Subject: APO 3.5m User's Committee meeting, 02/03/03

From: strauss@astro.Princeton.EDU

Submitted: Tue, 4 Feb 2003 10:06:00 -0500 (EST)

Message number: 655 (previous: 654, next: 656 up: Index)

  Apache Point Observatory 3.5m User's Committee Meeting
		February 3, 2003

Attending: Bruce Gillespie, Ed Turner, Michael Strauss, Alan Uomoto,
Jon Holtzman, Al Harper, Bruce Balick, Russett McMillan, Chris Stubbs,
Don York

Absent: Rene Walterbos, Mike Shull

****************Image Quality and Telescope/Mirror Motion***********

Russett McMillan recently <a>posted</a>
a summary of what is known about problems with the telescope
collimation, focus, and jitter that affect image quality.  There is
evidence from Shack-Hartman that mirrors are moving, and degrading the
image quality in the best seeing.  These motions are of two forms:
both jitter (many Hz), smearing images, and jumps (perhaps associated
with the telescope cooling) that take the telescope out of
collimation, on timescales of hours to days.  The Primary Mirror
Support System motions do not correlate with seeing, and are only weakly
correlated with wind.  

  Much of the discussion at this meeting was on the issue of telescope
focus.  With DIS, one can update the focus using guider images during
the exposures; for other instruments, Russett feels we spend a
significant amount of time out of focus.  With DIS, Russett does
indeed chase the focus quite aggressively on timescales of between
five and fifteen minutes; she has built up an intuition of how to
tweak the focus.  Note that the telescope doesn't stay exactly
boresighted during focus changes, which means that one can get image
jumps if one were to change focus during an exposure with, say,

  Al Harper reported having taken GRIM images with doubled stars
repeatedly recently; it doesn't sound like it is due to this focus
issue.  He and Russett will discuss this in more detail offline. 

  Russett bases her intuition of how to change the focus based on such
variables as the difference between air and telescope temperature, the
telescope elevation, and the season, but this is not yet in the form
of an objective algorithm.  She has attempted to fit available focus
data for trends in these various variables, thus far without success.
If this were to be incorporated into an automatic algorithm that the
TCC could use to update the focus, the air temperature would be
needed, a quantity the TCC doesn't currently have access to.  It would
be real work on Russell Owen's part to incorporate this. 

  Note that the TCC does currently have a zeroth order focus
adjustment as a function of temperature, which is unambiguously better
than nothing.  The TCC adjusts the focus only when slew commands are
issued, or when the OS issues a command for it to do so (i.e., never
during an exposure).  Bruce Gillespie reported that the WIYN folks
have tried hard to implement a higher-order open-loop focus
adjustment, without success; it may be that the only practical way we
could improve on the focus would be with a closed-loop approach.

 John Holtzman suggested that it might be worthwhile, with this in
mind, to give the observers more information so that they can adjust
focus as they see fit.  In particular, perhaps the observers should
have access to the guider images.  Given the nature of remote
observing, however, and the fact that it has taken Russett years of
experience to develop her intuition for focus updates, it is not
obvious that the observers will do a much better job.  

  Chris Stubbs suggested that the astigmatism corrector in the guider
could be removed; the resulting astigmatism in out-of-focus images
would tell one the sense in which the focus should be corrected.  He
will pursue this idea further.  Russett pointed out that there is a
fair amount of astigmatism already in guider images; the astigmatism
corrector is not doing a perfect job. 

***********Status of DIS: 
  The new blue optics have been put in (this improves the vignetting
seen earlier, and makes a better match of plate scale to the bigger
detector).  Throughput measurements have been made, which show 
confusing results.  There is some sense in which these weren't done
quite right, and they will be repeated in the next few days.  

  The current plan is to install the new red optics during the April
bright time, due to scheduling limitations, the confusion on the
throughput measurements, and some disagreement between Jeff Morgan and
the manufacturer on the focal length of the optics as delivered.

**The New Cloud Camera
  The new cloud camera has been in for some time, but the
documentation for this is still rather minimal.  This effort is being
led by the SDSS folks, but we need to get a little more information to
allow users to interpret the information.

**Telescope Offsets
In light of recent confusion over these issues, Russett has posted a <a
on how offsets work on the telescope.  

**Forest Fire Preparations
  In light of the recent fires that destroyed Mt. Stromlo observatory,
we discussed the very real fire danger that APO faces.  Bruce
Gillespie reported on plans to thin about 8 acres down the slope
(up-wind) of the observatory.  Dead trees and flammable ground cover
will be removed and/or chipped up; this work will start this
week. We hope that this would substantially slow any fire coming up the
ridge, giving people extra time to evacuate.   

  For the first time in recent memory, there were several corrections
sent to Michael Strauss to last month's minutes:

  George Wallerstein wanted to clarify that it was one of his research
grants, not him personally, that would contribute $30K for upgrades to
the echelle. 

  Jon Morse pointed out inaccuracies in the description of the request
for CIF funds for NIC-FPS.  Here is a corrected version:
  The CIF funds would be used to get one or two z filters (one which maximizes
throughput, one which mimics the Sloan z bandpass as closely as possible),
spare parts for the mechanisms and electronics that would be needed in the
years after NIC-FPS has been at APO, plus a final payment on the detector.

  Next meeting on Monday, March 10 at 11:30 AM East Coast Time. 


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