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I am considering buying a telescope with a large primary mirror that is about seven years old - it has a small spot of corrosion on the mirror, that the owner says is stable.

Should I be concerned about this spot spreading to the whole mirror or getting much larger?

What kind of care or treatment should I give it?

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Ask them to define stable, then consider entropy; in other words, yes, be concerned. –  Grant Thomas Jan 3 '12 at 17:52

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If the spot is small, it should be of no concern. More important is the overall quality of the mirror itself and the telescope it's mounted in. I would try to find out what caused the corrosion, since aluminum mirror coatings rarely show corrosion unless they are in a corrosive atmosphere, such as close to an ocean. But a small area will have no effect on the images.

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