Minutes
Einstein Observatory Users Committee Meeting
June 19, 1979
I. Status of Observatory
There have been no radical changes. Another attempt was made to reactivate star
tracker B, which showed no sign of life, reinforcing the conclusion that damage
to this tracker was permanent. The other trackers are operating well and showing
no signs of problems. CFA is continuing studies of one-tracker and no-tracker
options (navigating using only X-ray sources). The star tracker shutters are a
potential area of concern. Should there be a failure of one of these, the
photomultiplier tube has a built-in current limit which should handle overloads
(caused by looking at the bright earth) for a while. However, we would expect
fatigue to limit the lifetime in this mode of operation.
A. IPC
The gas leak rate, after stabilizing for a while, is still decreasing slowly
(more and more material is plugging the calibrated leak). We now operate at high
voltage step 4 during an observation. Should the gain continue to increase, we
may have to go to high voltage step 3. We cannot continue to change high voltage
steps indefinitely. Although the high voltage on the X-ray counter is decreased,
the high voltage on the anticoincident circuit is not. If the gas gain continues
to increase, the anticoincidence wires will eventually break down and this
counter will no longer be useful. The IPC is now kept at the focus for
approximately a week in order to get maximum time with constant gain.
The efficiency of the IPC has been recalculated., and theoretical efficiency and
the Huntsville calibration now agree to approximately 10% except for the
calibration point at 3.5 keV, which is still off by about 25%.
B. HRI
No problems; operation has been nominal. As expected, the gain of HRI #3 is
gradually decreasing. CFA will change the high voltage to compensate for this in
approximately two months. The only possible problem this presents is to projects
concerned with monitoring the long-term variability of sources.
C. OGS
Four more sources were observed; Cyg X-1 and Cyg X-2, MK 421, and SS Cygni.
SS Cyg was quiescent and too weak to obtain usable data. Quick look data for Cyg
X-1 and 2 showed a low energy cut off as expected and no obvious lines or
structure.
D. FPCS
The hardware is operating well. There have been no additional problems. A
calibration has been accomplished using the Crab Nebula.
E. SSS
There have been no additional problems with operation of this detector. The
lifetime prediction is still approximately 10 months. The temperature is expected
to rise at the end of September (a highly uncertain prediction), and operations
with this detector will cease.
II. Data Processing
The new tape drives and disks have been installed and are running well. The star
sensor map-mode software is now being tested. New IPC software is still not
finished. otherwise, quick look and production data are now being processed in
better than real time. The available FPCS and HRI data are essentially all
reduced. The IPC images have been merged in production, and an interim system for
finding sources and strengths is being used. IPC spectral processing is now done
off line in special cases, but is not performed in general.
Four CFA groups are developing the IPC software. (a) One group is studying area
and vignetting as a function of off-axis position. (b) A second group is
attempting to understand the IPC gain and resolution function. (c) A group is
working on the mapping of detector response as a function of position. Electronic
distortions and "striping" appear in data from long observations. These effects
must be known in order to detect faint sources. (d) A group is studying the
detector background which depends on position in the counter, position in the
orbit, ,time, and energy. They are developing a model to handle the rib-shadowed
regions, vignetting, diffuse X-ray component, etc. It is now difficult to use
this detector to study a weak diffuse source that fills up the field of view. It
is necessary to understand the background first.
CFA is planning an accounting system which will keep track of what observations
have been done, length of observation, sources seen, strength of sources,etc.
III. Momentum Management and Mission Lifetime
gas usage dependence on target position and scheduling. During passive
management, the number of pounds of gas used per orbit as a function of orbital
latitude showed the expected sin (28) behavior. In the worst case,if we were to
only look at targets having orbital latitudes between 30 deg and 60 deg, the
remaining lifetime would be approximately 1.2 years. In the best case, observing
only targets at orbital latitudes less than 10 deg
or greater than 80 deg (targets in the orbital plane or perpendicular to the
orbital plane), remaining lifetime would be 4 years.
Active momentum management (pairing) was studied orbit by orbit. Using this
technique, pairs of targets are selected to balance gravity gradient torques. The
two targets must have the same orbital latitude but with a 180 deg difference in
orbital longitude. Results show that pairing must be done well or no gas is
saved. The separation in orbital latitude and the difference in orbital longitude
must be no greater than 10 deg If pairs meeting this criterion can be selected,
the remaining lifetime will
be approximately 4 years.
The Goddard Control Center software cannot now adequately implement active
momentum management. We are, however, selecting pairs of targets by hand and will
continue in this fashion until the software can be changed to find proper pairs
automatically.
Observation scheduling is now governed by the following policy:
1. Long observations determine when a given instrument is at the focus. For
example, to do a deep survey with the HRI, we first determine when the orbit
plane is in the most favorable location. The HRI is then scheduled
to be at the focus during this time.
2. Passive momentum management is always acceptable. In the example above,
passive momentum management is being used.
3. Short observations, if not at good passive latitudes, will be paired.
4. If the target is important, and if there are no targets to pair it with, a
position will be selected and used as a dummy pair.
The orbit is now decaying because of high solar activity. Should the sun stay
active, re-entry might occur as early as mid 1981 giving two years of remaining
life. This is a topic of great concern. We are trying to get a better estimate of
mission lifetime
IV Time Allocation
The attached graph shows observation time allotted to different organizations on
a month to month basis. CFA and GSFC are over their allocations. The Guest
Observers, CAL, and MIT are under their allocation. The large fluctuations
reflect times when the IPC is in or out of the telescope focUS. Guest Observers
and CAL use predominantly the IPC. During the month of May, a lot of time was
devoted to an HRI deep survey which shows as a maximum in CFA usage and a minimum
in that of other organizations. The time averaged numbers are:
Percentage Allotted Excess
Organization Time Used Percentage Time Percentage Time
CFA 44 36 + 8
MIT 12 16 - 4
CAL 7 12 - 5
GSFC 26 16 +10
GO 11 20 - 9
The excess in GSFC time reflects the steady use of the SSS. There is general
agreement that, in view of the limited lifetime of this instrument and the high
scientific returns, we should continue to operate in this manner.
We will attempt to decrease CFA time and increase GO and CAL time to achieve
better balance. Starting immediately, a higher percentage of observing time will
be given to the IPC. Guest Observations will be given first priority; second
priority will be given to CAL observations; third priority to CFA, MIT and GSFC.
It is important to remember that only targets favorable for momentum management
will be observed. Thus, at any given time, we cannot observe all possible Guest
Observer targets.
V. Status of Guest Observer Program
At the May 31 NASA review, 43 proposals were reviewed and 26 accepted for
incorporation in the Einstein observing program, giving a total of approximately
160 Guest Observations now scheduled or in progress. The table below summarizes
the number of proposals considered.
Review Total Proposals Accepted Rejected
------------ --------------- --------- ---------
August 1978 68 43 25
November 1978 60 47 13
February 1979 63 46 17
May 1979 43 26 17
--- --- --
234 162 72
VI. New Red Book
Using the current observing efficiency of 46%, we estimate that observations in
the consortium Red Book can be accomplished by the end of March 1980. We are
preparing a second Red Book, which will carry the consortium plan on for another
six months. Our current schedule anticipates having all consortium observations
collected by August 15 and resolving conflicts and completing the new Red Book by
September 15. Guest Observer time will be held at 20% average until January 1980
and then gradually increased. Guest Observer time could be as high as 50% at the
end of 1980. Should the Guest Observer Program be limited by NASA funding, the
consortium will prepare observations that will utilize this time.
FS:jk
Distribution: (* attendees)
G. Branduardi
C. Canizares*
P. Charles*
R. Giacconi *
S. Holt
J. Hutchings *
K. Long*
A. Opp
E. Schreier*
F. Seward*
H. Tananbaum *