Minutes
Einstein Observatory Users Committee Meeting
May 22, 1980
Attendees: Canizares, Charles, Giacconi, Harnden, Harris, Hutchings,
Kaluzienski, Long, Marshall, Schreier, Seward
I. Spacecraft Status
Everything is working well. The only cause for worry is the gyros.
The spacecraft has six gyros and needs three to operate. One failed
soon after launch and a second in January. There are now four
operable gyros, but the situation is not as good as having one extra.
The noise depends upon which three gyros are operating and for some
combinations, the total noise level is unacceptable. There are now
three gyros operating satisfactorily. If there is another gross
failure, the fourth can be used as a replacement. However, there may
be difficulty due to the noise level of the system.
The X-ray instruments and the two star trackers are fine. The second
control IPC leak is operating well and clogging very slowly. The IPC
gain is effectively constant for several days. If the clogging
continues at the present rate, the leak will have deteriorated to the
status of the first leak in mid-1982.
II. Data Processing
A. General
Processing is currently 80 days behind the date of the observation--
about 20 days later than usual. Part of the problem is that NASA is
behind schedule in delivery of tapes to CFA. Also, CFA has devoted
computer time to star tracker calibration, IPC calibration, and
special data processing. We can stay current with data processing and
still devote two days a week to special processing. Demands have been
greater than this recently.
To catch up, CFA is trying to use the data analysis computer to do
production data during off hours. Some hardware is needed to do this,
and a proposal has been sent to MSFC to procure the needed items. CFA
has started to microfiche results of routine data processing. This is
expected to keep the paper output within the bounds of the one room
used to store Einstein data.
B. Aspect
Data are being processed with a new aspect routine incorporating
magnetic corrections in the star trackers. Production data, starting
with data taken in orbit after February 5, has 1-2 arc sec smearing
(compared to 8 arcsec previously), and the boresighting is correct to
~ 3 seconds. We have yet to incorporate star tracker C calibration,
redo the magnetic corrections, and then put in new boresighting. We
anticipate this will be complete by the end of June.
C. IPC
The IPC processing software is being overhauled. Eventually, data
will be processed with a better background estimation, more accurate
source locations, and with the ability to derive spectra.
Two days in orbit were devoted to mapping the IPC response to a
strong source in the central 4' x 4'. Observations were spaced at 25
arcsec intervals, and gain variations observed in this central region
were ±l channel. The preflight calibrations indicate gain variations
over the entire IPC of ±2 channels for point sources. The target date
for incorporating gain data derived from the new IPC calibration into
the automatic processing is September 1.
As you know, there have been significant difficulties with performing
spectral fits to IPC data. All previous formulations and programs
suffer from one or more of the effects which are listed in Table 1.
Most of these difficulties have been reduced as a result of a year-
long study of these problems; and "off-line" programs incorporating
some of the benefits of this increased understanding are scheduled
for completion on September 1, 1980.
III. Lifetime and Target Restrictions
Nominal reentry (200 km altitude) is now calculated to occur June
1982, worst case, September 1981. There is a reasonable possibility
then of another two years life for the Observatory.
In 18 months, we have used 1/2 the gas on board at launch. If we
maintain the current rate of gas usage, 0.23 lbs/day, the gas will
run out in February 1982. We must decrease the usage to 0.18 lbs/day
to last until June 1982. It presently appears that there is no
reasonable way to get below 0.12 1bs/day, and the only way to achieve
0.18 lbs/day is to restrict targets to those in low-gas-use situations.
Software is now available at CFAto choose possible target pairs for
active momentum management. However, as has been established in the
past, both pairing and passive management are highly dependent on the
angle between the orbit pole and the sun (beta-angle). Thus, there
are significant periods of time when no pairing is possible for a
given target. It is not yet apparent how far in advance the beta-
angle can be reliably predicted.
CFA will be implementing a new policy severely limiting observations
which are wasteful of gas, as follows:
1. All proposed new targets which have -20° < delta < 20°, or |delta|
> 60° will be treated as now. Active and passive scheduling
constraints will be rigidly adhered to.
2. All proposed new targets which have 20° < |delta| < 60° will be
conditionally accepted. They will not be scheduled unless they meet
one or more of the following conditions:
a. they can be scheduled passively after giving priority to already
approved observations;
b. they can be paired satisfactorily with other targets, after giving
priority to already approved observations;
c. they can be paired with a "dummy" target at the ideal pairing
position.
The time charged to the observation will then be doubled. Prospective
guest observations to be entered into this category must be judged
worthy of this additional time by the NASA review committee.
3. Specific entries - targets requiring very definite time slots,
such as coordinated observations - will, in general, not be approved,
since they prevent pairing. Such observations deemed extremely
important may be conditionally approved pending ideal passive
scheduling conditions;
4. All long duration targets in the 20° < |delta| < 60° category
existing in the observing catalog are currently being reviewed to
determine if acceptable scheduling windows are available within the
next year. Non-acceptable targets will be reviewed on a case-by-case
basis and discussed with the Users' Committee.
The Users' Committee recommends that the NASA Proposal Review
Committee should consider gas usage when reviewing future proposals.
IV. Consortium Additions to the Observing Catalog
A few new targets have been substituted by the Consortium in the OCA.
A list of these changes to the OCA was distributed to members of the
Users' Committee for comments. If there are no objections, they will
remain in the OCA as Consortium observations. The Consortium
observations currently fill up the observing time until April 1981.
In the future, it was decided that rather than issue a new Red Book,
Consortium observations will be added in the same way as guest
observations, but probably in six-month rather than three-month
increments. Interested parties will be kept up to date on the
Einstein observing plan by distributing the observing catalog
probably at three-month intervals.
V. Guest Observer Survey
Seventy-five replies were received to the questionnaire distributed
to Guest Observers. The overall response to the Guest Observer
Program was favorable. Unfavorable comments and constructive
criticism were tabulated. Most unhappiness was due to the existence
of the Red Book; inadequate information concerning: scheduling, data
reduction routines, guidelines for proposal writing; and delays in
the NASA funding process.
To improve the situation, CFA will: (1) update the Users' Manual and
distribute it widely, (2) take steps to inform all Guest Observers
about the present status of their observations, (3) distribute
information concerning new target restrictions, (4) distribute the
schedule for new aspect processing, (5) distribute IPC calibration
data when it is available, (6) improve the hard copy format of the
IPC data to be more understandable.
VI. Guest Observer Program
Ninety-five proposals have been received for the June review. During
the month of April, 36% of the useful observing time was devoted to
Guest Observations.
Three hundred and thirty Guest investigations have now been approved.
The total observing time is 7 million seconds and there are 1700
targets. Although many proposals are received from some groups, the
number of individual Guest Observers is approximately the same as the
number of Guest investigations, since most proposals involve several
investigators.
The observing time devoted to Guest observations is following the
time approved by the NASA Review Committee. The attached graph
compares the actual usage with that anticipated when the second year
Consortium program was compiled. During the first quarter of 1980,
26% of Einstein time was given to Guest observations. The average of
the last three reviews commits 38% of the time to Guest Observers.
The grand average over the first 15 months is 18%.
Frederick D. Seward
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TABLE 1
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Difficulties with IPC Spectral Data
(Listed in decreasing order of importance)
Effect
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1. Variation of detector gain with time.
2. Variation of detector gain with (Y, Z) position.
3. Uncertainty in the energy values of the pulse height channels.
4. Uncertainty in the spectral response (resolution) function.
5. Uncertainty in the efficiency of the telescope/detector.
Problem
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1. Real-time gain measurements (ALP plots) are not accurate enough
for spectral fits.
2. Even for "on axis" sources, gain varies significantly.
3. Boundaries are accurate at 1.5 keV, but lowest and
highest,channels are uncertain due to effect #2.
4. Width of the response function had been over (under) estimated for
point (extended) sources due to effect #2.
5. Theoretical effective area did not match measured values at all
energies.
Solution
--------
1. Production processing will compute BAL, to be used instead of ALP.
2. In-orbit calibration has mapped detector gain over central
4' x 4' region for use with point sources; existing data will handle
source of extent > 2' throughout field.
3. Lab data and reinterpreted flight cal. source data have led to an
improved channel mapping function.
4. Reinterpretation of calibration data has provided more accurate
function.
5. Corrections < ± 20%) have been applied where needed.