CO2 snow cleaning of optics:: curing the contamination problem

Cleaning large optics with carbon dioxide snowflakes is a process that has achieved general acceptance as a maintenance technique at astronomical observatories over the past decade. The technique is slowly spreading into the general optics community where smaller optics need to be cleaned, but removal and washing of precisely positioned, or deeply embedded, optics is time consuming and impractical. One obstacle preventing the more widespread use of CO2 snow cleaning has been the fact that the more inexpensive lower grades of liquid CO2, the source material used to produce the snowflakes, are often contaminated with oil. The origin offhis oil is believed to be the lubricant used in the compressor that liquefies the CO2 gas. Since liquid CO2 is an excellent solvent for oil, the contaminants eventually turn up in the liquid. Recent development of an inexpensive zeolite based filtering system, allows removal of oil contaminants with nearly 100% efficiency. The data presented in this report show the result of decontamination experiments for a variety of oil components. The snowflakes made from the purified liquid CO2 are capable of producing surfaces that are nearly atomically clean. This level of cleanliness can be important for the successful operation of some ultraviolet and infrared optical systems.