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It is often cold outside, especially in the evening. So when I take my Newtonian Reflector back in my house, it starts condense.

Can the water destroy my telescope? or the mirrors? Do I have to clean it?

My telescope is fully painted, but I'm afraid of the screws. Screws often rust.

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The tube itself is not that much of a problem, just wipe it with a soft cloth. The screws should be stainless steel, surface rust can be wiped off as well.

But moisture, which contains humic acids from plants and pollen as well as mold damage the reflective surfaces of mirrors. If it is an expensive one, leave it outside or in a shelter until the next day and store it with the main mirror pointing down. Don't expose it to a temperature change of -10 to +20°C. Do not clean optical surfaces mechanically if your unsure about the coating and the cleansing stuff. Wiping the aluminium surface will scratch it if it has no protective coating.

If it is a particular expensive bucket and you're in an area that experiences much moisture (where i live, water sometimes runs off the tube, really), consider heaters. Or try another day :-)

Amateur astronomer forums like e.g. cloudynights.com have much more info on this.

Hope that helps.

Edit: also: How do I remove fungus from a telescope mirror?

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    $\begingroup$ I think of aluminized mirrors as either having a naturally occurring Al2O3 native oxide on top or an additional factory dielectric coating. What exactly are "humid acids" and how can they reach the well-protected aluminum layer? Can a source be cited or is this just something you've heard? $\endgroup$ – uhoh Dec 29 '19 at 10:15
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    $\begingroup$ They are a typo i will correct, oops and thanks :-) You guys are awesome, really ! $\endgroup$ – user31179 Dec 29 '19 at 10:28
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    $\begingroup$ Looks like i am a bit behind things: even mass products have a protective coating for the reflective surface these days: gs-telescope.com/content.asp?id=142. But it's SIO2, which dust often is as well. So it is 1:1 chance of applying a scratch ... $\endgroup$ – user31179 Dec 29 '19 at 10:40
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    $\begingroup$ You could also consider adapting the photographers' trick of wrapping the camera in an airtight plastic bag before taking it inside, and leaving it there until it has warmed up. That way, it won't get in direct contact with indoor air while still cold. You'd probably need one of those big dumpster bags for a telescope (and some cord to tie the mouth of the bag around the mount) but it should be doable if the scope isn't too large. $\endgroup$ – Ilmari Karonen Dec 29 '19 at 18:14
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    $\begingroup$ Sure, but in case of a photographic setup that's a bit impractical. imgur.com/q36ijOD (image mine). The counterweight has 20 pounds. $\endgroup$ – user31179 Dec 29 '19 at 18:20

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