Dew/Helping the mirrors adjust properly

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timtest

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Hello, I have recently purchased an 8'' Dob (skywatcher) and have a quick question.<br /><br />First off I store my scope indoors. Now, for example one night I wanted to go out observing after I got home from work (its a cool night around 0ºC) so I put my scope in the garage so it will adjust to the temperature and be good to go when I get home. When I get home I notice it’s too cloudy and decide not to go out, so I bring my scope back inside. I look at the optics about an hour later and notice my primary mirror was still very fogged up (the cap has been on the scope the whole time), is this normal? <br /><br />My question is: Is it ok for my scope to go through that much of a temperature change so quickly? Or, if I go out observing in the winter time for a few hours is it ok to bring the scope inside right away? Is there a better way to let the optics adjust to the temperature change?<br /><br />If I’m frequently going out in cold temperatures (I live in Winnipeg, cold winters!!! <img src="/images/icons/wink.gif" /> ), what’s the best way to handle this matter? Being new to the whole amateur astronomy scene I want to be as careful with my new scope as possible. <br /><br />Thanks, timtest. <br />
 
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alkalin

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Anything brought from the cold to warmer ‘moist’ air is going to have a collection of moisture on it in a short period of time. So best is to leave it in a colder clime at all times, only to bring it to warmer temps if it does indicate haze or fogged surface, until the moisture is gone. Once returned to a colder air environment, keep the mirror covered until you use it, and when your finished then put a tight cover back on it, you just do not want anything including moisture to accumulate on the surface, but it should not be necessary to store it in a warmer place unless there is frost. If it is really cold, frost has a tendency to accumulate over time on the surface of anything unless you can keep it covered tightly. If the mirror is frosted, recovery by warming is necessary until frost is gone. And I think it is a good idea never to let the frost get very thick, otherwise it may damage the coating of the mirror surface. And I have to add that it is never advised to rapidly change the temperature of the mirror, even though it is probably low expansion Pyrex, which I do not know.<br /> <br />I do know that large telescope mirrors of today are made of very low expansion materials such as zerodur or cervit, which makes it possible to make very large mirrors without them cracking when they cool after the hot molding process. You are probably aware that temperature stabilization is essential to good imagery, and the less the coefficient of expansion of the mirror is, the better. The very high mountain locations of large scopes testify to their abilities despite adverse conditions such as freezing.<br /><br />There are other items to consider such as a ‘dew’ ring, a warmer to keep moister off the mirror, but still consider the down side; it might degrade image while you are using the scope.<br /><br />I first built a Newtonian about 50 years ago when I lived in the northern mid-west that could clearly show the planets; the rings of Saturn were impressive. I did not us
 
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