6         FUSE Instrument Technical Appendix

 

6.3.1.3  Stim Lamp Assembly

 

The detector Stim Lamp Assemblies (one for each detector) included the mercury vapor stimulation (or “stim”) lamps, a mounting bracket, and a pinhole aperture to coarsely control the amount and direction of light reaching the detectors. They were mounted to a structural bracket in the spectrograph cavity, approximately 1.25 meters from the detectors. Their location allowed direct, quasi-uniform illumination of each detector, with count rates of ~2,000 to ~12,000 counts per second, depending on segment. The stim lamps were not designed to provide a true flat field of the detectors, but rather were included to provide general diagnostics of detector health. Before launch, the lamps were used to provide detector aliveness tests while the instrument was at atmospheric pressure by illuminating the MCPs through the sapphire windows in the vacuum doors. On orbit, they were used regularly throughout the mission as a means of monitoring detector performance, especially gain sag (Section 4.4.2).

 

The stim lamps were powered through the detector auxiliary power supply, which also powered the ion pumps.

 

6.3.2   Normal Detector Operations

 

6.3.2.1  High Voltage Ramp-up

Ramping up the high voltage was a complex process with many built in safeguards to ensure that the detector remained safe. The ramp-up scripts:

• enabled the IDS rule which monitored the AUX current for out-of-bounds conditions
• set the HV levels to zero
• set the SAA levels to their proper values
• set the SAA integration and reduce times
• set the HV current threshold to accommodate possible turn on transients
• armed the detector powered on the high voltage
• reset the HV current threshold for more protection.

 

Only after all of these steps were taken would the HV be ramped above its turn on (2500 volt) level. The ground scripts used for ramp-up referenced a file which contained the proper voltages. Separate scripts used for raising the high voltage for testing purposes did not permit increases of more than 10 digital units at a time to guard against accidentally increasing the high voltage to an unsafe level.

 

4.n.2.2  High Voltage Management: Occultation Manager

 

The detector occultation manager was implemented during the mission in order to minimize the amount of gain sag at the location of airglow lines. At the beginning of the mission, the high voltage was normally left at FULL except when passing through the SAA. This meant it was always ready for a peak-up or an observation, but it also meant that any photons falling on the detector removed charge from the MCPs, even though they weren't necessarily being collected by the IDS. Once the differential gain sag problem became obvious, development began on an occultation manager, which lowered the MCP high voltage to SAA levels whenever it wasn't needed. This decreased the amount of unnecessary charge depletion by removing several sources of non-productive exposure:

(1)   Airglow lines illuminated the detectors during occultations and other times between exposures, causing significant localized charge depletion.

(2)   It was not uncommon for a slew to a new target and the acquisition of that target to occur well before an exposure began. This could be due to an intervening SAA or occultation, for instance. The target could remain in the aperture at full high voltage for many minutes before the exposure began. Before the implementation of the occultation manager, this time contributed to the detector exposure and gain sag, even though no data was collected.

(3)   Similarly, after an exposure was completed, the target sometimes remained in the aperture until it was time to slew to the next target. Before the occultation manager, this would also add detector exposure without the benefit of collecting any additional data.

 

The occultation manager was used starting on 20 November 2001, and used for the rest of the mission. It was only turned off during times when the high voltage was being ramped up, and during other special tests.

 

6.3.3    SIA tables

 

As described previously, the FUSE detectors were photon-counting detectors, and the DPU produced a location and pulse height for each detected event. These positions were passed via the Science Data Bus to the IDS, which could either send that information on to the spacecraft (after adding timing information) for eventual downlink to the ground (TTAG mode), or add it to a two-dimensional image (HIST mode), discarding the pulse height information in the process. Due to memory limitations, an image of all four 16384 × 1024 segments couldn’t be stored in IDS memory. Instead, only the regions around the primary spectra were saved in memory, and they were typically binned by 8 in the cross-dispersion direction. The primary exceptions to this procedure were the M999 stim lamp exposures, which were binned 2 × 2, and for which the entire detector area was saved.

 

For each HIST observation, a Spectral Image Allocation (SIA) table was used to define the detector regions which were saved to memory. The SIA tables varied with the aperture chosen, and they also changed during the mission as a better understanding of the location of the spectra was gained. SIA tables were actually part of the IDS, but because they define how the detector data is saved during histogram exposures, they are described here.

 

SIA tables consisted of an 8 × 64 array for each segment, with each array element referencing a 2048 × 16 pixel region on the detector. Each array element could be set to 1 to save the data from the associated detector region in IDS memory, or 0 to discard it. In most cases, the observation scripts for histogram exposures called the init_bulk_memory_f script, which loaded the standard set of SIA tables for all apertures. A special set of SIA tables for histogram stim lamp exposures were uploaded from the ground when needed. The standard SIA tables were changed several times during the first few years of the mission ( Table 6.3‑7 through Table 6.3‑12). These changes were made primarily to adjust the positions to ensure that all data from the aperture was being collected. Note that the masks (Table 6.3‑4) were eventually set to be the same as the SIA tables, but they did not match early in the mission.

 

The SIA tables were downloaded to the ground with the science data, and they are stored in the primary data unit in the raw FITS files. Although they are not used by CalFUSE, they can be examined to verify that the expected mask was used and was not corrupted. The typical SIA table consists of up to 16 rectangular regions turned on, which include, for each segment, the detector regions where the SiC and LiF spectra fall, along with the area around each stim pulse. In many cases, one or more of the stim pulse regions overlap with the spectral regions. The tables below describe the regions of each detector segment covered by the SIA tables during the mission.

 

It is important to note that since the SIA tables operate on the raw data coming from the detector, they are not in the geometrically-corrected FARF, but in a distorted frame which is not corrected for geometric distortions or thermal shifts and stretches. Thus, the SIA table regions were oversized to avoid losing data.

 

Approximate Start Date	All X Range	1A Y Range	1B Y Range	2A Y Range	2B Y Range	Table Names
11/20/1999	0 - 16383	160 - 319	128 - 287	288 - 399	400 - 495	siahirs1x8b
5/17/2000	0 - 16383	160 - 319	112 - 271	288 - 399	400 - 495	siahirs1x8c, siahirs1x8d

Table 6.3‑7 SiC HIRS SIA Tables used during the mission

 

 

Approximate Start Date	All X Range	1A Y Range	1B Y Range	2A Y Range	2B Y Range	Table Names
11/12/1999	0 - 16383	288 - 447	240 - 399	208 - 319	224 - 335	siamdrs1x8b
1/13/2000	0 - 16383	288 - 447	240 - 399	208 - 319	352 - 463	siamdrs1x8c siamdrs1x8d

Table 6.3‑8: SiC MDRS SIA Tables used during the mission

 

 

Approximate Start Date	All X Range	1A Y Range	1B Y Range	2A Y Range	2B Y Range	Table Names
10/11/1999	0 - 16383	16 - 175	0 - 175	384 - 495	480 - 591	sialwrs1x8b
3/4/2000	0 - 16383	16 - 175	0 - 175	352 - 463	448 - 559	sialwrs1x8c, sialwrs1x8d, sialwrs1x8e

Table 6.3‑9: SiC LWRS SIA Tables used during the mission

 

 

Approximate Start Date	All X Range	1A Y Range	1B Y Range	2A Y Range	2B Y Range	Table Names
11/20/1999	0 - 16383	624 - 783	608 - 767	544 - 655	592 - 703	siahirs1x8b
5/17/2000	0 - 16383	640 - 799	608 - 767	560 - 671	592 - 703	siahirs1x8c
3/27/2001	0 - 16383	640 - 799	592 - 767	560 - 671	592 - 703	siahirs1x8d

Table 6.3‑10: LiF HIRS SIA Tables used during the mission

 

 

Approximate Start Date	All X Range	1A Y Range	1B Y Range	2A Y Range	2B Y Range	Table Names
11/12/1999	0 - 16383	768 - 927	736 - 895	464 - 575	544 - 655	siamdrs1x8b siamdrs1x8c
6/20/2001	0 - 16383	752 - 927	720 - 895	464 - 575	544 - 655	siamdrs1x8d

Table 6.3‑11: LiF MDRS SIA Tables used during the mission

 

 

Approximate Start Date	All X Range	1A Y Range	1B Y Range	2A Y Range	2B Y Range	Table Names
10/11/1999	0 - 16383	480 - 639	464 - 623	624 - 735	656 - 767	sialwrs1x8b
3/4/2000	0 - 16383	480 - 639	464 - 623	624 - 735	672 - 783	sialwrs1x8c
5/18/2000	0 - 16383	496 - 655	480 - 639	624 - 735	672 - 783	sialwrs1x8d
5/1/2001	0 - 16383	496 - 655	464 - 639	624 - 751	656 - 799	sialwrs1x8e

Table 6.3‑12: LiF LWRS SIA Tables used during the mission

 

6.3.4  Single Event Upsets

The electronics for the two FUSE detectors were turned on six days after launch, on June 30, 1999.  On July 2, the Detector DPU code reported a change in the checksum value calculated by its Cyclic Redundancy Check (CRC) routine. Since this CRC calculation was done over a range of internal detector memory that contained the executing DPU code, this meant that the code had been corrupted. The following day, a similar error was reported on Detector 2, but neither had any apparent effect on the detector performance. The third reported CRC error, however, caused a loss of some Detector 1 telemetry. Only by rebooting the detector was the telemetry restored. These errors continued to be reported at a rate of one every few days throughout the mission. Their cause was later determined to be single event upsets (SEUs) in the DPU memory, due to radiation from the South Atlantic Anomaly (SAA). This susceptibility to SEUs was due to the choice of RAM in the DPU; although the memory chip chosen had a high threshold to total radiation dose and was immune to latchup, it had a relatively low resistance to Single Event Effects (SEEs).

 

Since the regions of memory being corrupted contained the DPU code that was controlling the detectors, this was potentially a very serious problem. Although the detector software was robust and it was believed to be unlikely that a single bit flip could cause any permanent damage to the detector, as a precautionary measure rules were developed to minimize the danger. An intensive investigation was conducted to formulate a plan for responding before the high voltage was first turned on. In order to minimize the risk to the detector, the decision was made to only operate the high voltage when a code image was loaded into both Lower Core (LC) and Upper Core (UC) memory. If an SEU occurred in the active memory region, a command was issued to jump to the alternate core on the next ground station pass, and an uncorrupted version of the code reloaded from the ground. If the error was in the inactive memory core, it was simply reloaded to repair it. If both cores were found to be bad during a ground station pass, the detector was rebooted, which shut down the high voltage on that detector. Since recovery from a detector reboot required close to 24 hours because the ramp-up steps were commanded from the ground, this was a major cause of inefficiency early in the mission. In addition, the IDS flight script det_hv_f was modified to reload several important detector parameters (SAA count rate limits, SAA integration times, SAA HV levels, SAA reduce time, HV current limit, and AUX current limit) after every SAA passage.

 

This procedure was performed manually until 8 January 2000, when version 16600 of the DPU code was loaded onboard, along with modified IDS scripts that autonomously detected and repaired the code. Although the code was initially loaded from IDS memory, starting in April 2000 IDS EEPROM memory was used to free up memory for IDS scripts.

 

Before the autonomous code repair was implemented, 122 SEUs had been detected on the two detectors and repaired manually. After these changes, the autonomous correction code permitted the IDS to identify an SEU and correct it autonomously within three minutes in most cases.

 

There were 1285 SEUs detected on the two detectors during the mission. These were divided as shown in Table 6.3‑13. The fact that they were roughly evenly distributed between Upper Core and Lower Core memory, and Detector 1 and 2, shows that the susceptibility was due to the choice of parts rather than a single bad chip (note that UC and LC for a particular detector were located on the same memory chip). No obvious correlations of the SEU rate with time were seen, aside from changes in the rate due to modifying the size of the memory area checked.

 

Detector	Lower Core	Upper Core
1	335	271
2	299	343

Table 6.3‑13: SEUs by detector and memory core.

 

Although most of the SEUs had no noticeable effect on the operation of the detectors, the consequences could be more severe when certain parts of the code were affected. In some cases, a detector watchdog reset occurred, causing the detector to reboot. Other times unusual behavior was seen, such as a loss or corruption of detector telemetry. Those cases usually required rebooting the detector from the ground to recover.

 

As expected, nearly all of the SEUs occurred while the detector was in or near the SAA. Figure 6‑11 shows the location of FUSE in its orbit when each SEU occurred. The figure shows that they were concentrated near the center of the SAA, and that the spatial distribution was similar for the two detectors. Only fifteen of the 1238 SEUs for which the location could be identified did not fall within the SAA contour shown.

 

The CRC calculation took up to 40 seconds to identify a bit flip, depending on where it is in memory. In addition, CRC values and diagnostics were reported in telemetry only once every sixteen seconds. As a result, the times reported are likely to be somewhat later than the actual time of the SEU, and the locations marked on the figure will be slightly misplaced from their actual position.

 

Figure 6‑11: Positions of SEUs during the mission. The dashed line shows the SAA region used by Mission Planning after 17 September 2003. No events occur below -25º due to the orbit of the satellite.

 

 

In addition to the DPU code, detector masks (Section 6.3.1.2.2.3) were stored in the same memory chips, and thus were also susceptible to corruption. The CRC check done by the DPU code only checked mask memory when a mask was changed, however, so it was not possible to tell when this occurred. As a result, there were instances early in the mission where a mask bit was switched off due to an SEU, causing a loss of events in that region of the detector. Once this effect was indentified, the observation scripts were modified to reload the masks before every observation. This greatly decreased the likelihood of this problem.

 

6.3.5   High Voltage Transients (Crackles)

During satellite-level testing, the detector high voltage power supply current periodically had excursions that were large enough to potentially cause damage to the detectors. To minimize the likelihood of a detector failure on orbit due to one of these high voltage transients, or ‘crackles’, UC Berkeley modified the DPU software to monitor the high voltage power supply currents on segments A and B (HVIA and HVIB), and the Auxiliary Power supply current (AUXI), at a sample rate of approximately 1 millisecond. After these modifications, whenever these currents were greater than or equal to a threshold (a ‘mini-crackle’), a diagnostic was issued by the detector DPU. If this threshold was exceeded for a particular length of time (the persistence), the high voltage for both segments of the affected detector would be turned off and an additional diagnostic issued.

 

In addition, a portion of DPU memory was set aside for use as a circular buffer containing the last 1024 samples (approximately 1 second) for each of these three currents, along with three histograms that stored their distribution.

 

The thresholds could be set via ground command, but because the maximum current of most crackles was so high, changing the threshold had a limited effect. The persistence, however, could only be modified by changing the DPU code. Each change required modifying the code followed by extensive testing. Section 6.3.1.2.2.7 describes the DPU code versions used during the mission, along with the thresholds and persistence values for each.

 

Figure 6‑12 shows the three currents during a typical crackle event, as saved onboard in the circular buffer. Although the details of each crackle event were different, the basic features were similar in most cases:

(1)   One of the currents (in this case HVIA, the high voltage current on segment A), showed some oscillatory behavior and then went off scale.

(2)   The AUX current followed a similar shape as the segment current, but had a much lower value. Since it did not reach its maximum, it often showed the oscillation more clearly.

(3)   The current draw of the other detector segment (HVIB) was also affected.

(4)    When the (HVIA) current eventually exceeded the threshold (182 in this case) for the persistence time (60 msec), the high voltage shut down, and the currents all dropped to zero.

Figure 6‑12: HVIA, HVIB, and AUXI during a crackle.

 

Figure 6‑13: Number of crackles and mini-crackles vs. time.

Table 6.3‑14 summarizes the numbers and types of crackles and mini-crackles for each detector segment during the mission, while Figure 6‑13 shows their distribution as a function of time. Recall that mini-crackles had no effect on operations, while crackles shut down the high voltage on both segments of the affected detector, which required a high voltage ramp-up from the ground. Since the thresholds and persistence values changed during the mission, the crackle rates at different times cannot be compared directly, but it is clear that the incidence of crackles is episodic, for reasons that were never understood. It is also thought that they were more likely to occur soon after a high voltage ramp-up.

 

Detector	Mini-Crackles			Crackles
	HVIA (27)	HVIB (28)	AUXI (16)	HVIA (2A)	HVIB (2B)	AUXI (2E)
1	695	67	114	6	18	0
2	456	0	239	75	0	8

Table 6.3‑14: Number of mini-crackles and crackles during the mission.
The values in parentheses are the diagnostic value (in hex) issued by the detector.

 

Crackle shutdowns were the most common reason that the high voltage on one of the detectors was not at FULL as planned during an exposure. Since the time to return the high voltage to its nominal value after a shutdown could be a day or longer, this was a major source of inefficiency. Plans to automate the high voltage ramp-up process were developed, but had not been implemented by the time the mission ended.

 

Similar current protection scheme is used on the GALEX and COS detectors, but the DPU code for those detectors provided more flexibility on setting thresholds and persistence values.

 

6.4           Fine Error Sensor Cameras

6.4.1   FES CCD Detector

 

The CCD detector was a modified version of a SITe SIA003A. This was a 1024 ´ 1024 pixel thinned backside-illuminated CCD die mounted on a 2-stage thermo-electric cooler (TEC) and sealed in a kovar package with a fused silica window.  The CCD quantum efficiency, full-well depth, charge-transfer efficiency, dark current, and readout noise were all in accord with pre-flight measurements.  Basic characteristics of the CCD are given in Table 2.7‑2.

 

Half of the CCD die was masked, so that the CCD could be operated in frame transfer mode. After a given exposure duration, the image would be transferred rapidly to the region under the mask and then read out slowly so that the readout noise would not affect the accuracy of the centroid calculation.

 

CCD thermal control was achieved through the use of an external radiator in conjunction with a thermo-electric cooler (TEC). The CCD package was coupled to the external radiator by means of a heat strap fabricated from flexible copper braid that was fused at either end into a copper block. In general, the spacecraft attitude was maintained so that the radiator of the active FES was shaded from the sun.  During in-orbit checkout the TEC setpoint was chosen to be -32C for FES-A and -30C for FES-B. Lower setpoints could be maintained at some attitudes, but the reduction in dark current did not justify the operational inconvenience of tailoring the setpoint for each attitude. With the advent of two-wheel and one-wheel operations later in the mission, which necessitated a much wider range of spacecraft roll angles, on-board scripts were developed that would autonomously adjust the TEC setpoint in response to the changing thermal environment. The resulting higher dark current and slightly-degraded FES performance was accepted, as the pointing performance was then limited by the torquer bars and not the FES.

6.4.2   Performance and Anomalies

6.4.2.1  FES-A Failure

The first in a series of intermittent spontaneous FES-A reboots occurred on day 105 (April 15) of 2005. When the reboots occurred, FES-A was left in Boot mode, and could not be commanded to application mode without first cycling the power.  In addition, the FES would frequently not recover unless it was left powered off for several minutes to an orbit. This led to the suspicion that some component in the controller had an intermittent problem with heat-sinking. We attempted to mitigate this problem by lowering the temperature of the FES-A surroundings and by having the IDS flight scripts autonomously handle the power cycling and rebooting sequence. We operated in this mode for several months, but ultimately decided to switch to FES-B on July 12, 2005. FES-A was not used for guiding during the rest of the FUSE mission.

 

The cause of this problem was not determined. FES-A had been subjected to survival temperatures (-5C), along with the rest of the FUSE instrument, between Dec 28 2004, when the Roll RWA failed, and March 22, 2005, when version T31 of the ACS software was loaded. The reboot problems began a few weeks later. It was suspected that this prolonged cold soak had precipitated the problem. 

 

6.4.2.2  FES-B Focusing

Following the initial focusing of the LiF2 channel for optimal spectrograph performance in IOC, the PSF of star images in FES-B was found to be 2–3 times larger than desired (the FWHM was 4–6 pixels instead of 2 pixels), indicative of defocusing of FES-B.  This caused the light from any given star to be spread over many more pixels than would be the case if FES-B were properly focused, reducing the contrast between the signal and the background. 

 

Although FES had no internal focus adjustment mechanisms, the focus could be adjusted by moving the LiF2 FPA and/or the LiF2 primary mirror along the Z-axis (focus direction).  If the FPA was moved, the only impact to FUV data in that channel was to reduce the throughput of the MDRS and HIRS apertures.  If the LiF2 primary mirror could also be moved, the spectral resolution of the LiF2 FUV data will be degraded.  FES-B was used successfully with these large star images during periods when the FES-A was being annealed. When FES-B was made the permanent guiding camera, however, the likelihood of guiding problems with faint stars was too large. FES-B was refocused by moving the LiF2 FPA by 400 microns (See section 6.2). The resulting image quality was comparable to that of FES-A.

 

 

6.5           IDS: Instrument Data System

6.5.1   IDS–ACS Interface

The IDS was responisble for managing FES operations, for attitude determinations based on FES data, and for initiating slews. In essence, the IDS and FES were an integral part of an extended attitude control system architecture. Communications between the ACS and IDS were handled by Fine Pointing Data (FPD) packets (IDS to ACS) and ACS status messages (ACS to IDS) transferred across the SDB. Measured quaternions, covariances, and configuration flags were sent to the ACS once per second in FPDs for fine pointing management. In return, the ACS provided status messages back to the IDS.

 

The spacecraft was responsible for the health and safety of the satellite. Therefore the ACS was free to reject individual FPDs or ignore the IDS altogether. The IDS did not command the ACS, but changed the status of FPD flags to in effect request ACS activities. The ACS monitored state changes in the FPDs and responded accordingly.

6.5.2   Observation Sequencing and Fine Guiding

Slewing and target acquistion was controlled by scripts executing in the IDS. The scripts controlled sequencing of commands, while complex functions such as processing of FES images or attitude determination were performed by the embedded guidance task software in the IDS.  Except for centroid computations performed by the FES when in tracking mode, all fine attitude calculations were performed by the IDS. 

 

The main steps in a basic target acquisition sequence were as follows:   

1. The IDS sends a new attitude to the ACS, initiating a slew, and then waits for the slew to complete.
2. The IDS commands the FES to obtain & deliver a full FOV image
3. The IDS processes the image and determines locations of up to 30 stars. 
4. The IDS commands the FES to centroid 3 to 6 of the brightest stars at 1 Hz, computes quaternions, and transmits them to the ACS. These quaternions are flagged as “unknown”, because the stars have  not yet been identified. These data were used to maintain a precise fixed spacecraft attitude while the absolute attitude was determined from the FES image. (see Section 8 for a discussion of quaternions).
5. The IDS compares the objects in the image to the star catalog uplinked for this observation and determines the absolute attitude.
6. The IDS sends the absolute quaternion to the ACS, which initiates a slew to remove the residual attitude error.
7. After the slew, the IDS commands the FES to centroid pre-selected stars surrounding the target, at a rate of 1Hz or 0.5 Hz. As each set of centroid data is received by the IDS, it computes quaternions from reported star positions, and sends them to the ACS for input to the attitude-determination filter.
8. At the end of the target visibility period, the IDS commands the FES to stop tracking stars, and the IDS stops generating FPDs. The ACS maintains coarse tracking using gyros and TAMs.
9. For multi-orbit observations, return to step 2 above when the target is next unocculted by the Earth, and the satellite is not passing through the SAA.

 

More complex acquisition procedures were needed for some observations. The main variations are described briefly here.

 

Offset acquisitions. In cases where the star field in the vicinity of the target was too sparse for the star-ID process to work, or if there were a very bright source in the FES field of view, the first five steps of the acquisition sequence above would be performed at an offset field. After the field was identified, the correction slew request would include the offset needed to move directly to the desired target position.

 

Peak-ups. Observations using the MDRS or HIRS apertures would include a peakup procedure folowing step 7 in the basic acquisition sequence. Peakups are described in Section 4.2.2.

 

FES-assisted acquisitions. In these acquisitions, the target would be left at the reference point position, away from the apertures, and the FES would be commanded to measure its position as well as that of the guide stars. The IDS would then compute the actual position of the target and then command a short slew to place the target in the proper spectrograph aperture. This mode was initially envisioned for objects such as comets that might have poorly-known coordinates, but in practice is was often used for routine observations.

6.5.3   IDS Thermal Control

6.5.3.1  Thermal Design

The IDS was responsible for the operation of the instrument thermal control system used in normal operations. Temperature data from instrument thermistors were collected from the IPSDU, averaged, and then used to control 32 heater zone switches in the IPSDU. Overall management was specified by a user-defined thermal control table. The table identified which thermistors were associated with each thermal zone and the upper and lower temperature thresholds for the zones. Up to 4 thermistors were monitored for a zone and, when the average temperature fell outside the zone thresholds, the heater was turned off or on. By executing one full cycle of the algorithm every 16 seconds, the IDS guaranteed that the temperature of each thermal zone was maintained to +/- 0.25 C of the desired temperature. Space for two tables was provided to facilitate the loading of a new table while under thermal control of another.

6.5.3.2  Thermal Performance

The deadbands in the thermal control tables were set at +/-0.25 C. The control system was able to maintain temperatures within this deadband, with the exception of the equipment panels which were controlled to within 1C, which was adequate for the electronics modules mounted on these panels.

Under nominal three-wheel operations instrument temperatures were well-controlled about a daily mean temperature that exhibited no long term trends.  In particular, the mirror bench temperatures on the shaded (+X or LiF) side of the instrument exhibited no change in daily mean temperature over the mission. During the subsequent two-wheel phase with roll-offsets the temperatures were also, in general, well-controlled.   However, for some of the heater zones in the lower sections of the instrument a gradual increase in the daily mean temperature was evident - including the SiC1 mirror bench.

The behavior of the zones in the upper half of the instrument was slightly different. This includes the spectrograph structure, the GMA shrouds, and the gratings. Each of these zones maintained constant or nearly constant temperatures until the pitch wheel failure in December 2001. Following restoration of 3-axis stabilization, the mean temperature in each of these zones was slightly (<0.1C) higher.  The temperatures of these zones exhibited a few slight mean temperature changes/offsets later in the mission and the SiC grating temperatures increased gradually by 0.05C over the last few years of the mission.

The LiF grating temperatures were well-controlled. The combination of controlling the grating pie-pans in conjunction with their thermal mass was successful in providing a stable environment for the optics.  Under normal operating conditions the grating temperatures were controlled to within 1C.  However, the grating temperatures, in particular for the SiC temperatures, varied with solar beta angle.

The zones within the instrument were controlled very well throughout the mission. The slight changes that were seen were presumably due to minor degradation of the multi-layer insulation with exposure to solar UV radiation and atomic oxygen. The distinct step in temperatures seen at the time of the roll wheel failure in particular may be the result of such exposure, as this was the first time in the mission that exterior surfaces of the instrument not covered by silver teflon blankets were exposed to the Sun for an extended period.

The temperatures of the telescope baffles were not controlled, as there were no functional or survival requirements to heat the baffles. The baffle mounting to the structure was designed to decouple mechanical stresses from differential thermal contraction of the baffles with respect to the structure. However, the significant variations in telescope alignment found on orbit that correlated with the Solar beta angle and orbit pole angle, and with orbit phase, suggests that this decoupling was not adequate. The temperatures of the door closure and unlatch HOPs, located near the top of the baffles, are seen to vary on orbital timescales by 4-5 C on the LiF side and by 6-8 C on the SiC side.  These temperatures vary with spacecraft attitude as well: typical variations early in the mission were 3-5 C on the LiF side, and 12-15 C on the SiC side. Typical temperatures at the tops of the baffles were –40 C and below on the LiF side, and –20 to -40 C on the SiC side.

6.6           Instrument On-orbit Performance

6.6.1   Telescope Focus

6.6.1.1  Post-launch Focus Assessment Details

 

The FUSE instrument was focused pre-flight with the provision for in-flight focus adjustments for the mirrors and FPAs.  On-orbit focus adjustments were expected due to (1) the unavailability of a laboratory FUV source with a light beam collimated to roughly one arcsecond accuracy and (2) the anticipated focus changes associated with gravity release and changes in the positions of the optical elements resulting from moisture desorption from the optical bench structure.

 

The on-orbit instrument focus procedure was essentially a two-step process. First, the telescope was focused by adjusting the mirror to FPA distance for each channel. Then each spectrograph was focused by adjusting the distance from the telescope mirror to the spectrograph grating for each of the four instrument channels.  The FPAs were then re-adjusted to maintain the previously determined telescope focus.

 

The telescope focus was determined through a series of knife-edge tests performed by scanning a target across the edge of  the FPA slit. The knife-edge test was repeated for a set of FPA positions to determine the location along the optical axis where the light cut-off was sharpest.  Then the FPA for each channel was moved to place the aperture at the best telescope focus as indicated by this test.  The FPA motions executed to attain the telescope focus for each channel  are presented in Table 6.6‑1.

 

 

FPA	FPA Motion Z-axis (microns)
LiF1	no change
SiC1	-99
LiF2	+48
SiC2	-81

Table 6.6‑1: Initial in-flight telescope focus adjustments made November 23, 1999.  Adjustments in the focus (Z) direction are limited to 10 micron increments of the FPAs.  Small residual errors account for the slight departures from integral 10 micron changes in the adjustment values above.  The true uncertainty in the magnitude of the computed focus adjustment was at least 30 microns.

 

Two programs were executed to determine the spectrograph focus. The first of these programs, I817, was executed during the December 7 – December 9^th, 1999 time interval. Multiple stellar spectra of HD208440 were acquired through the LWRS aperture for each of 5 mirror positions stepped in 150 micron increments along the optical axis.

Data from program I817 and the earlier knife-edge tests enabled a robust determination of the best grating to mirror distance (the spectrograph focus) for the LiF1 and LiF2 channels. However, the signal-to-noise of the I817 data was relatively low for the SiC channels resulting in a less robust determination of the best spectrograph focus position for the SiC1 and SiC2 channel mirrors.

 

On December 12, 1999 the mirrors and FPAs were adjusted to the best spectrograph focus for each channel based on the data obtained December 7^th-9^th. LiF2, SiC1, and SiC2 previously had their FPAs adjusted to achieve the best telescope focus (mirror to FPA distance) as determined from the knife edge testing. For these channels, both the FPA Z position and the mirror focal distances were adjusted to focus the spectrograph and maintain the best mirror-to-FPA distance (i.e. the telescope focus) determined by the knife edge scans. For LiF1, the adjustment of the mirror location for spectrograph focus matched the required adjustment for mirror to FPA focus; hence the mirror only was moved. The FPA and mirror position adjustments made on December 12^th 1999 are presented in Table 6.6‑2 for each channel.

 

Channel	Initial FPA Z Postion (microns)	FPA Z Motions Executed (microns)	FPA Z Postion Dec 12, 1999 (microns)	Initial Z Mirror Postion (microns)	Z Mirror Motions Executed (microns)	Z Mirror Postion Dec 12, 1999 (microns)
LiF1	64	0	63	293	-200	93
SiC1	-100	-140	-238	-149	-150	-299
LiF2	62	150	212	25	150	175
SiC2	-77	none; -150	-225	82	-150	-68

Table 6.6‑2: Spectrograph focus adjustments executed on December 12^th, 1999 as a result of the I817 post-launch programs.

 

Given the low signal-to-noise of the spectrograph focus measurements conducted as part of program I817, a second spectrograph focus test was executed two months later as part of program I819. The target, WD0439+466: the central star of a planetary nebula, was observed in the LWRS slit. The spectrum exhibited many narrow molecular hydrogen lines, which were nearly ideal for focusing.  Quality data combined with careful analysis resulted in a much better measure of the spectrograph focus. On March 16, 2000, the spectrographs were brought to their best focus position by moving the telescope mirrors along the optical (Z) axis. The magnitude and direction of these motions are provided in Table 6.6‑3. These motions were generally in the opposite direction from the first spectrograph focus adjustment, but since the data were of significantly better quality, they were considered far more reliable. Corresponding adjustments were made to the FPA Z positions on March 24^th 2000 to maintain the mirror-to-FPA separation for LiF2, SiC1, and SiC2; the LiF1 FPA Z position was adjusted by -100 microns (see Table 6.2‑1 for the history of FPA Z motions). These focus positions remained the default throughout the remainder of the mission.

Mirror	Z Mirror Motion (microns) Executed December 12, 1999	Z Mirror Motion (microns) Executed March 16, 2000	Net Mirror Focus Adjustment (microns)
LiF1	-200	No change	-200
SiC1	-150	+125	-25
LiF2	+150	-60	+90
SiC2	-150	+250	+100

Table 6.6‑3: Spectrograph focus adjustments executed as a result of the I817 and I819 post-launch programs.
The spectrograph focus adjustments implemented March 16^th, 2000, were used for nominal operations for the remainder of the FUSE mission.

6.7           Telescope Alignment Performance

During the IOC shortly after launch, it was realized that the telescope mirrors underwent periodic motions that shifted the target’s image at the telescope focal plane and thus its spectrum in both X and Y on the detector. A source in either of the SiC channels could move as much as 6 arcseconds in a 2 kilosecond time interval. This motion had two effects on the data: first, flux was lost if the source drifted (partially or completely) out of the aperture; second, the was spectrum shifted on the detector, degrading spectral resolution for observations using the LWRS (30 arcsecond) aperture.

 

6.7.1   Initial Alignment

The procedure for the initial post-launch channel alignment was to perform a spiral search while guiding on the target using FES-A with LiF1 and the LWRS aperture. The FUV count rate was monitored while using the mirror actuators to locate the target position in the three non-guide channels. This method required use of a moderately bright FUV point source in a fairly isolated star field to provide sufficient counts and prevent confusion from nearby field stars.  Overall, this method worked well and was used to initially align channels to a 15-arcsecond  precision.

The data were analyzed to determine the mirror rotations required to remove the measured alignment errors at that attitude for each of the three non-guiding channels relative to the LiF1 channel.  As further alignments and observations were attempted, it was found that all of the channels were moving with respect to each other, with misalignments as great as 40 arcsec.

Finer co-alignment was then performed by stepping the roughly co-aligned images across the edge of the LWRS slits.  This method allowed for co-alignment to approximately 2 arcseconds.  Early attempts to use this method were hampered by the fact that the measurement and adjustment were not closed loop.  Scans were made and mirrors adjusted, but no confirmation of the image position was made.  FUSE would then slew to another target, which often resulted in co-alignment being lost due to changing the thermal environment of the instrument.  Early in the mission the source of the misalignment was unknown and the lack of a closed loop mirror adjustment initially masked the thermal misalignment problem.

6.7.2   Mirror Motion Anomaly

The FUSE  “Image Motion” was defined as the variation of the alignment of the SiC1, LiF2, and SiC2 channels with respect to the reference LiF1 channel used for guiding.  Although these motions were generally quantified as being associated with the non-guiding channels, they were caused by the motions of LiF1 in combination with the other channels.  The observed image motion was attributed to the thermal changes induced in the instrument by changes in boresight beta angle and orbital pole angle.  The boresight beta angle is the supplement of the angle between the satellite-sun line and the instrument line of sight and the orbital pole angle is the angle between the orbit pole and the instrument line of sight.

To mitigate loss of data due to channel misalignment while retaining observing efficiency, the LWRS aperture became the primary observing aperture for most programs.  For MDRS and HIRS observations, two peakups per orbit were executed to maintain channel alignment during an observation and obtain full spectral coverage while using these apertures.

6.7.3   On-orbit Mirror Motion Mitigation Strategy

A two-part strategy was developed to maintain alignment of the four mirror channels.  This strategy included both a predictive component based on empirical modeling of changes in mirror position as a function of boresight attitude and a periodic re-baselining of the alignment. 

6.7.3.1  Mirror Alignment: Baseline Maintenance

The re-baselining component of the alignment maintenance was achieved by performing xy-scans on a stellar target and performing near real-time analysis to determine the direction and amplitude of the corrective mirror motions required to attain alignment at that particular attitude. The alignment baseline was valid for a range of beta and orbital pole angles in the vicinity of the target.  Alignment within a particular attitude range could be maintained for approximately two weeks, assuming that targets were scheduled within the allowed beta/pole zone.  Approximately every two weeks, xy-alignment scans were performed, the data analyzed, mirror motions calculated and executed to remove the accumulated thermal and temporal drifts and maintain the mirror co-alignment at the current attitude.  For targets within a beta/pole range with a different thermal environment over an orbital period, xy-alignment scans were required to establish the mirror co-alignment baseline at that attitude.  It was also necessary to establish an alignment baseline for every hemisphere crossing of the satellite.

6.7.3.2  Mirror Alignment: Predictive Modeling

 

Given the sensitivity of the thermally induced mirror motions to the boresight attitude, maintaining channel alignment by performing xy-scans for each target would have been prohibitively inefficient.  As a result, a model was generated that predicted the motion to be executed for each mirror to maintain channel alignment for each target observed.  This predictive model was part of the planning and scheduling process.  The mirror motion scripts were uploaded to the satellite and executed as part of the command sequence for each observation. 

This predictive model for the corrective mirror motions was derived from the analysis of xy-scans taken at multiple attitudes in combination with the analysis of peak-up data using the 4-arcsecond wide MDRS and the 1.25 arcsecond wide HIRS aperture.  The xy-scan data provided the magnitude and sign of the target mis-alignment in both the x and y direction. The peak-up data provided sign and amplitude information in the x-direction, but no y information was available.  In addition to the dependence of the mirror mis-alignment motion on spacecraft attitude, temporal dependencies were also found but not completely nulled by the predictive model. 

6.7.3.3  Channel Alignment Operations

 

Empirically, it was determined that the secular drift of the predictive model relative to observed target position in the different channels required obtaining xy-scans every two weeks to re-establish the model baseline. Extreme attitude changes, such as hemisphere crossings which were driven by the sun angle and typically resulted in large beta changes, were not well-modeled and required xy-scans at the target attitude to determine the appropriate mirror motions for channel alignment. In addition, mirror motions were a strong function of attitude for targets at large beta angles

 

Executing the scans, analyzing the data, and uplinking the corrective mirror motions impacted observatory efficiency. Since the channel misalignment resulted from thermal changes in the instrument environment local to the mirrors, it was imperative that the mirrors were in thermal equilibrium prior to executing the xy-scans.  Thermalization periods of 6 -12 hours were required depending upon both the beta angle and the magnitude of change in the beta angle between targets.  Analysis of scans taken with extreme attitude changes exhibited an exponential dependence of the target (i.e. mirror) position as a function of time, consistent with the thermal non-equilibrium hypothesis. Achieving full positional equilibrium could require 11 hours; however, the target was usually within the 30 arcsecond wide LWRS aperture and monotonically approaching equilibrium within 6-8 hours.  As a result, to improve efficiency, observations were typically initiated before completion of the full thermalization period.  For time-tag observations the data are processed by the CalFUSE pipeline as photon lists.  Signal-to-noise permitting, this facilitates the comparison of data in temporal bins to remove secular drifts from the data.

The image motions for the two SiC channels were primarily in the X  (dispersion) direction, although small corrections in Y were also occasionally required.  For the LiF2 channel, the motion was exclusively in Y (perpendicular to dispersion), and significantly larger than the Y motion in the SiC channels.  Y motions required corrective mirror adjustments. In the X (dispersion) direction the FPA was used to accommodate small changes in the predicted position of the target because FPA adjustments were simple to implement and resulted in no loss of observatory time.   Due to the limited travel range of the FPAs, larger X motions corrections required mirror motions to achieve co-alignment.

Analysis of the  xy-scan data and comparison of signal levels obtained in each of the channels during routine observations enabled the formulation of a set of rules to maintain alignment between alignment re-baselineing observations.

These predictive model and alignment rules were incorporated into a planning spreadsheet that was executed prior to completing the detailed scheduling of targets. The primary alignment considerations were to restrict the changes in beta and pole angle to within 30 degrees of the previous target.  In addition, large attitude changes required extending the observation time on a target to include the thermalization time for that attitude and delta attitude to ensure obtaining full spectral coverage, if needed.  Hemisphere crossings and observations at high beta angle required long thermalization times and dedicated alignment activities.  Long-term temporal dependencies were not well-modeled necessitating the need for re-baselining  alignment scans approximately every two weeks.

Alignment activities were required at both low and high beta angles.  Motions of the LiF2 mirror at high beta angle (>90 degrees) were not well modeled an the consequently observations requiring LiF2 data were not executed at high beta angles without a dedicated alignment activity.  Due to the paucity of data at high beta angles, we were not able to improve the predictive model in this regime.  Pole angle was a less important consideration than beta angle for maintaining alignment.  Observations at a single beta angle for a large range of pole angles supported this conclusion. 

Later in the mission, planning constraints necessitated by operational changes arising from failures of the FUSE reaction wheels and associated with spacecraft safety took precedence over alignment constraints. This periodically resulted in either increased re-baselining activities to achieve mirror co-alignment, which was operationally inefficient, or acceptance of limited spectral coverage as a result of mis-aligned channels.  To ensure that observations were acquired with the necessary spectral coverage, Guest Investigators were required to specify the channels essential for their science program. This information was propagated through the MP database and factored into the scheduling and detailed sequencing of observations.

6.7.3.4  Orbital Motion

Image motion was also found to have several time dependencies.  A very repeatable orbital motion was found, and was well mapped for CVZ targets (Figure 6‑14 This source of this motion was attributed to the thermal cycle of the orbit.  For some targets, a 24-hour period motion of as much of 8 arcsec full amplitude was observed.  This motion appeared to be roughly sinusoidal in shape. There were also apparent long-term time dependences for some of the motions, but a paucity of continuous data inhibited characterizing this temporal dependence.

Figure 6‑14: Orbital Dependency of Image Motion for Selected Targets

6.7.3.5  Impact and Evolution of Image Motion Corrections

Image motion impacted FUSE science data in several ways.  (1) Channels were misaligned for targets at beta and pole angles near the edge of the range of nominal operations.  To mitigate this, the predictor set was continually expanded and improved.  As a result, channel loss decreased over time.  (2) Re-alignment activities reduced the time available for science data collection.  Thus, realignment activities were streamlined to use significantly less time than required early in the mission. (3) Motions of the spectral image could result in the loss of spectral resolution, especially for long exposures at high and low beta angles.  Consequently, short histogram exposures (<500 s) were used for bright targets. Time-tag data is corrected for this motion in data processing using an empirically derived correction.  A more detailed discussion is presented in the the CalFUSE pipeline paper (Dixon, et. al 2007). The bright limit for time-tag data was increased to allow more targets to use this mode.  (4) Tracking orbital motion for medium and high resolution aperture observations required additional FPA motions, and impacted efficiency. 

6.7.3.6  Target Peak-up Strategy for MDRS and HIRS Observations

To maintain co-alignment of the channels at the arcsecond level required for the MDRS and HIRS apertures, an on-board peak-up procedure was employed.  This procedure slewed the telescope in small steps to move the images across the narrow aperture.  The FUV count rate at each point was used to calculate the position of the image centroid for each channel.  Next, FUSE was slewed to the peak position for the guiding channel.  Then, each of the other three FPAs were moved to the calculated location for each of their peak count rates.  In performing this operation, it was quickly determined that the images moved over an orbit while pointing at a single target.  Consequently, procedures were developed and implemented to perform peakups twice per orbit to maintain alignment when using the narrow apertures. The inclusion of mid-orbit peakups resulted in reduced observing efficiency, but enabled full spectral coverage using the MDRS and HIRS apertures.

6.7.4   Channel Alignment: Observations and Analysis

As discussed in Section 4.2.3.1, the real-time channel alignment process was time consuming. Large overheads were incurred as a result of the time required to thermalize at the target attitude, execute the scans, wait for ground station passes to downlink the data, perform the scan analysis, generate the mirror motion scripts, and then uplink the corrective mirror motions at the next available pass opportunity. This need for ground station communications for data downlink was both a time and a scheduling constraint. To eliminate overheads and streamline this process as much as possible, on-board detector counter data was acquired instead of science exposures.  Figure 6‑15 depicts  xy-scan counter data used to analyze an observation.  The first peak in the counts/second (occurring at between 400-100 seconds) is the y-scan; the second peak is the x-scan. The four channels are each graphed using a different color. From inspection of the graph, all channels are aligned in the y-direction.  However for the x-scans, the SiC data (blue peaks) are offset from the LiF1 guide channel data indicating a  misalignment of the SiC1 and SiC2 mirrors.

Figure 6‑15: xy alignment  scans as depicted using the Alignment Tool Graphical Analysis.  These data are discussed in Section 6.7.4.

With the loss of reactions wheels, the pointing jitter increased. Now, one of the primary observation scheduling constraints was the availability of torque authority, and, in one-wheel mode, using target sequencing to manage the spacecraft momentum. Minimizing the changes in beta angle and pole angle between subsequent observations became a secondary consideration. This change in planning strategy resulted in larger predictive mirror motions and larger deviations of the mirror position from the predicted position.  As a result of the larger alignment errors and the increased difficulty in executing maneuvers in one-wheel mode, the alignment scan pattern was changed both in its spatial coverage and from a step and dwell pattern to a series of continuous scans. This in turn necessitated a new alignment assessment tool, the Channel Alignment Tool (ChAT) shown in Figure 6‑16. In addition to the xy-scan pattern, diagonal scans were also executed to increase the diagnostic capability in one-wheel mode where the amplitude of pointing excursions was much larger than previously experienced when pointing was controlled using three reaction wheels.  The erratic scan trajectories in Figure 6‑17 and Figure 6‑18 exemplify the difficulty of fine-pointing with only a single wheel.

The graphical interface provided by ChAT for interaction with the alignment scan data greatly facilitated analysis of the scans.  Regions of pointing excursions and high background were now easily identified and excised from the alignment analysis. 

Figure 6‑16: Example of Channel Alignment Tool (ChAT) Results

Figure 6‑17: (Revised) One Wheel Mode Alignment Scan Pattern

Figure 6‑18 One Wheel Mode Alignment Scan Pattern Results

Figure 6‑19: Additional ChAT Sample Results

Confirmation of the successful execution of the commanded mirror motions was inferred by one of four methods: examination of the command sequence to verify that it had completed properly, execution of another set of X- and Y-scans, analysis of a follow-on peak-up activity, and/or analysis of the count rate of the next sufficiently bright target at a comparable attitude. Depending upon the availability of ground station contacts, addition scans or peak-ups could require several hours or more to perform the scans and downlink the data. Consequently, a confirmation of mirror motion execution was rarely performed.  In practice, confirmation of the count rate from the next bright target was the primary method used to confirm that the mirrors had been moved to their desired positions.

Examination of the telemetry over the eight year lifetime of the mission never revealed an instance where the mirror motion command only partially executed.  With one exception, the infrequent instances where the mirror motion did not appear to execute could be traced to a manual tracking error somewhere in the alignment maintenance process. Consequently, steps were taken to automate the procedure as much as possible.

For the first five years of the mission, LiF1 was the guide star channel and the LiF1 mirror actuators were not moved.  In 2005 due to anomalies with FES-A (see Section 4.5.3) LiF2 became the guide star channel.  The restriction on moving the LiF1 mirror was then lifted, and LiF1, SiC1 and SiC2 mirrors were moved to align with LiF2.

6.7.5   Mirror Motion Accuracy

There was no explicit test of the accuracy of the mirror motions and the mirror motions were not telemetered.  It could be inferred from multiple realignments that the inaccuracies of the motions are small compared to both the requirements and measured orbital motions.  Peakup test data showed motion accuracy about Ry for the SiC 1, LiF 2, and SiC 2 mirrors of better than 2 arcsec for small (< 6 arcsec) motions.  Moderate size motions (<30 arcsec) have been shown to have accuracies of better than 4 arcsec. This had significant positive impact on operations after the loss of the reaction wheels, when prioritized scheduling as a function of spacecraft momentum management was needed.  This required additional mirror motions for a greater number of observations and hence, greater flexibility in the mirror motion scripts.

6.7.5.1  Mirror Motion Tracking and Actuator Performance

Due to the thermally-induced relative motions of the four telescope mirrors (LiF1, LiF2, SiC1, and SiC2) with respect to one another (Sections 4.1.1, 6.6.1.1, and 6.7.2 - 6.7.4) the total number of mirror motions executed was much higher than the ~900 motions per actuator that was predicted before launch.

The actuators were designed for 1.0 × 10⁶ motions, far more than could reasonably be done in even a very long extended mission. As a result, although the number of actuator motions for each mirror were tracked, there was no cause for concern in their usage for channel alignment.  Furthermore, any loss of a single actuator on any number of mirrors is a soft failure that will cause a minimal loss of data. 

 

Figure 6‑20: Time series of individual mirror motions executed to maintain co-alignment of the LiF channels.

Figure 6‑21: Time series of individual mirror motions executed to maintain co-alignment of the SiC channels.

 

The mirror actuators were not run over their entire range of motion in flight.  However, the defocus tests, which were performed to assess the feasibility of executing bright target observing programs, did move the actuators to within 20% of their full range of travel for the LiF2, SiC1, and SiC2 mirrors.  The LiF1 mirror was not defocused, since as the guide channel at that time, the risk was not justified.  No adverse effects were noticed and the mirrors returned to their nominal in-focus positions as commanded.

The secular  drifts of the actuator position associated with mirror co-alignment over the lifetime of the mission is shown in Figure 6‑22 and Figure 6‑23.  The position of the actuators for each mirror, the overall repeatability of the system, and the ability of the model to keep the mirror motions within a well defined range is illustrated. The use of LiF2, rather than LiF1, as the guide channel in early 2006 is evident from the plots.  In April 2006, the predictive model became less robust as is indicated in the frequent use of larger actuator motions to obtain co-alignment.

Figure 6‑22: The range of motion for each of the Lif1 (top) and LiF2 (bottom) mirrors illustrating that although to co-alignment position for each actuator exhibits a secular drift with time, this change is small and well within the range of travel for each of the actuators.

 

Figure 6‑23: The range of motion for each of the SiC1 and SiC2  mirror actuators.

6.7.6   Spectral Motion Anomaly: Thermal & Mechanical Analysis

During IOC an investigation team was assembled to determine the cause of the observed spectral motion. The functional form of the motion with orbital phase was suggestive of differential thermal heating of the instrument.  A full FUSE thermal math model was correlated to the thermal balance test data obtained during thermal vacuum test at GSFC (November 1998, January 1999) and input into a structural Finite Element Model (FEM).  This model was then used to infer temperature induced image motions.  Results of the analysis showed that, as expected, the temperature of SiC side of the instrument and spectrograph increased as beta angle increased.  These results were in agreement with flight data depicting the SiC mirror bench and spectrograph heater duty cycles decreasing by 11% and 7% respectively as the instrument's exposure to the sun increased.  LiF mirror bench duty cycles were only minimally affected by sun angle changes.  SiC baffle temperatures at the upper attachment flexure changed as much as 28 degrees Celsius when the beta angle changed from 70 to 105 degrees.  The corresponding change to the LiF side was only about 1 degree Celsius.  Quasi-steady-state temperature variations at these points, caused by orbital influences, were 1 degree Celsius.

Since the spectral motion was first discovered using airglow lines, the possible sources of the motion were isolated to the gratings, FPAs, and detectors.  Because the SiC and LiF lines incident on a common detector behaved differently, the detector was exonorated as the cause of the observed motions. The grating bench was eliminated as the cause of the spectral motion because the amplitude and directionality of the spectral motions are larger in X than in Y and it was expected that any motions caused by the grating bencyh would be larger in Y than in X. Neither were the FPAs regarded as a significant contributor to the spectral motion since observations using the LWRS slit, which is much larger than the image of a typical astronomical source, show similar motions in the spectrum of a point source and in airglow lines that fill the slit.

The FUSE Grating Mount Assemblies (GMA) were also investigated for any potential contribution to the in-flight spectral motions.  The existing thermal distortion analysis was reviewed for orbital variation for the GMAs.  A overview of the design was also performed.  The review of the GMAs was limited to the mount assembly since the FUSE on-orbit performance shows no evidence that the grating glass itself could be distorting.  During the instrument design phase, a thermal model of the GMA was constructed and coupled to the satellite model to predict an on-orbit temperature map for the grating assembly components.  That analysis predicted that for each individual piece part of the GMA, the orbital temperature change laterally in IPCS X and Y was negligible (< 0.02 Celsius).  The orbital temperature change in the mount in the IPCS Z is ~0.2 degrees Celsius, and the worst case orbital temperature change between the inner and outer tube temperatures is 0.2 degrees Celsius.  The FEM distortion analysis showed that translations of the center of the grating are within short-term stability requirements.

However, the model analysis was not regarded as conclusive given the large size of the mount and the relative flexibility of titanium, the primary material of construction. A test simulating the on-orbit environment was regarded as the best verification of the mount's thermal stability.

Consequently, pre-launch tests were conducted to determine if the gratings distorted with temperature.  The gratings were heated using their shrouds while monitoring their spectral images for changes.  The glass temperature was raised from 21 to 28 degrees Celsius while the shroud temperature peaked at ~40 degrees Celsius.  A post-launch re-examination of this data found that the image moved 19 pixels on the detector (~114 microns) for this temperature change in the direction of IPCS +X for the SIC1 channel.  Concurrent observations using the LiF1 channel, without heating its shroud, exhibit only a ~3 pixel motion.  These results and the magnitudes of the motions are supported by a FEM/thermal analysis done for FUSE by the University of Arizona.

The resulting hypothesis was that the direct conductive path from the shroud to the grating mount hardware was heating the "wedge" non-uniformly, possibly leading to a grating rotation.  While not conclusive, this test data illustrates the potential sensitivity of the grating mount assembly to temperature change.

Subsequently, an on-orbit test was conducted to test the hypothesis that the heaters may have been thermo-electrically coupled to the spectrograph GMA.

To evaluate this hypothesis, the two grating shrouds (LiF2 and SiC2) on the +y spectrograph axis were permitted to drift to a lower temperature while the other two grating shrouds (LiF1 and SiC1) were held at their nominal set point of 23 C. A temporary set point of 19 C was chosen for the heaters. This temperature was chosen to permit acquisition of Lyman beta airglow spectra for 2 orbits without the heaters cycling while keeping the gratings at or above the spectrograph temperature, a contamination related issue albeit not a severe concern after ~15 months in orbit.

Although the time required for the GMA shrouds to reach their test thermal set-point was significantly faster than expected, a few conclusions could be drawn. The motion of the spectrum in the dispersion direction was not alleviated by operating with the GMA shroud heaters off.  Changing the temperature results in an offset in the zero point position of the spectrum on the detector in the dispersion direction.  And, the functional form of the relative motion of the spectrum was the same at the lower grating temperature as it is at the nominal temperature. The spectral motion exhibited after the GMA shrouds were returned to their nominal temperature was the same as it was before the temperature was lowered.

 

Figure 6‑24: LiF2B data over an orbital period illustrating that the spectral motion observed with the GMA shroud at 19 C (top) is shifted/offset by ~5-6 pixels from the data acquired with the GMA shroud at the nominal 23 C (bottom).