Subject: Dust closure guidelines

From: Bruce Gillespie

Submitted: Tue, 5 May 1998 17:54:08 -0600

Message number: 275 (previous: 274, next: 276 up: Index)

Effective immediately, the following telescope dust closure rules and
guidelines will be followed by all users of APO telescopes, both by
operations staff and visitors.  I expect these rules to be tweaked based on
future experience and differing science requirements (particularly for
SDSS).  At the present time this affords a relatively unambiguous
prescription that should improve the amount of on-sky time while at the
same time limiting dust exposure to a tolerable level.


	o always use flashlight tests and/or other visible indicators to
	  corroborate high dust counts, but base operating decisions on
	  measured dust counts as prescribed below.

	o notify Mark Klaene if telescope is exposed to a dust "event" so that
	  optics can be cleaned if necessary the next day (including

	o when dewpoint depression is equal to or less than 5 degrees C, be
	  extra vigilant in monitoring dust levels--dust will fuse to the
	  optics if condensation forms.

	o be aware of trends and the derivative of the dust counts.  There is
	  often only a short warning period between acceptable dust counts and
	  a major duststorm.  With more experience, we may be able to devise
	  appropriate automated alarms that are sensitive to rates of change.

SDSS 2.5-m and Monitor Telescopes:

	o using tower dust monitor, close telescopes when the > 1 micron dust
	  counts exceed 3000.  Do not reopen until the counts are below 3000
	  for at least 15 minutes.

	o when tower readings are between 2000 and 3000, monitor dust levels
	  at least every 15 minutes to avoid dust event "spikes."

3.5-m Telescope:

	o use dust monitor in enclosure when telescope is open.

	o when > 1 micron dust level is in the 2000 to 3000 range, operate
	  enclosure pressurization fan and close louvers while telescope is
	  open, checking enclosure dust level every 15 minutes.

	o if level inside enclosure exceeds 3000, close telescope.  Reopen if
	  outside dust sensor reads less than 3000 for at least 15 minutes.

	o if telescope is open, do not use pressurization fan if dust levels
	  are less than 2000 because it can degrade the seeing.

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