Compton Observatory Science Report #118
Thursday, May 6, 1993 
Eric Chipman, Compton Observatory Science Support Center

Questions or comments can be sent to the Compton SSC.  Phone 301/286-7764,
e-mail SPAN GROSSC::CHIPMAN, Internet chipman@grossc.gsfc.nasa.gov.  

The Compton spacecraft continues operations.  This is a big
week for the spacecraft, as the test and calibration burns of
the propulsion system are taking place.  

There were three maneuvers this week.  One to the engineering burn
attitude on Tuesday, another back to a science attitude on Wednesday,
and a third to the calibration burn attitude on Friday morning (May 7).

The first maneuver was from 1345Z to 1416Z on May 4, covering an angle of 
28.4 degrees, with a final boom angle wrt the equator of 28.4 degrees.  This
maneuver aligned the spacecraft z-axis with the planned thrust direction for 
the test firing of the propulsion system.

There was a second maneuver on Wednesday, May 5 to the attitude of period 
219.4, where the z-axis target is the Galactic Center and the x-axis target 
is MCG-2-58-22.  The times of the maneuver were 1355Z to 1439Z, and the
maneuver slew angle was 141.8 degrees.  The HGA boom angle wrt the
equator after the maneuver was 13.2 degrees.

The third maneuver, on May 7, was from 1320Z to 1348Z, covering an 
angle of 81.8 degrees.  The Boom angle from the equator is 32 degrees.

The test firing on Tuesday was a success, in that the thrusters burned 
correctly for a short time.  However, a miscompare of a gyro direction
sensor beyond the planned tolerance caused the spacecraft to enter a 
safehold mode from which it was brought out a few hours later.  Science 
was begun again as planned after the maneuver of May 5, for about 2 days 
up to the test of May 7.

Today's test sequence went perfectly.  The tolerance on the gyro compare was
increased somewhat, and no miscompares were found during today's 16-minute
sequence of firings.  The orbital altitude was raised by about 5 km, as planned.

BATSE
-----
During viewing period 219 BATSE folded-on-board pulsar data will be collected
for the Crab pulsar, and 31 ms resolution single sweep data will be collected
for pulsar searches. The single sweep data will be collected from LAD's 0 and
6 separately.

A shipment of IBDB's for 69 gamma-ray bursts and 119 solar flares was sent
to the Science Support Center for archiving last week.  (This is expected
to be available in the archive by Friday or Monday).

As of May 5, BATSE has detected 637 cosmic gamma-ray bursts out of a 
total of 2223 on-board triggers in 743 days of operation. There have been 644
triggers due to solar flares with emission above 60 keV.  

COMPTEL
------- 
The COMPTEL experiment was placed in a "safe" mode, with all primary
detector high voltages turned off, prior to the test burns of the
spacecraft propulsion system on Tuesday, 4 May.

The COMPTEL operations group reports that the instrument is operating
normally after switch-on of the detector modules, and continues
observations of scientific targets.  The telescope will be placed in a
similar safe, low-voltage configuration during future planned
maneuvers involving the spacecraft thrusters.

EGRET
-----
EGRET operations were normal this week except for adjustments for
the reboost test.  As noted last week, we are working hard to 
finish the work on Phase 1 and expect a series of papers to be
submitted over the next few months.  Meanwhile, four more preprints 
are being mailed this week.

Work with guest investigators for Phase 2 is increasing with data
being ready for them or work starting with them as they chose between
three and four months after the observation as promised in spite of the
major computer problem in March. 

OSSE
----
OSSE is once again in Science Mode today (6 May) and operating normally.  The
OSSE team would like to thank the GRO Flight Operations Team for their quick
action in response to the spacecraft safehold on Tuesday.  OSSE had been
placed in its Safehold Mode on Monday evening in preparation for the thruster
test, and was not affected by the spacecraft contingency.  OSSE was commanded
to return to Science Mode on Wednesday, about 5 hours after the maneuver to
the target attitude for viewing period 219.4.  OSSE will once again be placed
in Safehold Mode tonight in preparation for Friday's calibration burn.

For viewing period 219.4, the Z-axis target is 1E1740.7-2942 (GI: N. Gehrels)
and the X-axis target is the Sy galaxy MCG-2-58-22 (GI: J. Schachter).  The
Sun is not available on OSSE's scan plane, so the slewing response to BATSE
solar flare triggers is disabled.

Phase 2 Guest Investigator R. Nemiroff and collaborators visited NRL last
week to analyze their data from the unidentified COS-B source 2CG010-31.

Data from viewing period 7.5 will be delivered to the GRO Science Support
Center Archive this week.  The targets for this pointing were the galactic
plane at 25 degrees longitude and the active galaxy NGC 5548.

OSSE team members have completed the technical reviews of Phase 3 proposals,
and the reviews have been delivered to the Science Support Center.