Most of the work on particles and optics contamination was done by John Varsik while he was at NSO, Sacramento Peak. (He is now at Big Bear.) He found that the particles between 30 and 40 microns in diameter are the dominant scattering contaminates on dirty mirrors. See the URL at the bottom for the details. The number density of airborne particles falls very roughly as d^-4 where d is the particle diameter. Consequently, the density of 1 micron particles is a million times larger than 30 micron particles and it is correspondingly much easier to measure the density of 1 micron particles than 30 micron particles. John Varsik has looked for commercial instrumentation to measure 30 micron particle density without success. It would be possible to design and build such an instrument, but my group does not have funding or resources to pursue this at present. In the meantime, we must rely on surrogate measures of 30 micron particle density, e.g., 1 micron counts or visibility of a flashlight beam. Neither is particularly satisfactory since both sense particle smaller than the dominant size for contamination and the power law exponent is quite variable. Flashlight beam visibility is most sensitive to ~3 micron particles, depends on the observer, flashlight and battery charge, is qualitative and requires active participation by the observer. To some extent, the limitations of these techniques are complimentary. I believe that both should be used until something better is available. Walter Siegmund http://www.sunspot.noao.edu/~varsik/dust/absindex.html APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 183 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