MINUTES
Einstein Observatory User's Committee Meeting
May 20, 1982
Attendees: Phil Charles (GO), Martin Elvis (CFA), Dan Harris (GO/
CFA), John Hutchings (GO), Steve Kahn (CAL), Tom Markert (MIT), Frank
Marshall (GSFC), Steve Murray (CFA), Bob Novick (CAL), Fred Seward
(CFA/GO), Harvey Tananbaum (CFA). No representative from NASA was
able to attend.
I. Communications to Observatory Users Since Last Meeting
A letter was mailed September 16 describing the schedule for data
reprocessing and listing operating procedures and rules recommended
for the Data Bank by the User's Committee. A questionnaire was
included concerning HRI data. Observers were asked to specify data
for high priority processing and to indicate data for which they were
willing to waive their exclusive six months rights after reprocessing.
The only reasonable requests received for high priority processing
were those connected with optical observations. If the reprocessed
data were available by January, southern hemisphere telescope time
already reserved could be used to search for optical counterparts to
X-ray sources in the LMC. If these data were processed in the normal
time sequence, optical observations would be delayed for one year.
Since the HRI reprocessing did not get started until the first of
January, only half the observations requested were reprocessed in
time for the optical observations. Very few observers waived their
six month exclusive rights to the data, and, because only a small
fraction of data were approved for early release, there is not an
appreciable impact on the Data Bank schedule.
A second letter was mailed March 16 containing a list of HRI
observations and a date of Data Bank availability for each
observation. For those observations which had already been
reprocessed, this was the reprocessing date plus six months. The Data-
Bank date was predicted for the observations not processed. Because
of an error in the computer program, 25% of the HRI observations were
inadvertantly omitted from this list. The error is being corrected,
and the Data Bank schedule for all HRI observations will be
recompiled and mailed this June.
An IPC questionnaire asking for high priority observations and for
observations for which the observer waived the six month delay was
also mailed. We have had virtually no response to this. CFA will
remail this questionnaire later this summer when the schedule for IPC
processing is more definite.
II. Status of HRI Reprocessing
After several false starts, HRI reprocessing was begun approximately
1/1/82, and almost all 1979 observations have been reprocessed. 1978
and 1981 (after the severe gyro problem) have been reprocessed to
generate boresight information. These two groups of observations will
have to be done again after the proper boresights have been
calculated. Reprocessing proceeds at a rate of 10 days of
observations processed in every 7 days of computer time.
Gains in exposure time of 25% or more are found for about 60% of the
observations. The attached Fig. 1 shows the number of reprocessed
observations as a function of the ratio of the new to old exposure
time. On the other hand, approximately 15% of the observations were
found to contain time with the Earth in the field of view. Since the
new processing contains a more accurate ephemeris for removing Earth-
blocked data, exposure times are less than those in the original
image. Signal to noise, of course, has improved.
Positions of X-ray sources have 90% confidence errors of 4.2
arcseconds compared with approximately 10 arcseconds previously. We
have employed the reprocessed data to bootstrap to a new set of
boresight parameters that give a 3.3 arcsecond 90% confidence radius.
These parameters have now been included in the standard system and
will be used for all the 1980 observations. Periods at the beginning
and end of the mission will be processed after the 1980 data, and it
is expected that all the HRI data will have been reprocessed by
August 31.
Other processing activity has included completion of aspect
processing for the SSS. The processing of the North Ecliptic Pole
Survey has been held up by the lack of adequate nap information
supplied by the OCC. The Committee spent Some time discussing this
problem. There were 178 short (a few hundred seconds) IPC
observations done for GSFC of the area around the north galactic
pole. Three days of data are involved. Because a different pointing
technique was used, no aspect solution exists for most of these data.
CFA can now reconstruct where the satellite thought it was going to
point but not where it was pointed. Efforts will be made to find
individuals formerly associated with mission operations who might
help resolve this problem.
Figure 2 is a histogram of observed guide star separations. Two guide
stars were used to lock the telescope on each target. The distance
between the two stars derived from the star tracker data compared
with the calculated separation is an indication of the quality of the
aspect solution. This quantity is less than 4 arcseconds for 75% of
the data and less than 6 arcseconds for 92% of the data. If the
separation is high, it is an indication that the star sensors found
the star fields confusing. The guide star separation observed is
listed at the beginning of the computer output of each reprocessed
HRI sequence. If this number is unacceptably high, more than 4-6
arcseconds, and if the science warrants more effort, the problem can
be resolved by measuring star positions on PSS prints, calculating
the light centroid, entering it in the computer, and reprocessing the
data. It is up to the individual observers to be aware of this
problem and to initiate a second reprocessing.
An example was shown of the output of reprocessed HRI data. The
format has been extensively changed to include useful information,
such as the above
mentioned guide star separations, and the format is calculated to be
more
easily understood by the casual user.
CFA will generate a data format description to accompany Einstein
data given out on magnetic tape. There has been some confusion in
this respect. For example, with the advent of new processing, TGR
files replace TBM files.
III. Status of IPC Reprocessing Plans
CFA is getting ready to completely revise the IPC data processing
software. Plans are to correct for temporal and spatial gain
variation and non-uniform IPC background. Data will be presented in
energy coordinates rather than pulse height coordinates, and a
spectral fit will be done for strong point-like sources within 2' of
the detector axis. This is possible since a detailed gain calibration
exists for this region of the detector.
We have developed a model for the IPC background. This background
suffers from non-uniform illumination of the calibration source,
variable charged particle background (which can be somewhat predicted
using the output of the anticoincidence detector) and a variable soft
diffuse X-ray background due to telescope vignetting. After
subtracting these various background components, a global DETECT
program will search for sources. A local DETECT program will also be
used which will be capable of finding point-like sources embedded in
diffuse emission.
All data will be processed using the new distortion algorithm which
has already been used in processing the 1981 IPC data. This algorithm
is capable of locating sources within the ribs to an accuracy of 20
arcseconds.
CFA is now in the preliminary design phase for the new software, and
detailed design will start in June. The output format for IPC data
will be completely redone along the lines of the new HRI format. We
expect to first attempt IPC reprocessing on September 1. Depending on
problems encountered, the actual IPC reprocessing is estimated to
start between October 1 and January 1. If the IPC reprocessing
proceeds at the same rate as real time data acquisition, it will take
18 months to reprocess all IPC data.
Dan Harris gave a draft of the new IPC format to committee members
for inspection and comment. Comments should be phoned or mailed
directly to Dan Harris.
IV. MPC Reprocessing
Reprocessing of MPC data is necessary. The channel boundaries have
been changed to produce good fit for calibration sources. Because of
this, the old spectral fits are probably incorrect for sources with
strengths >1 UFU. The background algorithm has also been revised to
take into account a time dependent calibration source and the natural
background. Because of the variable calibration source in the
background, MOP by MOP processing is necessary.
The level of effort on MPC reprocessing depends on the level of NASA
funding to CFA. If HEAO cuts occur as expected, all MPC data cannot
be reprocessed as a matter of course. We plan to reprocess data on a
case by case basis as requested. The two people working on the MPC
programs now are a graduate student and a CFA post-doc, neither of
whom is fully funded
by NASA.
V. NASA GO Support and "Dear Colleague" Letter
(communicated by L. Kaluzienski by phone to F. Seward)
The "Dear Colleague" letter has been prepared in draft form.
Realistically, the author will require one month to complete the
letter and an additional two months will be required for internal
NASA circulation. Thus, the earliest the letter could go out is
within three months.
The long-term NASA funding plan is that 400K per year is available
for outright guest observer support for all three HEAO missions.
250-300K of this would be available for Einstein. Since this money is
uncertain, NASA is reluctant to announce funds in the "Dear
Colleague" letter.
Since the end of the mission, approximately ten proposals have been
received and approved for collaborative data analysis. The number of
proposals expected for use of the Data Bank is unknown. If enough
proposals are received, the proposal review committee will be
reconvened, and proposals will be reviewed as the Einstein Guest
Observer proposals were.
The Committee thought that the NASA "Dear Colleague" letter should
contain a description of technical improvements in the reprocessed
Einstein data. These are very important for some of the observations,
and potential Data Bank users should know of them.
There was some discussion concerning the time of issue of the "Dear
Colleague" letter. If this were done as soon as possible, the letter
could be mailed September 1. Since time is required to write
proposals, proposals would not be received much before January 1,
1983 and a proposal review could be initiated if necessary for, say,
March 1. At this time, all HRI data will be in the Data Bank, and 2-4
months of IPC data will have been reprocessed. Potential Data Bank
users will have a schedule (distributed by CFA) listing expected
dates of availability of all the IPC data at this time, but this will
not be available until the IPC data reprocessing is actually started.
Thus, proposals can reasonably be written concerning the IPC data,
but a March 1 first review is early for potential IPC users.
VI. Review of Rules for Data Bank Use
These were discussed at the last meeting of the Users' Committee,
August 10, 1981, and distributed to the Einstein observers. The
Committee had no changes or modifications to these rules and
considered them still satisfactory. The Committee thought it would
also be desirable if the Data Bank could be queried without formal
proposal. Thus, a simple request involving neither funding nor
support from CFA personnel could be handled with no bureaucratic
restrictions. Records will be kept of Data Bank users, but there
should be no delays for simple requests: e.g., a telephone call
asking whether a particular object is an X-ray source or not.
VII. Future Communication with Users
CFA will distribute (1) a corrected Data Bank schedule for HRI
observations on or about June 15, (2) a Data Bank schedule for IPC
observations on or about January 1983, and (3) a new Users' Manual
soon after the new IPC data format is firm, again on or about January
1983.
VIII. Future of Users' Committee
The Committee thought that changes of membership, unless wanted by
the individuals concerned, would be undesirable. The next meeting
will be scheduled coincident with the January 1983 AAS meeting in
Boston.
FDS/jkf