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Message: <blockquote data-quote="Binastro" data-source="post: 3542770" data-attributes="member: 111403">What should be cleaned off quickly are fingerprints on optics, as these can lead to etching the glass or coatings if left.Another problem, relating to the half covered objectives is Drooping eyelids.A friend recently had surgery on both eyelids, as they were seriously affecting his vision and driving. He said he could only see partly because of this problem. Related is the use of binoculars inclined up for astronomy or indeed down. I have found that sometimes only half the aperture is used. This is clearly seen in out of focus star images.This also applies to telescopes used at high angles without a star diagonal or prism.</blockquote>