I've seen amateur reflecting telescopes with tremendous crud on the mirror and the image still looks pretty good.
Truss-type reflectors are totally open to atmospheric contamination, including dust, smog, insects, etc.
That makes me wonder, how often are large professional reflecting telescopes cleaned?
I assume there's a light cleaning period, a heavy cleaning period, and a recoating period.
Does anybody know what these are? Does the contamination in the intervening months/years mainly affect light grasp, not resolution or distortion?