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I am a little worried about bringing the scope back indoors when I'm through for the night. I'm thinking of 2 stages. First into the 25°F garage with dew strips still on for about an hour, and then put the scope into a cloth bag with corrector plate pointed down and bring it into the house. I'm a little worried that it will get soaked when I do this. Any thoughts? Thanks, Brett

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Hi Brett, for several winters I've brought my 8" SCT from the 10F deck back into the 82F living room w/o any temperature or moisture problems.
If you're concerned about the temperature difference affecting the optics, re-acclimate it in stages as you described. But to prevent the scope from getting soaked from the humidity in the house, place it into a large plastic lawn and leaf bag with plenty of airspace. Make sure the bag's puncture-free and closed securely, and remember to open the bag next morning to allow the scope to breathe again.

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Carol - I bring the scope in, point it down, put cover lid on and leave it. I have seen some posts that leave the lid off with mirror exposed to dry pointing down? I'm worried about dust attaching to the mirror whilst still wet. Am I doing worse by covering the optics?

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Hi Dave, the only time I use the plastic bag is when it's cold enough for eyeglasses to fog up when entering the house. On not-so-cold nights when the scope is merely dew covered, I wipe the outside off as much as possible and then do exactly what you said, and have never had any problems.

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Carol, I used the scope twice last weekend and used the plastic bag trick the first night and it worked great. I had read about it somewhere else and it makes sense. When you trap extremely dry air in a bag from outside and bring it in the sealed air in the bag never hits the dewpoint so you have no dew/frost.
The second night my scope tube frosted up while observing and I worried about sealing a frosty scope in a plastic bag so I didn't do it. I brought the scope in and sat with a hair dryer and towel until it was warm. My mirror did dew up but cleared within an hour. What would you do if your scope was frost covered? Maybe I should get a 12v hair dryer to remove the outside frost at the observing site and then seal it in a plastic bag.

Brett

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I'm glad it worked so well for you, Brett!
As far as frost.. first, cap the corrector plate and visual back. Then use a plastic Teflon pan scrubber to scrape the frost off of the OTA, base and fork arms, and wipe them down with a piece of rough terrycloth. Put the scope in the bag, and remember to open it up in the morning. The small amount of frost that might remain has never been a problem.


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8"SCT ~ 120achro ~ 90Mak ~ 80ST ~ 11x70s ~ 22x100s

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All a dew heater has to do is to keep the corrector at ambient to prevent dew (assuming the dew point is below ambient).

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True, and it makes me wonder if some of these electrical solutions might be using a sledgehammer to throw the egg out with the bathwater.

There is a great deal of column space here encouraging sufficient "cool down" time to be provided for CATs, with warnings of poor imaging resulting if the scope doesn't reach ambient. Sometimes it's stated that CATs (especially Maks) are such heat-sinks that they can never match ambient if the temperature is fluctuating. With this in mind, strapping a heating element to your scope or blasting it with a hair dryer doesn't seem particularly conducive to optimal viewing. Granted, a dewed up corrector is also somewhat short of ideal - but the arbitrary laying on of heat seems likely to wreck things for high magnifications.

Even the most sophisticated controllers seem to be used rather bluntly:
http://www.cloudynights.com/accessories2/dewbuster.htm

10 degrees above ambient?! It'll work for dew control, but what would Jupiter look like at 250x? Is the level of sensing/control available not sophisticated enough to keep the corrector at or near ambient?

Having been in some amazing dewing conditions where nearly every surface is soaked, it has struck me that the primary mirrors of long focal length Newtonians always avoided misting up, as did the insides of unheated refractors. My guess based on this experience is that the typical dew shields seen on CATs are simply too short. I'm not even convinced the supposed ideal length of 2x the diameter of the objective/corrector applies for bigger scopes (seems suspiciously 'linear' to me).

For my five inch Synta Mak, I'm looking to construct something just over twice as long as the OTA. It'll be cheap and cheerful, unlike complicated electrical anti-dewing devices. I'd rather spend the equivalent of a high quality eyepiece on a high quality eyepiece.

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Peter

10x50 binocs
ED80 'frac
10" f/4.3 Newtonian alt/az
Bryce

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