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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