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I like this quote (from the URL I posted above):
Dirt happens, and in moderate amounts it has remarkably little effect on performance. In his book Star Testing Astronomical Telescopes, Harold Richard Suiter analyzes the effects of dirty optics mathematically. His conclusion? "The maximum amount of dirt [that a perfectionist] should tolerate on the optics is about 1/1000 of the surface area, [which is] the size of a single obstruction about 1/30 of the diameter." In other words, on a 10-inch mirror this is the equivalent of a completely opaque dirt blot 1/3 inch across. That's quite a pile of crud before there's any detectable effect on contrast.
"Don't decide to clean mirrors on the basis of shining a light down the tube at night," advises Suiter. "All mirrors fail such a harsh inspection." After you've done what you can to prevent dust, ignore it.
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