Subject: GRIM focus -- potential problem

From: Russet McMillan

Submitted: Tue, 9 Mar 1999 16:44:00 -0700 (MST)

Message number: 341 (previous: 340, next: 342 up: Index)

There have been some oddities lately with the focus on GRIM changing
widely (>400 steps) from night to night, and also changing on short
timescales with only slight changes in temperature.  The cause for
this has not been pinned down, and extensive on-sky tests won't be
possible until the next engineering time is scheduled, near the end of
this month.  It's still possible that the variations seen so far were
due to temperature change or windshake, although the observed
temperature variations seemed too small to cause such a radical focus
change.  Other instruments have been more consistent in focus, so this
may be a problem limited to GRIM.  The difficulties observed so far
were seen at f/5, where the depth of field does tend to make the
instrument focus more sensitive.

Until proper tests can be done, users are encouraged to take notice
and report large changes in focus at any focal ratio, and allow time
for extra focus runs if they should be necessary.  The guider is the
obvious instrument to use as a check against GRIM, so it would be good
to let the observing specialist set up and focus the guider, even if
no guiding is planned for the evening.  If the GRIM focus changes but
the guider focus does not, that could help us narrow down the problem
to the instrument.

Non-GRIM users should also watch out for focus variability in other
instruments, in case the problem is not instrument-specific.

				    Russet McMillan.


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