Subject: Wk of Oct 21 mirror washing

From: Ed Turner

Submitted: Sat, 5 Oct 2002 00:43:54 -0400 (EDT)

Message number: 620 (previous: 619, next: 621 up: Index)

Unfortunately a very sudden and unexpected rain shower on the night of
Sept 28 managed to splatter the primary and tertiary mirrors quit
significantly and to significantly degrade the newly aluminized
primary mirror surface.  After considerable investigation and engineering
discussions the decision has been taken to try to clean these two surfaces
as soon as possible, and particularly before the spotting on the surface
is so firmly set that it cannot be removed by washing.  It is hoped that a
prompt cleaning will remove 80-90% of the effects of the rain.

This cleaning, which requires removal of the optics from the telescope,
will be undertaken during the week of Oct 21 unless weather conditions
prevent it (see below).  The nights of the 21st and 22nd are already
scheduled for EN01 so no scheduled science time will be lost on those
nights.  However, the process is expected to require 3 to 5 nights of down
time to complete, including recollimation of the telescope on the sky,
generation of a new pointing model and so forth.  Thus scheduled science
operations on Oct 23-25, inclusive, are also affected with a decreasing
liklihood later in the week.  Of course, if the weather is sufficiently
uncooperative, the on-sky recomissioning work could be pushed back to even
later dates.

The potentially affected programs, in order through the three days, are UW04,
PU08, UC01, PU08, UC01, PU08 for Oct 23-25.  Beyond that CU04 has time on
Oct 26.  The PIs and observers for these programs will need to keep a
close eye on the progress of the cleaning and on-sky recomissioning during
that period to determine whether or not their scheduled time has been
pre-empted by this "emergency" engineering work.

NOTE - The PI of any program that loses time in this manner may apply to
me for compensation from the scheduled EN01 time on Nov 19-20, if that
would be useful for its science goals.

Also NOTE that if it becomes unseasonably cold at APO during that week or
is expected to do so, we may have to cancel the washing altogether; the
cleaning cannot be safely carried out at temps near or below freezing.
Thus, even the programs scheduled for Oct 23 might be uneffected.

Please direct any questions or comments to me.

Ed Turner

PS - The 3.5-meter OSs estimate that adopting a sufficiently conservative
closure policy to ensure that such surprise showers occur much less often
than once per year would cost 3 to 5 nights of observing time per month.
This price seems too steep to pay.  Some possible alternative technical
solutions (distributed rain warning sensors) are being investigated.

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