No, it doesn't work like this at all. The topical archive has some
information on how a dew shield works, and how dew forms. It can be
confusing, but dew does not "fall", it simply condenses out of the air. The
dew shield helps prevent dew from forming on the corrector plate (the glass
plate at the front of an SCT). Unlike on a Newtonian telescope, the tube of
an SCT is closed by the corrector plate on the skyward end, and nothing can
fall onto the mirror. On a Newtonian telescope, the whole optical tube acts
as a long dew shield, thus you don't hear about people using dew shields on
them.

It's like your mirror when you are in the shower. The moist air condenses
on the cold glass of the mirror. Even though the mirror is vertical, the
condensation doesn't fall onto it, it simply forms on it.

Outside, as the night air cools, it holds less moisture. The temperature at
which the air will start condensing out moisture is the dew point. You can
see this temperature on meteograms from the National Weather Service. The
tricky part is that your telescope can get colder than the air around it.
Because this colder temperature might be below the dew point, dew condenses
onto the surfaces (optical or not) of the telescope.

A dew heater applies heat to the optical surfaces to keep them warm and thus
above the dew point. A dew shield keeps the corrector plate warm, but via a
different method entirely. The various descriptions in the topical archive
explain this well if read as a group. Look under "Dew Zapping; Dew
Formation & Its Prevention". There are four articles.

Philip Freeman

> ----------
> Sent: Thursday, February 15, 2001 3:13 PM
> Subject: [M]: Dew sheilds
>
> I think I understand that a dew shield is for keeping dew from settling on
> your mirror (i.e. falling into the OTA and landing on the mirror). I
> assume
> it can't keep condensation from occurring, and that's what a heater is
> for.
> I think.
>
> Here's what I don't understand. At some angle up, dew, or rain drops, can
> fall into the opening. At 90 degrees, straight up, it all falls onto the
> mirror, shield or not. At 45 degrees (tangent = 1), assuming a 10"
> diameter
> opening, it falls into the tube, but lands on the inside 10" of the wall
> near the open end. At 75 degrees (assuming a 36" long OTA, tanget = 3.73)
> some water hits the edge of the mirror. So at DECs of over 75 degrees the
> mirror will always get wet. Am I right, or is it that most observing is at
> DECs of less than 75 degrees, or with OTA lengths greater than 3.7 times
> the
> OTA opening diameter?
>

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