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New and Improved Mirror Cleaning Procedures (1Mar93)
(from KPNO, NOAO Newsletter No. 33, 1 March 1993)

"It is all done with mirrors" has no greater truth than in a modern
astrophysical observatory. And the cleaner the mirror the better! KPNO
is now cleaning mirrors in an improved fashion with the carbon dioxide
snowflake method which has been found effective at other sites.

High signal throughput and low scattering performance are affected by
dust which accumulates on optical surfaces. A clean aluminum surface
has a reflectivity of roughly 90% at 5000 A; when a mirror begins to
look dirty, the reflectivity is 80% or so. Regular cleaning increases
the lifetime of the optical coating and the surface polish, which
reduces the handling risk involved in re-coating and re-figuring.  In
the CO2 snowflake process, liquid CO2 is forced through a nozzle and
expanded onto the mirror surface. Dust is removed as the snowflakes
slide over the tilted mirror surface on a cushion of CO2. This
procedure works only when used frequently for it is not effective when
the dust is allowed to adhere to the coating. CO2 is also not effective
in removing dust which has become glued to the surface by water
spotting.  Estimates are that CO2 cleaning restores the mirror
reflectivity to roughly 90% of that of a fresh coating.

Our plan is to clean all of the primaries and secondaries with CO2 on a
monthly basis. The process is very fast and not at all messy. Most of
the time goes into setup. (One has to be careful in the winter not to
try to CO2 clean when the humidity is high.) Mirrors will be washed
annually to remove dust which has become stuck to the mirror. We plan
to monitor mirror reflectivity to determine if we should change our
current scheme of re-coating every two years.

Associated with cleaning the mirrors, we are also trying to keep the
domes and telescopes cleaner to stop dirt which accumulates in the
domes from blowing onto the mirrors. We are also cleaning the optical
path above the detector to minimize dust which can fall down on filters
and CCDs. Resources limit how much we can do in this regard, but
awareness of the problem is an important first step in getting the best
performance from our telescopes and instruments.

Bruce Bohannan, Jim Hutchinson,
Jon Settlemyre, John Scott

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