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: 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 weekends). 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. APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 275 in the apo35-general archive. You can find APO the archive on http://www.astro.princeton.edu/APO/apo35-general/INDEX.html APO To join/leave the list, send mail to apo35-request@astro.princeton.edu APO To post a message, mail it to apo35-general@astro.princeton.edu APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO