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I went out last night in another attempt to see some DSOs. I have a Skywatcher 150p sitting under an Altair 10x60mm RACI finder scope. It was pretty clear and the temperature dropped quickly to slightly below zero.

Increasingly I was seeing less through the Altair than I could see with the naked eye, and the reason was that the eyepiece fogged up. I'd sort that out only to find that the lense at the front end had a thin coating of frost over it too. However, I was much more reluctant to touch that part of the scope, so I packed it in for the night.

Is this a common issue, does it only happen when temperatures drop below zero, and are there any answers other than a heated dew shield (which I imagine would be difficult to find for the Altair)?

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Yep, will usually fog up then freeze. I made a dew shield from an old camping mat for my scope and then made smaller versions for both my R.D.F and finder. Seems to keep the objective end clear. Still have occasional issues with the other end fogging up, best I have found there is to keep it capped until you want to use it.

HTH

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Hair dryer. Even a little travel one would be good. Or dew heaters, as you say.

Otherwise, a dew shield like David suggests.

Like him, I keep mine capped until used.

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Before Christmas last year I did an all night imaging session. I took the picture below of the telescope as seen below. You might be able to see that the telescope & dew shield is caked in ice. This is another reason why I use a robotic telescope- no freezing off my bits unless absolutely necessary!

I would invest in a good quality heated dew shield and a controller to regulate the heat to various devices like eyepieces and your finder. I very rarely use a visual eyepiece so a desiccant in the CCD camera removes moist air that could form frost and fog up the camera surfaces. Even with the best dew heater the primary on my telescope sometimes fogs up & I have to use the Starizona "thermal stabiliser" to pull a gentle airflow through the optics. I dry the telescope off with a fluffy towel before covering it after the observing session.

post-36426-0-68699600-1421689324_thumb.p

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Yeah, the last two times I've had my dob out the tube has been caked in ice. Mirrors stayed clear, though. I don't understand dew/frost

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