Dust Closure Policy

Effective 5 May, 1998

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.

General:

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

• 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.
• 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:

• use dust monitor in enclosure when telescope is open.
• 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.
• if level inside enclosure exceeds 3000, close telescope. Reopen if outside dust sensor reads less than 3000 for at least 15 minutes.
• if telescope is open, do not use pressurization fan if dust levels are less than 2000 because it can degrade the seeing.

Bruce Gillespie, Site Operations Manager