Quote:
Originally Posted by gregbradley
I have gotten dew on the mirrors of the CDK17 about 3 times in 3 years. Its not really a problem. As far as auto when it hits ambient that is also a nonissue as the mirror always seems to be a tad above ambient and the closest is .1C difference I have seen. Maybe once in a blue moon it goes below ambient. That is more ideal anyway as you don't want a boundary layer on the top of the mirror.
Talk to Paul Hease or Bird about thermal effects on mirrors. They cool theirs to get it below ambient a few degrees to get rid of thermal currents.
Ideally there should be fans blowing across the mirror. The latest CDK17 has this. Mine does not. It gets rid of the boundary layer. I was thinking of rigging up some fans facing across the mirror. 2 should do it.
Ron Wodaski talked about thermal currents and sideways fans at AIC this year.
Auto dew heating could be handy on some nights but you most likely will find dew only occurs on those nights early in the morning. I have not really lost any imaging time because of dew or at least minimal amounts.
I think the main thing is turning on the fans early enough to allow the mirror to get quite close in temp to the ambient. If its more than 1C then it will be hard to focus and won't be at its sharpest. .3 to .6 is OK. .1 is sweet. It takes the CDK about 1.5 hours to get close. I am sure it will be similar for this scope.
Greg.
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Interesting - thanks for the info Greg! Surprised dew hasn't bothered you that much. Dew used to cause all sorts of problems for my Tak before I put a dew heater on. On many occasions the whole observatory was dripping wet and the objective fogged over before 1:00am.