2.1 Instrument
2.1.1 Overview of Optical Layout
The FUV instrument (Figure 2.1.1-1) is based on the Rowland circle design and consists of four separate optical paths or channels. A channel consists of mirror, a focal plane assembly (FPA), a diffraction grating, and a portion of an FUV detector. The channels must be co-aligned so that light from a single target properly illuminates all four channels, thereby maximizing the sensitivity of the instrument. This is accomplished with actuators on the mirror assemblies and the FPAs.
The four channels are composed of two nearly identical pairs. As shown in Table 2.1.5-1 each channel has a bandpass of about 200A. Thus, at least two channels are required to cover the entire ~290A grasp of the instrument. All four channels cover the 1014-1077A band, which accounts for the peak in the effective area curve in this region (Figure 2.1.1-2). This multi-channel design allows the coatings of the mirrors and gratings to be selected to maximize reflectivity in each of the "sub-bandpasses." The two mirrors and two gratings of the short wavelength pair of channels are coated with SiC. The two mirrors and two gratings of the long wavelength pair of channels are coated with aluminum and a LiF overcoat.
The visible light portion of the instrument is used only for field acquisition and guiding. It consists of CCD-based Fine Error Sensors (FESs), which image the sky in a 21 x 17 arcminute field around the target. This is done by means of a reflective front surface on the FPAs of the LiF channels. Only one FES is operated at a time; the other provides redundancy.
2.1.2 Telescope Mirrors
The four telescope mirrors are identical off-axis paraboloids. The blanks are made of Zerodur, with a triangular rib structure on the back to provide a lightweight but very stiff substrate. Each mirror is attached to the front of a honeycomb sandwich intermediate plate by means of tangential blade flexures, which minimize mounting induced distortions. Three stepper-motor actuators are attached to the rear of the intermediate plate, which allow tip-tilt-focus adjustment of the mirror.
The off-axis angle of the SiC mirrors and the LiF mirrors differ slightly. This angle is defined by aperture stops place over the surfaces of the mirrors. Otherwise, the mirrors are identical except for the coatings.
Thermal control is maintained by heaters attached to the intermediate plates, which radiatively couple to the rear of the mirror substrates.
Some of the specifications of the mirror assemblies are given in Table 2.1.2-1
Table 2.1.2-1
Mirror type Off axis parabola
Substrate material Zerodur
Size of clear aperture 387 x 352 mm
Focal length 2245 mm
Off axis angle (to optical center) 5.3668deg. SiC mirrors
5.4678deg. LiF mirrors
Coatings 2 mirrors with ion beam deposited SiC
2 mirrors with LiF over aluminum
2.1.3 Focal Plane Assemblies
At the focus of each telescope mirror is a Focal Plane Assembly (FPA) which acts as the optical entrance aperture for each spectrograph channel. An FPA consists of an optical flat mounted on a precision two axis flight-adjustable stages. As summarized in Table 2.1.3-1 four slits are cut into the flat:
1) A high resolution aperture (HIRS: 1.25 x 20 arcsec) to ensure that maximum resolution (25,000 - 30,000) is maintained even if the telescope imaging or pointing stability are below expectations.
2) A high throughput aperture (MDRS: 4 x 20 arcsec) which gives maximum sensitivity. If the telescope image quality and pointing stability meet their specifications, this slit will also provide spectral resolution of approximately 25,000.
3) A large square aperture (LWRS: 30 x 30 arcsec) which can be used for observations of faint extended objects and for coarse alignment of the channels.
4) A pinhole aperture (PINH: approximately 0.5 arcsec in diameter) which can be used for observations of extremely bright objects.
The geometrical arrangement of the slits is shown in Figure 2.1.3-1.
Table 2.1.3-1
Aperture Keyword Dimensions Throughput
(arcsec) (approximate)
high resolution HIRS 1.25 x 20 0.67
high throughput MDRS 4.0 x 20 0.98
large square LWRS 30 x 30 1.00
pinhole PINH 0.5 (diameter) ~0.10
The slit throughputs are computed assuming nominal telescope, pointing jitter, and pointing accuracy specifications.
The pinhole is designed for observations of very bright targets, for which FUSE has not been optimized. Its flatfield and wavelength calibration will be different than for the other slits, and it will not be photometrically calibrated, since its throughput depends strongly on the pointing stability. Its use will not be initially supported.
Each FPA can be moved independently in two directions. Motion tangential to the Rowland circle, which is roughly in the dispersion direction and perpendicular to the slits, is for co-aligning the channels and to perform focal plane splits for high signal/noise ratio observations of bright targets. (FP-split mode is not currently available). Motion perpendicular to the plane of the FPA mirror is for focusing the slits with respect to the spectrograph grating and detector.
The front surface of the FPAs on the two LiF channels are reflective in visible light. The Fine Error Sensors image these surfaces, viewing guide stars in the region around the science target, in order to pass pointing error information to the Attitude Control System. The LiF channels are used because Al+LiF has greater V-band reflectivity than SiC.
2.1.4 Spectrograph
The spectrograph has a Rowland circle design, with a diameter of 1652 mm. Light entering the spectrograph through an FPA slit illuminates one of four diffraction gratings. The gratings are holographically ruled on spherical substrates made of fused silica. A grating disperses the light onto an FUV detector. Each of the two detectors has a sufficiently large 2-dimensional area to record the light from a pair of gratings. These two spectra are displaced by a few mm in the spatial direction so there is no overlap between the spectra. Each detector records the light from one SiC channel and one LiF channel.
Although the holographic rulings allow for correction of astigmatic optical aberrations, the image of a point source still has a vertical extent of 150 to 1000 microns (~ 15 to 100 arcsec) on the detector, depending on wavelength. This means that there is virtually no spatial imaging capability, since the HIRS and MDRS slits are only 20 arcsec in length.
Table 2.1.4-1 lists some of the design parameters of the spectrograph and diffraction gratings.
Table 2.1.4-1
SiC LiF
Rowland circle diameter 1652 mm 1652 mm
Ruling density at grating center 5767/mm 5350/mm
Grating angle a 24.0deg. 25.0deg.
Grating angle [beta] 9.31660deg. @ 986Å 9.76612deg. @ 1107Å
Grating dimensions 266 mm (dispersion) x 275 mm (spatial)
Grating type first generation, type II holographic
2.1.5 Detectors
[N.B.: This section will be updated if the anode on FL03 differs from the anode on FL02.]
Two microchannel plate (MCP), double-delay line detectors are used in the FUSE instrument. In such a detector an incident photon strikes a potassium bromide (KBr) photocathode, which covers the top surface of the high-voltage biased Z-stack of MCPs. The MCPs provide electronic amplification of this initial photoelectron, resulting in a cloud of ~1e7 electrons at the bottom of the stack, which impinge on an anode. The events are located in the spectral direction on the anode by measuring the time required for the electron cloud pulse to propagate in the x-direction. Events are located in the spatial direction by measuring how the charge in the cloud is split by the interleaving wedges of the anode in the y-direction.
The total charge that can be extracted from the MCPs is a limited resource which is managed carefully. High S/N ratio observations of any target, bright or faint, deplete this resource and may cause gain sags in the illuminated region of the detector. Section TBD describes the bright object limitations in greater detail.
The active area of a detector must be approximately 190 x 10 mm in order to capture the full spectral range from a pair of gratings. This requires the use of two MCP stacks in each detector. Each stack is 80 x 10 mm, and they are placed end-to-end in the long direction, with a ~8 mm gap between the stacks. The two detectors are displaced slightly in the dispersion direction so that no wavelength interval falls in the gaps of both detectors. The actual wavelength coverage of the four channels is given in Table 2.1.5-1.
Table 2.1.5-1
Channel Segment A Segment B
SiC 1 1092.6 - 1001.5 994.3 - 902.3
LiF 1 991.0 - 1090.3 1098.1 - 1196.3
SiC2 914.8 - 1006.7 1013.9 - 1104.8
LiF2 1183.1 - 1084.7 1076.9 - 977.5
The photon event locations in each detector MCP segment are digitized into 16,384 x 1024 pixels. The digitized region extends slightly beyond the perimeter of the segment, so the pixel size is approximately 6 x 10 microns. The detector resolution, however, is about 25 x 50 microns. A spectrograph resolution element is 40 microns (for R ~ 25,000).
The detector normally records the (x,y) location of every arriving photon and its pulse height; this is called time tag (TTAG) mode. In addition, time stamps are inserted into the photon address data stream, nominally at 1 second intervals (but user-selectable from 0.0625 to 256 seconds) to give UTC time information for the arriving events. Doppler correction due to the orbital velocity of the satellite is done on the ground.
The amount of memory required to store the time tag data obviously depends on the flux of the target and duration of the observation. Each TTAG event requires 4 bytes. As an example, an object with f(1000A) = 1e-12 erg/sec/cm2/A observed through the high throughput slit (MDRS) will give approximately 700 counts/second from all four channels combined, or 5.6 Mbytes in a typical one orbit integration of 2000 seconds. These data are stored in the instrument data system (IDS) memory, and then forwarded to the spacecraft memory. The IDS can store 32 Mbytes, and the spacecraft can store 140 Mbytes.
If the total count rate from the two detectors exceeds 32,000 counts/second the bus between the detectors and the IDS cannot keep up, and events are randomly discarded. Spectral integrity is maintained, but photometric accuracy is compromised.
At fluxes exceeding approximately f(1000A) = 3e-12 erg/sec/cm2/A it is more memory efficient to build a histogram image of the spectrum on the detectors rather than record each event individually. These histograms are nominally binned to 1 x 8 detector pixels, which results in a minimal loss of spectral resolution. Only the region of the detector illuminated by the spectrum is stored in memory. This histogram requires 3.6 Mbytes of memory. To avoid Doppler smearing of the spectrum, 4 equal length histograms will recorded each orbit, requiring a total of 14.4 Mbytes per orbit.
Since the single FUSE ground station is out of contact with the satellite for a ~12 hour block per day, and is in contact for only about 10 minutes per orbit otherwise, management of this memory to avoid filling it up is of central importance to Mission Planning. This memory constraint will limit the number of observations of bright targets that can be scheduled.
2.1.6 Fine Error Sensors
Each of the two LiF channels has a Fine Error Sensor camera which images the 20 x 20 arcmin field in the neighborhood of the science target. Visible light is directed to the FES CCD from the mirrored front surface of the FPA. The FES will determine the centroids of up to six guide stars in the field with an accuracy of 0.2 arcseconds and send this information to the Attitude Control System once per second. This will result in a satellite pointing accuracy of 0.5 arcseconds.
The FES limiting magnitude is mv=13.5, sufficient to find at least one guide star in 85% of the fields at the Galactic Poles, and virtually 100% of the fields at lower Galactic latitudes.
Each FES has a three-position filter wheel. In the first position is a nearly zero-loss glass blank, which serves only to maintain the proper optical path length in the system. In the second position is a neutral density filter with an attenuation of 99.6% (6 magnitudes), for use when acquiring bright targets or when guiding on bright stars near the science target. In the third position one FES contains a V-band filter and the other FES contains an R-band filter. However, their use will not be supported at this time. There is no shutter on the FES.
The imaging properties of the FES are modest. The stellar point spread functions are typically 4 arcsec FWHM, depending on field angle and color. Nevertheless, an image of the field will routinely be provided to users to confirm that the proper target was acquired.
The FES consists of a pair of mirrors which re-image the FPA onto a 1024 x 1024 pixel frame transfer SITe CCD, which is masked to a 512 x 512 pixel image area. The pixel dimensions are 24 x 24 microns. The CCD is cooled to -60C with a 2-stage thermoelectric cooler coupled to a radiator on the side of the satellite. Only one of the two FESs is used at a time, and its radiator will be kept on the shaded side of the satellite at all times. The other FES will not be used unless dictated by a decline in performance.
2.1.7 Instrument Data System
The Instrument Data System (IDS) is a 68020 computer with a coprocessor and associated hardware and software. It is responsible for controlling all instrument functions, including thermal control, actuators on the mirror assemblies and FPAs, and detector and FES operations. It accepts data from the FUV detectors and FES, and stores it in instrument memory. It also passes engineering housekeeping data and science data to the Spacecraft Data System.