The issue of gypsum dust was raised during the last User's Committee Meeting. Recent high winds have carried some of this stuff (which is CaSO4.2H20) up from White Sands and deposited it on various devices including our optics. There was concern expressed that this, plus a bit of water, would produce a potion that might damage the mirror coating. I would like to suggest that, while it might make a mess needing washing off or other cleaning efforts, it is unlikely that any significant chemical reactions with the aluminized surface will occur. Gypsum is relatively INsoluble in water. The concentration of Ca++ or SO4-- in a saturated solution is very very small. The cation won't effectively compete with aluminum for the oxygen in the Al2O3 surface nor will it react with such naked aluminum as i might encounter. Neither will the sulfate displace the oxygen. Aluminum oxide is very inert stuff; that's why costs so much to refine bauxite. In short, we should blow the gypsum dust away, or wash it off as needed but not worry too much about it eating holes in our coatings. Pollen, however, might be another matter. -k- APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 272 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