Over time, your reflector's mirror is going to become dirty. It's not a matter of if, but when. The trick is knowing when you actually need to actually clean the mirror. The surprising thing is that a thin layer of dust will impact your viewing so little that it's not worth some of the risks involved in cleaning. Cleaning your telescope's primary mirror should only be done when the mirror is so dirty that it could possibly affect your views.
Even most smudges will not affect your views (I know from experience!) This being said, I would recommend that you not clean your telescope's mirror unless it is really dirty or grimy (as mine was) or it really, really just irks you to have a dusty mirror.
Cleaning your mirror is not as scary as it sounds, however, and can be done by any amateur astronomer with a bit of patience and some distilled water! Just follow this guide, and I'll show you how to do it safely. (Speaking of safety, don't do this outside on a sunny day, or in a sunbeam indoors, you might catch a magnified sun reflection in your eye!)
Things you will need:
Cleaning the mirror
If you decide to do this step, wet the cotton ball first. Then lightly drag it across the mirror surface. Don't put any pressure on it, just kind of let the weight of the cotton ball do the work. Use one cotton ball for one pass, wiping from the center outward, then throw it away. Reusing cotton balls can drag dirt across the mirror, potentially scratching it.
Now you have a clean primary mirror, ready to be reassembled into the mirror cell in the reverse order that it was removed. When you tighten the clamps, don't tighten them down too much, as this can distort the mirror and affect your views. Just tighten them until they make contact, then a little tiny bit tighter.
If you follow this advice and work patiently, your mirror should come out sparkly clean and without any damage!
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Disclaimer: I cannot take any responsibility for any damages, to you or to your property, occuring before, during, or after performing this telescope mod or resulting from any advice taken from this website. Do so at your own risk.