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From: DOXSEY@avion.stsci.edu
To: jay@stsci.edu, mcclure@stsci.edu, mackenty@stsci.edu, miller@stsci.edu,
NORMAN@stsci.edu
Subject: GHRS safing last night
Date: Thu, 06 Feb 1997 15:27:39 -0500 (EST)



GHRS safing

The GHRS safed last night, at 11:59 PM local time. The
safing was a result of a check on currents and voltages which is
performed by the NSSC-I. The current draw on the main 28 volt
input line to the GHRS was in excess of the check limit on
3 successive 1 minute samples, which led the NSSC-I to safe the
GHRS. At the time the HST was in a target occultation period
and the GHRS was idling between segments of a side 2 visit.

Review of the engineering data at the time indicates that there
was about a 2-3 ampere increase in the current into the GHRS. There
is a dc-dc convertor in GHRS LVPS 2 which feeds power to many different
items, the carrousel, the amplifiers in the detector electronics box,
the shutter, etc. A number of the voltages sagged a bit during this
period. The likely cause is a short circuit, either in the power
supply itself or in one of the outboard boxes that it powers.
The engineers are doing a second look at the details of the power
supply design and the actual voltage response to see whether
the problem can be isolated to a specific output line, or is inside
the LVPS and generic to it. In the former event, it may be
possible to reconfigure GHRS and operate without the offending
box. In the latter case, it will be impossible to operate
side 2. You may remember, we have had previous problems with
the carrousel operation from side 1 and the side 1 standby
power supply. If the side 2 LVPS itself has failed, then we may
not be able to use GHRS again.

We are currently in a two day period using other SIs.
There will be a meeting tomorrow morning to hear the results
of the engineering analysis. We will also discuss whether
or not to try to turn the side 2 LVPS on for a few seconds
to see if it is ok (The safing event turned off all the
outboard boxes, so turning on just the LVPS could help
isolate the problem). If this test is carried out and
works, then we may bring the GHRS up just to run two
Side 1 observations (side 2 LVPS is needed to run side 1
observations), a Mars observation and the calibration of
the GHRS with COSTAR mirrors removed.

Given the upcoming servicing mission, one might ask
"why bother" with the testing. It will be important to know
whether the GHRS is still a viable spectrograph, in the event
that STIS does not perform as expected during the SM-2
functional test. The GHRS has been viewed as a backup
in the event of problems with STIS. Caution is important,
though, because we want to avoid blowing the fuses on
the power lines from the HST to the GHRS, as these same
fuses and lines will be used by STIS. The fuses are
replaceable in orbit by the astronauts, but we dont want
to change them if we can avoid it, it is just another
thing that could go wrong.