ROSAT Status Report #6:

July 5, 1991

International Users' Committee

The ROSAT IUC met at MPE on June 12-13. The primary agenda item was the approval of the AO2 observing program. Discussions were also held regarding the spacecraft status and the plans for recovery from the gyro failure; these are summarized below. Several important policy decisions were made by the IUC regarding the short and long term conduct of the guest observer program.

A consequence of the Y gyro failure is that the satellite is now operating under severe pointing constraints, forcing very long observations of one target per day (as compared to an average of about 7 per day under normal operations). Hence, although only 15% of the planned AO1 pointings for May-August have been made, ROSAT is continuously taking good science data. This will result in an oversubscription of A and B pointing time during the second half of the AO2 pointing period, even assuming the satellite reattains a 60 percent pointing efficiency. According to the previous IUC policy, all scheduled category "C" targets from AO1 that were not observed were to be carried over into the AO2 observing period as category "B" targets. If this were done, an enormous oversubscription would result in the second half of the observing cycle, and a major consequence would be that many AO2 "A" and "B" targets would not be observed in favor of AO1 category "C" targets. In order to prevent this from happening, and to ensure that such a thing will not happen if ROSAT is unable to achieve high observing efficiency once "normal" operations resume, the IUC approved the following policy changes for category "C" targets:
- All category "C" targets that were scheduled during AO1 but not observed will be carried over into the AO2 season not as "B" targets, but as "C" targets.
- If a carried over AO1 "C" target is not scheduled in the AO2 season, it is then dropped and the PI must reapply for time in a subsequent proposal round.
- Those "C" targets whose observation was started but which were not observed for the SCHEDULED amount of time (not the requested time) will be handled on a case-by-case basis. As there are very few such targets from AO1 belonging to US PI's (approximately six), these will be completed.
- In all future observing seasons, a scheduled "C" target that is dropped will not be carried over. Thus, the appearance of a "C" target on the long term timeline no longer guarantees it will be observed, and PI's should plan as a matter of course to repropose "C" targets in subsequent proposal rounds.
The IUC agreed that approximately one week will be spent completing the all-sky survey. This activity will commence on approximately August 2. The German program will be charged for the time: the amount charged will be the product of the number of days times the average good daily exposure time achieved in the reduced pointing mode (approximately 30 ks per day).
The IUC agreed to lengthen the observing cycle interval from six months to one year, starting with AO3. Due to the uncertain status of the satellite, specifically actual date and the efficiency to be attained when normal operations recommence, it was agreed that the AO release date should occur contemporaneously with the release of the long term AO2 timeline. The proposals will be due no sooner than early October, and will slip essentially day-for-day as the resumption of normal activities slips past August 1. Another change that was agreed is that proposal forms will be provided for the justification section as well as the targets, in order to help enforce the page limit, and to provide the review panels with a more uniform set of documents.

Spacecraft Status

ROSAT has been operating quite successfully in the reduced pointing mode. Only three targets have been missed since May 22; these were missed due to a lack of guide stars for the tracker. Over the past month or so, the solar activity has been quite high, so increased radiation belt contours have been used to control the PSPC high voltage. This is to prevent an automatic shutoff of the high voltage, which can only be reset during ground contact, and hence results in the truncation of observations. The typical exposure attained during this mode has been 30 ks.

The attitude operations during reduced pointing mode are worth an explanation. All slewing and target acquisition is carried out during ground contact, to ensure that the erratic X-gyro does not force ROSAT into safe mode. Thus observations are limited to those targets for which guide stars are visible during ground contact; this translates to targets with declinations higher than +20. Further, as the missing Y-gyro signal is provided by taking the vector difference between the S (diagonal) gyro and the Z and the X gyros, the X-gyro error is propagated into the Y direction as well. In order to prevent satellite from going into safe mode because of too large a sun angle, the allowable range of target sun angles has been reduced to plus or minus 5 degrees. Thus the available targets at any given time is quite restricted, and every available target with requested exposure larger than 10 ks has in fact been scheduled.

Slews between targets are always performed in the same direction. A week's observations will start at one end of the narrow band at declination +20, sweep to higher declinations (in steps 20 degrees or less), and then down to +20. Then a "long slew" is performed, during which the satellite is rotated approximately 240 degrees, across the earth, back to the other side of the sky.

Activities leading to a return to normal operations are underway. The ultimate goal of these activities is develop the capability of running ROSAT without gyros, relying on star tracker, sun sensor and magnetometer signals for attitude information. The sun sensor and magnetometer can each in principle provide attitude information in two axes to an accuracy of one degree. The star tracker field of view is being widened to allow it to acquire targets in a cone of radius 1.5 degrees. The implementation of the new onboard software will be done in three steps:

New safe mode software incorporating the sun sensor signal will be implemented. This is essential, because the current safe mode requires three gyros, and if another fails, then it becomes "unsafe." This software should make possible a relaxation of the sun constraint. It should be available late this week.
Software allowing a wider tracker cone angle will be implemented. This software has already been written for ASTRO-SPAS, and needed only to be converted from "C" to assembler language. This software might make automatic target acquisition again possible, thus allowing observations of southern targets. Implementation of this is also pending.
Software replacing the gyro signals with the magnetometer and sun sensor signals will be implemented. Originally scheduled for testing and implementation in August, it is now anticipated to be ready sometime in September. Once this software has been implemented, "normal" operations will resume.

AO2 target list:

Final checks are currently being performed on the approved AO2 list. The list will be made available sometime early next week.

SASS:

SASS production processing of the pointed verification data for the PSPC and the HRI have now been completed. These data are now on their way to the PI's who own them. Processing of AO1 pointed observations will commence no later than July 8th.

The observing schedule is as follows, with PI name and original target priority and original scheduled time following the target name:

         New ROSAT Timeline
        __________________

        30 Jun -  1 Jul  UK   1E1346+266         Mason (2) 11.0         
         1 Jul -  2 Jul  US   Abell1795         Arnaud (3) 50.0
         2 Jul -  3 Jul  WG   Alp Cep           Schmitt (2) 7.0
         3 Jul -  4 Jul  US   II Peg            Huenemoerder (3) 10.0
         4 Jul -  5 Jul  US   PG0027+260        Kahn (3) 10.0


         5 Jul -  6 Jul  US   3C294             McCarthy (3) 10.0
         6 Jul -  7 Jul  WG   LHS2924           Schmitt (3) 30.0
         7 Jul -  8 Jul  US   Mkn478            Halpern (3) 6.0
         8 Jul -  9 Jul  US   HD203467          Grady (2) 1.8
         9 Jul - 10 Jul  WG   1E2259+586        Dennerl (3) 40.0
        10 Jul - 11 Jul  UK   Lambda And        Bedford (3) 6.0


        11 Jul - 12 Jul  WG   1404+226          Ulrich (2) 10.0
        12 Jul - 13 Jul  US   1404+286          Marscher (3) 5.0
        13 Jul - 14 Jul  WG   NGC7635           Wendker (3) 10.0
        14 Jul - 15 Jul  WG   M31               Truemper (2) 10.0
        15 Jul - 16 Jul  WG   M31               Truemper (2) 10.0
        16 Jul - 17 Jul  WG   NGC383            Boehringer (3) 20.0
        17 Jul - 18 Jul  US   0133+207          Wills (3) 6.5

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