MINUTES
EINSTEIN USERS' COMMITTEE MEETING
January 23, 1981
1. Status of Spacecraft
Gyro 4, turned off August 27 and unresponsive to commands since that time, was
successfully turned on December 6. This was the result of repeated attempts,
approximately once per day, to start the two good gyros turned off August 27.
Since 7 December, operation of the Observatory has been "normal". Some of the
time during September, October, November, was used conducting observations with
gyro 5 which was for a while capable of intermittent noise-free operation. In an
attempt to operate with only two gyros, a system was developed to use one of the
solar sensors as a pseudo-gyro. Several attempts were made to control the
Observatory using this mode, none of which were successful, and much of the gas
supply was consumed.
On December 7, 25 pounds of attitude control gas remained. By observing only
targets in positions such that gravity gradient torques are minimized, gas usage
has been lowered to average 0.1 Ib/day for the past five weeks, a factor of 2
better than the best previous gas consumption. It is worth noting that there is a
large uncertainty in the residual amount of gas.
The first end-of-life prediction is May 1, 1981 when the gas supply will
probably be depleted. Re-entry is now calculated as October 1982 with a 20 worst
case May 1982, approximately one year later than the lifetime set by the limited
gas supply.
The instruments and star trackers are working well, and the aluminum filters
have been used successfully for the first time to conduct observations. There is
a problem with one thruster sticking. The protect mode has been changed so, in
the event of an emergency, the gyros are not shut off as they were previously.
After the gas runs out, the Observatory will probably spin at the rate of
several degrees per second. There will be enough power in daylight for
engineering tests, but the gyros will not operate and aspect will be unknown. No
useful astronomy data is expected after the gas is exhausted.
2. Distribution of Time
Observatory time for the remaining life has been portioned among the various
users as shown below. The times given in seconds are for five months, from
December 7 to May 7. We at first set aside 15% of the time as Director's time,
but no requests for this time were received which fell into the category of
unforeseen scientific opportunities for which special
allowances should be made. Any new observation of first-rate scientific
importance which clearly cannot be accommodated within institutional programs
should be brought to the attention of the Director. Such observations could be
carried out but would necessitate a corresponding reduction in other programs
already planned.
Table 1. Division of Observing Time
CFA .285 1.68 x 106 sec
GSFC .095 .57
MIT .126 .75
CAL .095 .57
GO .401 2.39
Director .000
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TOTALS: 1.000 5.96 x 106 sec
3. Mission Planning (Scheduling of Observations) The goal is to minimize
gas usage without unreasonable restrictions on science results. To do this,
targets are observed only if close to the plane or pole of the orbit. If a target
is at a declination such that this never occurs, the target can be paired with
another so that gravity gradient torques cancel and gas usage is minimal. Pairs
are not always acceptable to the GSFC program which arranges targets in a
specific sequence for observation, and it is time consuming to choose acceptable
pairs. For these reasons, we can pair only a small fraction of those targets at
unfavorable declinations. Specific entries are discouraged. Targets are rated on
a new priority system: 3) must do; 2) highly desirable; 1) desirable; 0) probably
won't get done, but will be used as filler when there are not enough higher
priority targets to use up the time. There is a finite probability that a
priority a target will be observed, and the "owner" will be charged for the
observatory time used. If you do not want a target, it must be deleted from the
OCA. Gas usage will be taken into account so a program gets, for example, 10% of
the gas rather than 10% of the time. High gas use targets of high scientific
importance can be observed, but the program will be charged accordingly.
A few hours of data were lost January 17 because of a loss of ground
support--preempted by shuttle. (Einstein tape recorders can hold 7 hours of data.
A longer interval between tape recorder readouts results in data loss.) CFA has
asked for enough stations to cover Einstein during times of high usage by other
programs, and we are investigating the possibility of not putting Earth occulted
data on the recorder. In this way, the
recorder's capacity for astronomical observations would be increased.
4. Guest Observer Program
The last Guest Observer Review was held December 22. Sixty proposals were
considered, and only 12 accepted. Realizing that any new observations would bump
guest observations already scheduled, the committee only accepted ~ 65
kiloseconds of additional time (~1-1/2 days). The total number of Guest proposals
submitted since May 1978 was 699. The total number accepted was 459, or 67%. As
of July 1980, Einstein has completed 130 Guest programs. All observations have
been done and all data from these observations processed.
During the month of December, 40% of useful observing time was used by Guest
Observers. On the first of December, the total time in Guest observations not yet
attempted was 4.5 x 106 sec. The total guest allocation for the five months
remaining was about 2 x 106 sec. The total time in Guest observations given a
high priority by the proposal review committee was 1.5 x 106 sec. We estimate
that if the Observatory operates until May 1, we can complete approximately
two-thirds of the high priority Guest observations, and about one-third of the
low priority Guest observations. The requirement for minimal gas use eliminates
most intermediate latitude targets from consideration. Fred Seward is setting
priorities for the Guest observations remaining, and has written a letter to all
Guest Observers requesting priorities for their individual investigations.
5. CFA Computer
Both CFA computers have suffered from a high fraction of down time (~20%) for
the last few months. This is more than usual for systems of this type. CFA and
Data General have investigated the problem, and have concluded that the
environment in the computer room is not well controlled. The air conditioning air
flow is not satisfactory in the room, and temperature fluctuations are high.
Planned improvements are: 1) improving the air flow; 2) restrict access to the
computer room to a few knowledgeable people; 3) Data General will re-cable the
system; 4) thorough preventative maintenance will be done on the system.
CFA is asking MSFC to replace tape drives. Existing drives are unreliable, and
it is difficult to have the unit serviced.
6. Data Processing
NASA is sending tapes six weeks after the observation. CFA processing currently
lags one month behind CFA acquisition of the tapes. Data processing is delayed,
as usual, by special processing requests. Additionally, mission planning has
become more sophisticated. The computer is tied up 1-2 days each month with
selection of targets to be observed.
7. Reprocessing of Data
All Einstein imaging data will be reprocessed at least once to incorporate the
latest knowledge of detector characteristics and backgrounds.
a. IPC
Processing of IPC data will include automatic calibration source analysis,
correction for time variability of detector gain, a new calibration of detector
distortion with consequent improvement in derived source positions, a better
estimate of detector background, automatic spectral fitting for strong sources on
axis, and a clearer presentation of the IPC data. At least six months will be
required to incorporate all these changes. The improved positional analysis will
be on-line in approximately one week.
b. HRI
HRI data will be reprocessed incorporating better aspect determination and star
tracker boresights. The present program already makes a star tracker magnetic
correction accurate to about 0.5 arcsec. Final boresights are now being derived
and test objects analyzed to arrive at the proper 90% confidence radius. We
estimate that this radius will be between 3 and 4 arcsec after the analysis is
complete. This new program is not yet on-line. We are still working on fiducial
light corrections which measure how well the HRI is positioned after the focal
plane transport assembly is moved (a 1-2 arcsec error).
All these calibrations are for HRI #3. The first three months' data were taken
with HRI #2 which will require a different calibration. Quicklook data are
processed through a different system with different calibrations. Locations are
not as accurate in quicklook data as in production data. We estimate that we can
start processing HRI data incorporating the new corrections in about 2 weeks.
Since the computer is busy full time processing flight data, reprocessing old
data cannot start seriously until Einstein ceases observing.
8. The Einstein Data Bank
The reprocessing and release of Einstein data has been mentioned several times
in general terms. With the end of the mission near, we would like to generate a
specific plan including dates when observations are made available to others.
At best, the mission will last until June 1981. It should be possible to start
reprocessing some data at that time. The reprocessed data will go into a data
bank which can be used by the scientific community through the mechanism of Guest
Observer reviews. Thus, anyone could apply to use these data, but approval by a
NASA review committee would be necessary. This protects us from ridiculous
requests and ensures that CFA support is not overextended. It also allows users
to apply for financial support for their program. We would plan to give the same
services to data bank users that are currently supplied to Guest Observers. We
expect to participate in the review process as we now do, but there will be no
veto because of consortium conflicts.
We would distribute a "Yellow Book" as soon as possible (September 1981)
containing a list of all Einstein observations, the observation date, and the
expected date of availability. Thus, potential users would know when specific
objects would be reprocessed and in-the-bank, and proposals for use of these data
could be started. Exceptions might be made to this schedule as long as the
expected release date is contained in the Yellow Book.
The Users' Committee felt that observers should have exclusive use of data after
reprocessing, and that there should be a delay between reprocessing and data
release. Most of the Committee felt that a six month delay was appropriate with
minority opinions varying from three months to one year.
It was suggested that data be reprocessed in order of celestial coordinates
rather than in the same sequence in which data were taken. It was also pointed
out that an additional computer would allow reprocessing of data to proceed
faster than real time. CFA is investigating this option.
9. Miscellaneous
Future satellites will have imaging capabilities. EXOSAT is planned to be
launched in 1982 with a life expectancy of two years. The German satellite,
ROSAT, is expected to be operational in 1985 and 1986 and might undertake pointed
observations, although preliminary plans call for a scanning survey mission with
the imaging instrument.
The Users' Committee felt it should continue in operation to oversee the
functioning of the Data Bank. The next meeting will be in May-June 1981.
Attendees: C. Canizares (MIT), P. Charles (GO), W. Forman (CFA), R. Giacconi
(CFA), J. Grindlay (CFA), F.R. Harnden (CFA), D. Harris (GO/CFA), J. Hutchings
(GO), K. Long (CAL), F. Marshall (GSFC), J. Schwarz (CFA), F. Seward (CFA), H.
Tananbaum (CFA)