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. APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 341 in the apo35-general archive. You can find APO the archive on http://www.astro.princeton.edu/APO/apo35-general/INDEX.html APO To join/leave the list, send mail to apo35-request@astro.princeton.edu APO To post a message, mail it to apo35-general@astro.princeton.edu APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO