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Old 13-07-2018, 05:33 AM
fsr
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Join Date: Nov 2015
Location: Hurlingham, Buenos Aires
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Quote:
Originally Posted by gb44 View Post
I had an 18inch plate glass closed tube newtonian.( also on a Titan) I did find that I could induce primary mirror dewing with the cooling fan and that switching off the fan was the solution. I did have a fan to blow across the primary front surface
but didnt use it much. It did help a bit with seeing. Such dewing was a rare thing.
GlennB
Well, dewing occurs because hot air can transport more humidity than cold air. So, if hot and humid air hits a cold surface, such that the air is cooled to a temperature which cannot hold the humidity it carries anymore, it will release the extra water as droplets on the cold surface. I can see how a fan can be a problem if the air begins to dew over the mirror, as you're forcing more air to hit the mirror. Up to that point, the air should heat the mirror slightly, or to be more precise, stop the mirror from falling too much under ambient temperature, but if it fails to keep the temp over the dew point, then it's blowing air at ambient temp over a mirror under the dew point.

So, it seems that the options are: use a dew cap, if the scope has a lens on the front; keep the dew cap and the scope from loosing too much temp; keep the mirror/lens over the dew point by heating it.

I found this article about dew in scopes. Very informative, and good ideas: http://www.asterism.org/tutorials/tu...ngOfOptics.pdf
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