Subject: grim


Submitted: Thu, 11 Apr 1996 11:57:03 -0600

Message number: 13 (previous: 12, next: 14 up: Index)

Contents: (1) Report on a new technique
for observing a rapidily time-varying event
with GRIM, a stellar occulation  (2) problem
with multiple exposures

(1)  Yesterday morning we attempted to observe
a stellar occultation of a star by Uranus.  The
IR was desireable for this observation since Uranus
is quite dark beyond 2um while the star is quite
bright in K.  High time resolution (< 1 sec) was
required.  Taking repeated images of the star is
not practical because of the long (~5 sec) time
between images.  Scott Severson had experimented
with decreasing the number of flushes between 
exposures, but this did not make a significant
difference in the dead time.

So we developed a telescope control macro that
moved the telescope in a box pattern while each
grim exposure was in process.  The telescope pauses
30 times to form ~30 stellar images in a box spaced
over the grim frame.  Since the time of each telescope move is
written to the hub.log file, a unique time can be associated with
each stellar image.  By requesting 40 exposures from
GRIM, each 28 sec long (but see (2)) the event can be
covered at 1 sec time resolution with some gaps.  The dead time is
5 sec every 28 sec rather than 5 sec every 1.2 sec.
Since occultations are rapidly time varying events, this
gave a much higher chance of catching important
sections of the event.  

This worked well in tests.  Unfortunately clouds
obscured most of the event so we did not get much useful
data.  It would be helpful if the time stamps were written
more accurately than 1 sec.

If anyone is interested in this technique, email me
directly and I can provide more details and some lessons learned.

(2)  We have found that when more than 35 exposures
are requested in the "expose" command box, that 
GRIM often hangs on the 35th exposure.  It is not clear
to me where the problem lies, but it is repeatable.
APO APO APO APO APO  Apache Point Observatory 3.5m  APO APO APO
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