COMPARISON OF LASER AND CO2 SNOW FOR CLEANING LARGE ASTRONOMICAL MIRRORS

Contaminants on large astronomical reflecting surfaces (hereafter ''mi rrors'') can significantly degrade their reflectivity, IR emissivity, and Light-scattering properties. We will show data that the emissivity and scattering can degrade appreciably after just a few days or weeks . A safe, effective, and inexpensive cleaning technique, preferably on e that can be used in situ, is especially important for 4- to 8-m clas s mirrors. Two cleaning methods CO2 snow and pulsed ultraviolet (UV) l asers, offer the potential to satisfy these needs. Our primary purpose is to compare the two methods, and highlight their advantages and pro blems. Also, since the UV-laser cleaning technique is a new one, we de scribe how it works and how it may be implemented. We found that UV-la ser cleaning removes contaminants that standard CO2-snow-cleaning tech niques do not remove as well or not at all. After 2-4 weeks of exposur e, Al-coated-mirror samples placed under the mirror covers of the UKIR T telescope on Mauna Kea and the 4-m telescope on Kitt Peak were clean ed about twice as effectively by the UV laser than the CO2 snow, based upon the amount of contamination left on the mirrors. For long exposu re times, the laser cleaning also restores the thermal emissivity bett er than the CO2 snow. While a CO2-snow delivery system can be less exp ensive than a UV-laser-cleaning system, the operation costs for cleani ng a large-diameter mirror with CO2 snow can be substantially greater than the costs to run the laser. Unlike CO2 snow, laser cleaning can b e applied no matter what the local humidity. The ease of both methods facilitates frequent use; however, the cost of the CO2 may eventually limit its usage.