument as designed is shown in Figure 5.

Figure 5 The design spectral resolution as a function of wavelength. Effects of structure and component distortion will lower these values slightly.

It meets this goal in at least one channel above 930 Å. Work is currently underway to include the effects of misalignments due to distortions during an observation. This will lower the resolution somewhat, but the 30,000 should still be attained over most of the region.

Observations of faint objects will require high effective area in addition to high resolution. The expected effective area at launch is shown in Figure 6.

Figure 6 The expected effective area of the FUSE instrument. The contribution from the SiC and LiF channels separately and together are shown. The gaps in the detector have been omitted for clarity.

The complicated shape is due to a combination of the varying number of channels contributing at different wavelengths and the effects of the LiF cutoff below 1050 Å; the effects of the detector gaps have been omitted for clarity. This effective area should allow observations of objects as faint as 5 x 10-14 erg cm-2 s-1 Å-1 with a S/N of 20 in 100,000 seconds.

Since the instrument is optimized for observing faint objects, it will be necessary to limit the flux for the brightest targets, such as those observed by Copernicus, in order to avoid overwhelming the detector and data system. Brighter objects (greater than 3 x 10-10 erg cm-2 s-1 Å-1) will require use of the pinhole aperture, or placing the target off the edge of one of the larger slits.

5. SCHEDULE

Start of Phase C/D                            November 1995                         
Start of Spectrograph Integration             February 1997                         
Start of Instrument Integration               December 1997                         
Start of Satellite Integration                March 1998                            
Call for Proposals                            Early 1998                            
Launch                                        November 1998                         

6. ADDITIONAL INFORMATION

Additional information about the FUSE mission and the science program may be found on the FUSE homepage on the World Wide Web. The URL is:

http://fuse.pha.jhu.edu


(Last updated July 3, 1995)
sahnow@pha.jhu.edu