At 12:10 AM +0000 2/12/02, Roger Wesson wrote:
I recall an article in an astronomy magazine some time ago that dirt on a
lens or telescope objective mirror will not affect image quality noticably
until it covers an area equal to a blob one-tenth of the lens diameter
across. So, for a 50/1.8 lens, diameter of the glass is 28mm, so you could
have a lump of muck 2.8mm wide stuck to the lens without it affecting image
quality.
I don't recall their criteria for determining what loss of image quality was
acceptable. If I can dig out the article I'll provide more info.
A friend/colleague worked briefly at Lick Observtory in northern CA, where
a generation or so back one or their peons went crazy, took a .45 into
the prime focus cage and started shooting at the main mirror. Not only
didn't the mirror crack (foot-thick annealed glass is pretty close to armor
plate) but after they painted the little holes black some people claimed
the seeing had actually improved slightly.
Considering that you don't see front mirror of a cat lens in the in-focus
section, I'm not surprised that 100f area would be around the threshhold.
Planar waveforms are amazingly robust.
paul
--
Paul Wallich pw@xxxxxxxxx
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