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When Dew Gets in Your Eyes (1Sep92)
(from KPNO, NOAO Newsletter No. 31, 1 September 1992)

Historically, Kitt Peak has used 90% humidity as an upper limit for
telescope operation. There are, however, two difficulties with this
criterion: (1) we really do not have a reliable way to measure
humidity, particularly when it is below freezing, and (2) humidity is
not what we want to know in order to keep water from condensing on the
optics or on anything else that might drip onto the optics. The
quantity to measure is the dewpoint.

We are now experimenting with a dewpoint hygro-thermometer which we
have installed near the 2.1-m telescope.  (A second one will soon be
located on the catwalk level of the 4-m). This device measures the
dewpoint to an accuracy of 0.5 degrees RMS by chilling a mirror to the
point where water begins to condense, just what we want to prevent
happening to our telescope optics. (Under freezing conditions the error
increases to two degrees RMS.) What we are now evaluating is the amount
of "depression"--the difference between air temperature and the
dewpoint, not the astronomer's attitude--which we should tolerate for
safe operation. Our goal is to find a quantitative measure which will
determine when we should open and when we should close so that we can
maximize telescope time without causing damage to our optical
coatings.

Alex Macdonald, Paul Harding, Bruce Bohannan

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