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Message: <blockquote data-quote="Binastro" data-source="post: 3586758" data-attributes="member: 111403">My Zeiss 30x60 gives good star images.But I don't have the electric drive, which was detachable.The interesting thing about the drive is I think it ran on 4 batteries, but 3 would give half speed.I would not try to access collimation screws.Generally mirror scopes are compact, but most are not waterproof.The military use them for long distance work.I don't know if they ever use birdwatcher's spotting scopes.Small refracting spotters are used for rifle target practice.I am at home using any type of scope, but for long distance views the colour errors of refractors are nearly absent with the mirror scopes.The resolution of a mirror scope is empirically found by deducting the secondary mirror diameter from the main mirror. I.e a 6 inch mirror scope with a 25% obstruction delivers the resolution of a very good 4.5 inch refractor.Obstructions above 25% cause contrast reductions.Planetary Newtonians, say 10 inch f/9 have obstructions as small as 1 inch across or slightly larger.There are also unobstructed mirror scopes, but these are difficult to make well.Mirror scopes usually take longer to temperature stabilise than refractors, although fast refractors seem to be worse than long focus refractors.</blockquote>