UV throughput was restored following the 6 hours CCD decontamination on day 318 (see Figure F2).
Science and Calibration observations continued to execute successfully.
For a picture of the Faint Object Camera click here.
Proposal FOC 6161 on day 316 ( November 12, 1995 ) was executed to verify aperture locaton of the F/96 camera.
On day 319 at 01:39 FOS cycle-5 focus, x-pitch, and y-pitch engineering test was run. After warm-up to the nominal high voltage, the trim current was set to zero, and the HV was set to a lower value. Spectra were then obtained with the 0.1-PAIR aperture, G190H, and the Pt-Ne lamp. The high- voltage was then varied in 200 volt increments +/- 600 volts from the nominal setting with spectra taken at each setting. This will allow for optimal focus determination.
On day 320 at 05:32 the last of four dwell scans were completed in support of a FOS interactive target acquisition. The science data was received and the offset parameters were uplinked in preparation for the scheduled observations (Prop 5824) on day 324 at 02:21 and 23:13.
1.2 Summary of major problems
Due to an FGS malfunction a loss of lock occured during an observation of proposal FOC6067 on day 95.333. The Take Data Flag dropped during exposure time and terminated the observation earlier than scheduled.
On Day 317 at 03:15:39 the NSSC-1 issued the status buffer message RFOS 290 P=24504S. This error message was associated with a FGS LOL (FGS 1 was dominate). The FOS was in the middle of executing a Peakup/dwell scan on Tethys (a moon of Saturn) in preparation for several rapid mode observations of Saturn's atmosphere and rings. The parameter 24504 indicates an error path was taken in the FOS RTCS YPKUP. This error occurs if the TDF never comes back up after the slew between dwells. The TDF remained down long enough to allow FOS event flag 13 to be set thus preventing the shutter from reopening until the end of the OBS set. The interesting aspect of this set of observations was that the target acquisitions were in a different OBS set then the actual observations, thus FOS event flag 13 was reset and the aperture door was allowed to open after the Target acquisition had failed. FOS event flag 13 processing was initially introduced as an over light protection scheme. In the above described case this protection scheme failed. Typically the only object bright enough to damage the detectors which FOS observes, is Mars, and than only if the mirror is used. Other objects such as Saturn and Jupiter could also trip the over light, possibly safing the instrument. The two most common reasons why a set of observations are split into two OBS sets are the phase dependent observations and the moving target observations (like a planet), where the target changes. One possible fix could be that the reset of flag 13 is associated with the end of a scheduling unit as opposed to the end of an OBS set. Discussions of the possible response and level of effort to fix this problem are ongoing.
On day 327 at 0431 a series of peakup were scheduled on the dwarf nova HT Cas (prop 6035, Blue side) which was experiencing an out burst which was expected to increase the stars magnitude several times. The initial concern was that this star was going to go into a super outburst condition, thus increasing in magnitude to a level which could trip FOS's overlight limit (3 million counts) during the target acquisitions portion of the proposal in which the mirror is used. Preparation were made to manually force the aperture door closed before the observation began. Two days before the observation was scheduled to begin, it was determined using ground base observations, that the flux levels were dropping and no action would be required in regards to the overlight protection, but an additional concern had arose that the target would be in eclipse during the third part of a three part mode 2 TA (peakup). ESB personnel began exploring possible ways to force the third peakup acquisition to fail without compromising HST's pointing. It was determined that by inhibiting FOS TDF RTCS, a clean failure could be forced. It was however, determined by the FOS Instrument team, after modeling the third peakup scan in conjunction with the expected time of target eclipse, that there was a good chance the TA would succeed. A post observa- tional review of the science and engineering data revealed the target was within but not centered in the FOS's aperture (1.0 arcsec aperture). An investigation is still ongoing as to why the target was not centered, but the targeting was good enough for the desired science.
A HOPR was submitted on proposal 5441 which was executed in 1994 on day 247. The main thrust of the HOPR was the unexplained variability of the target. FOS instrument Scientist requested the FOS engineer restore and analyze the Jitter images. Subsequently, jitter was ruled out as a cause of the variability.
1.2.2 Several recent GHRS target acquisitions have ended with a failure of the flight software to successfully perform target centering in the X direction. Each failure represents the use of the extended locate feature of the acquisition software to center a bright object. Since the calibration overhead is higher for the attenuated mirrors than for the normal mirrors, some observers have tried to save time by using the normal mirrors with extended locate for bright targets. When using the normal mirrors, a bright target will have an extended core with little contrast. The fine X centering algorithm will move the image across the central diodes until the maximum counts shift from one side to the other. The maximum movement allowed is a little more than one diode width. If the image core is wider than two diodes, a movement greater than what is allowed would be required for the bright shift to occur. As a result of these errors, acquiring bright targets with the normal mirrors and extended locate will no longer be allowed if these targets can be acquired with an attenuated mirror.
1.2.3 Monitoring of GHRS carrousel reset activity is continuing. During the month of November there were 10 reset events for 783 commanded positions. Figure GHRS-F4 contains a plot which shows the accumulated number of times the carrousel is commanded to a new position and compares this rate to the accumulation of carrousel resets. This plot shows that the rate of resets was proportional to the number of times the carrousel was moved for the first few years of the mission. Over the last year, however, we see the rate of carrousel resets increasing. While the rate of reset events is still small, this trend represents a deterioration of the carrousel mechanism. Figure GHRS-F4 also includes a bar chart which associates reset activity to specific locations on the carrousel. This chart shows that the rate of reset events is higher at the lower region of the carrousel step scale. The rates shown in the region of the G140M grating are mis-leading since this is a little used optical element. A single reset in this region is rated higher than it would be in a more commonly used region.
U2UH1201 was affected on day 322 due to a LOL in the baseline acquisition. U2OV6X01-04 and U2S06501-02 were lost due to STR mis-configuration.
2. Observatory Performance
1) For acquisitions on single GS:
The acq will fail to gyro mode.
2) For acquisitions on an GS pair:
a) Attempt to achieve coarse track on both guide stars in order to perform a coarse angle check. If coarse track cannot be achieved, the acq will fail to gyro mode.
b) If the above coarse angle check passes, try to go to fine lock mode on single GS. The primary GS will be tried first. If fail to achieve FL on the primary GS, the secondary GS will be tried. If fail to achieve FL on the secondary GS, the acq will fail to gyro mode.
c) If the above coarse angle check fails, the acq will fail to gyro mode.
No acquisitions in coarse track or fallback to coarse track will be allowed. The scheduled Coarse Track acquisition on a single guide star at 1995.326:20:56 was part of an engineering test.
The sky distribution of pointings in this month is shown in Fig. 2.1.
Fig. 2.2 shows the monthly average pointing miss for primary guide start acquisitions and reacquisitions. The pointing miss is measured from the location of the guide star found during search compared to the predicted position (start of the search). Table 2.1 describes the statistics of guide star acquisitions. It takes into account both primary acquisitions and reacquisitions. "No lock" means that coarse track cannot be established or maintained. "Degraded mode" refers to the cases where the guiding mode falls back to coarse track when the commanded mode of the find lock cannot be established or maintained. "Search rad exc" refers to cases where the guide stars are not found.
The distribution of guiding modes by Science Instrument during scheduled exposures is given in table 2.2. For each scheduled exposure, the actual guiding mode is obtained from the engineering telemetry. The scheduled exposure time is subsequently summed up by guding mode for each SI to produce the distribution.
The full-width at half-max (FWHM) of jitter during observations are plotted as a function of the magnitude of the dominant guide stars in Fig. 2.4. the jitter is obtained from the motion of the dominant guide stars in the FGS. The rms of jitter along V2 and V3 axes is also calculated for each observation. The average of FWHM and rms of jitter over all observations in each month is given in Fig. 2.3 and shows no obvious trend.
For each observation, the PMT sensitivity is calculated for each FGS in fine lock based on the PMT count rates and magnitude of the guide stars. The sensitivity is expressed in total counts of the 4 PMTs per 25 milli-seconds normalized for a 13th magnitude star with the FGS filter in pupil position. Fig. 2.5 shows the average sensitivities of each month since Janurary, 1991. The is no obvious trend. The variation of the sensitivities appears compatible to the error of the guide star magnitude.
3. Observatory Trending
WFPCII Tables: T1, T2, T3 and Figures: F3-F9 show the November instrument statistics and profiles for cycle usage, power and temperature. All values are nominal and within limits unless otherwise noted.
table t1 shows the cycles of various mechanisms and power supplies.
table t2 shows the lvps, mechanism, and TEC voltage and current outputs.
t3 shows the bays, optical bench, Bulkheads, Cold and Hot junctions, Camera Heads, Attach points, AFM, and Radiator temperature values.
The f/48 relay is considered to be operational restriced use to the use of long slit spectrometer observations.
Serveral plots of selected monitor points critical to the performance of the Instrument are an integral part of this report:
The SDS error log has been modified to plot any occurrence of a possible SDS TWO Bit Error.
Several Tables are inserted to keep track of operational statistics in particular of limited lifetime items:
For a picture of the optical path and the FOC mechanism click here.
