I dug up my old 6" Dobsonian telescope from storage and the mirror looks like the following:enter image description here

The picture is down the tube, so you can see the spider holding the secondary. To my eye, the spots look like corrosion and not cleanable, but does anyone actually know what they are?

It was stored relatively carefully - not totally sealed but in a house, with the tube covered with a plastic bag. So huge temperature changes are not likely, but it does quite humid here. It was last in action > 10 years ago.

The only other thing I know about the scope is that it was made by Criterion, sometime before the 1980s.


The spots could be mold or fungus. If a gentle cleaning with distilled water, a little detergent, and sterile cotton (no abrasives!) fails to remove them, there are several vendors who can strip the old aluminum coating and apply a new one. If you don't want to go that far, you can probably find a buyer who would. Criterion scopes have a considerable fan base.


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