Subject: Mirror washing canceled

From: Ed Turner

Submitted: Mon, 21 Oct 2002 10:49:31 -0400 (EDT)

Message number: 626 (previous: 625, next: 627 up: Index)

The plan to wash the 3.5m primary mirror this week, described in the
message quoted below, has been canceled due to low temperatures in the
dome.  It would be risky and probably ineffective to attempt washing under
these conditions.

This means that the scheduled science programs for Oct 23-25 (UW04, PU08,
UC01, PU08, UC01, PU08) will be carried out as scheduled.

Ed Turner

On Sat, 5 Oct 2002, Ed Turner wrote:

> 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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