Minutes
Einstein Observatory Users Committee Meeting
October 19, 1979
I. Status of Observatory
The satellite and detectors have been operating well with no major problems.
The IPC gain change is caused by a temperature change during the first day at
the focus of the telescope. We have changed the temperature profile of the IPC to
minimize IPC gain variations. We are also making IPC MOPs longer (up to ten
days) to give long periods of operation at relatively constant gain. Two days of
calibration measurements are planned using LMC X-I to map the IPC response. With
these data, it is anticipated that IPC source positions can be determined with
uncertainties of only 30 arcseconds inside the ribs and 60 arcseconds outside.
The two trackers are still operating well. The single biggest concern is
possible failure of the bright-object-protection shutters which operate ~ once an
orbit when the bright Earth is seen. We are still studying aspect determination
with no star trackers. In this mode of operation, positions of known X-ray
sources would be used to update the gyros every 12 hours.
For the past few months, there was no ice on the SSS detector, and consequently
the low energy response was very good. Data taken October 12 appeared to be noisy
and by October 18 the resolution was'" 300 eV or two times normal. The SSS
appeared to be failing but not as suddenly as predicted by calculations. (The day
after the meeting, October 20, noise level in the SSS had increased still
further and within a few days, a decision was made to cease operations with this
instrument.)
II. Observatory Lifetime
Current gas usage is 0.26 lbs/ day. Gas usage was 0.5 lbs/ day at the start of
the mission and has steadily decreased as we have gained experience operating the
observatory. Currently, we are not observing any targets having orbital
latitudes between 300 and 600%Gâ¢%@ It should be possible to observe
targets in this forbidden high-gas-usage range by pairing with other targets to
cancel the build up of gravity gradient torques, but there are still problems
implementing this mode of operation. At the current rate of gas usage, the gas
will last 2.3 years from the present date (until the end of 1981).
Atmospheric density is high because the Sun is very active. If solar activity
were nominal, re-entry would occur ~ February 1982. If solar activity is 2 sigma
above nominal as it has been recently, re-entry will occur April 1981. The
uncertainty in these calculations is mostly uncertainty in predicting the level
of solar activity. We will continue to conserve gas to be sure that gas
consumption does not limit the lifetime.
As of August, the satellite altitude (290 km at the start) has decreased ~ 10
km. The suggestion was made that drag might be minimized by choosing targets in
the plane of the orbit, so at least half the time the cross-sectional area of the
satellite in the direction of motion would be minimal. No action is currently
planned to implement this suggestion.
III. Aspect
A. Absolute Accuracy (Boresight)
The calibration of one tracker, Tracker C, has changed from pre-launch
calibration by'" 10 arcseconds. Taking this into account and using the pOSitions
of 35 measured X-ray sources, the absolute accuracy of the Einstein aspect
solution is now five seconds RMS.
B. Relative Accuracy
A problem has been discovered in HRI observations of some strong point sources.
The problem manifests itself in two ways: sometimes appearing as a double peak
distribution with ~ 6 arcseconds separation; sometimes as a linear smearing of
extent ~ 3 arcseconds. Several ideas are being investigated to find the source of
this difficulty.
1. HRI internal stability is being measured with the UV calibration array.
2. Star tracker stability relative to the instrument has been measured with
fiducial calibration lights. Shifts in the structure of ~ 1.5 arcseconds have
been detected.
3. There is a seasonal shift of ~ 3 arcseconds due to thermal changes which has
been measured by comparing fiducial light data taken over a long time base.
4. We are investigating the possibility that light scattered into the star
trackers is causing small shifts in measured star poSitions.
5. Encoding errors have not been ruled out.
6. Magnetic field effects are probably not important. The trackers are well
shielded and a pre-launch ground test in a Gauss field produced no observable
effect.
IV. Data Processing
We are up to date (processing data'" six weeks old) except for a net total of 68
days of data which is mostly SSS processing. This is not as time consuming as
image data processing since only aspect and MPC data need be handled.
The map mode works well. Some data have been reprocessed to retrieve information
obtained when the trackers were detecting stars but not locked on the expected
guide stars and tracking.
We are now processing IPC data using a program which oversimplifies the IPC
background and response function. It is inadequate for detecting weak sources and
doing spectral analysis. In the future, we hope to understand the IPC well
enough to develop a more satisfactory program which will take into account the
eccentricities of this detector. Probably all data will be eventually
reprocessed, and we anticipate processing will last "'five years after the end of
the mission.
We consider the Einstein images and other data to be the property of the
Observatory. After a suitable time lapse to allow principal investigators to
analyze and publish data, the data base will be open to all. In particular, the
Consortium plans
to eventually produce a catalog of Einstein X-ray sources.
The Guest Observer Program should continue after the mission using the Einstein
data as a resource. Guest Investigations for analysis of small bits of the X-ray
data have already been approved It is important that NASA continue to fund this
program to maximize the scientific return. We consider the::continuing
participation of a large number of scientists necessary to extract the important
information from the data.
V. Guest Observer Program
There have been five NASA reviews of Guest Observer proposals. The table below
summarizes the results of these reviews.
Total Guest Fraction of
# Proposals Observer Time Three Months
Review Accepted Approved (104 sec) Observing Time
------ ------------ ------------------- ----------------
1 43 57 .16
2 47 64 .18
3 46 94 .25
4 28 55 .15
5 44 120 .33
---
Total 298
The observing efficiency is ~ 46% and has been used to calculate the above
fractions.
The fraction of the observing time spent in Guest Observations and for each
consortium institution is shown on the attached graph. In June, an effort was
made to increase the amount of time spent on Guest Observations by having the IPC
at the focus of the telescope for longer periods. Since then, usage has been
consistently above 15% but not yet above 20%.
The time accounting system that has been used is a system which charges short
observations for actual useful observing time, but charges long observations for
the amount of time spent pointed at the target. Consequently the charges for long
times include time lost during Earth occultations and in the South Atlantic
Anomaly. We hope to eventually have a more accurate system, but right now our
system overcharges the long observations. Since most G. O. time consists of short
observations, this is one reason why the indicated Guest Observer time has not
risen over the desired goal of 20%. Because of the short life of the SSS
detector, it has been operating at a rate 10% higher than its allocation. This
percentage comes from all other organizations involved including the Guest
Observers. A third factor is momentum management in which the only observations
done are close to the plane or pole of the orbit. During the last two months, we
have been doing every available Guest Observation, but sometimes do not have
enough to achieve 20%. We have entered observations approved in the last review
as fast as possible, and are currently observing some of these targets. Guest
Observers now have a backlog of observations comparable to the Consortium
members, and momentum management is no longer limiting G. O. time.
The NASA review committee, realizing that Guest Observer usage will increase, is
being more liberal with the average time assigned to each Guest Investigation;
In the original reviews, in order to insure that there would be time available
for many
to participate, most proposals were limited to ~ 2 x 104 seconds unless there
were strong scientific reasons for longer observations. This unofficial limit
during the last review was running 4-5 x 104 seconds. However, the committee
tends to scrutinize proposals for large amounts of observing time more closely
than those requesting short times and is currently rejecting proposals for which
they feel scientific justification
is inadequate rather than limiting the time available to that proposal.
VI. Next Year's Consortium Observing Program: The New Red Book
We consider it essential to have a large backlog of observations. In order to
fill the satellite observing time, it is necessary to have many observations to
choose from, so we have prepared our future observing program and are in the
process of entering it into the computer. The assumptions on which these
observations are based are as follows.
Because the satellite observing efficiency is 45% rather than the pre-launch
expected efficiency of 65%, the observations listed in the old Red Book will not
be finished until April 1980. We plan to hold the integrated Guest Observer time
to 20% until April 1980. We then project that the Guest Observer allocation will
increase linearly reaching 50% in April 1981 and remain at 50% for the remainder
of the lifetime of the mission. The Consortium observing program is designed to
fill the rest of the time. Should the Guest Observer demands be greater or less
than this projection, the Consortium program would be adjusted accordingly.
By the end of November, we plan to have the future Consortium observing plan
ready for distribution to potential Guest Observers. We will also send them a
list of all observations, done and planned, Consortium and Guest, ordered by
right ascension so potential targets of interest can be easily located.
VII. Next Meeting
The next meeting of the Users Committee will occur in connection with the
Cambridge HEAD meeting at the end of January, exact time and place to be
communicated later.
FS:kJ
Attendees
P. Charles
G. Clark
W. Forman
R. Giacconi
S. Holt
J. Hutchings
L. Kaluzienski:
K. Long
S. Murray
E. Schreier
F. Seward
H. Tananbaum