Subject: GRIM exposure times

From: James Rhoads

Submitted: Sun, 26 Nov 1995 23:52:29 -0500

Message number: 3 (previous: 2, next: 4 up: Index)

	This might be of general interest to GRIM II users:
During the November shutdown, the known GRIM II exposure times bug (described
in http://astro.uchicago.edu/home/web/bjr/GRIM2/Bugs/expTim.html) appears
to have taken a somewhat new form.  The old form was that actual
exposure times were 1.09 sec longer than the requested exposure time.
The time recorded in the header was the requested one, not the actual
one.
	Now, the exposure times sent to the MC with "nexpose it=x" are
modified to x - 1.09 sec.  (So for example, a requested exposure time
of 5 sec gives a reported  OPENTIME= 3.909000  in the image header.)
This brings the actual exposure time in line with the requested one to
pretty good accuracy (my crude check of counts/sec from dome flats
suggests true times may be OPENTIME + 1.0 rather than OPENTIME + 1.1,
but that is chi-by-eye and should not be taken too seriously).
	The drawback is that a requested exposure time below about 1.2
seconds seems to hang the instrument, requiring a "stop inst" command to
be issued to wake it up again.
	I have only checked this for the Sun mcnode interface, but I
suspect it is true for the Mac Remark interface also, since Remark sends
stuff to the MC.
						James Rhoads.
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