Our estimates of the throughput of the 3.5-m mirrors have been based on measurements using a Microscan scatterometer (SMS, Portland OR). It may be that these measurements include a component of the scattered light, in addition to reflected light. The result is an overestimate of the reflectivity of these optics by perhaps 10% per surface. John Varsik of the National Solar Observatory has made preliminary measurements of the particle size distribution on a mirror surface that has been exposed to contaminates in a manner similar to the 3.5-m primary mirror. He uses a microscope to image the mirror surface. His results suggest that the particles between 30 and 40 microns in diameter are the dominant scattering contaminates. These particles remove light in two ways. As tiny bits of (mostly) transparent silicate crystals, they refract and reflect light over a fairly large angles. By disturbing the wavefront, they defract light over a solid half angle of lambda/diameter or about 1 degree. The Microscan has an aperture of about 1 mm about 30 mm from the surface. Thus, it appears to include light over a solid half-angle of 1 degree or more as reflected. Consequently, it may count diffracted light as reflected (plus a bit of the refracted light). The diffracted light will not enter a spectrograph slit because of its very small solid angle. How big is the effect? If the Microscan measures 80% reflectivity, i.e., 10% loss from the coating and 10% detected scattering, the actual scattering may be 20% and the actual reflectivity 70%. For two surfaces, the reflectivity may be 49% rather than 64%. This is a significant effect. A couple of final comments: (1) Jim Gunn has designed a multispectral reflectometer. We may wish to think a bit more about the reflectance solid angle for this instrument. (2) The particle counter that monitors air quality at APO does not sample enough air to acquire good statistics for particles larger than about 1 microns. If particles between 30 and 40 microns in diameter are the dominant scattering contaminates, we should be counting these particles instead. --Walter Siegmund APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 78 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