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Old 4th May 2012, 11:39 AM
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rich rich is offline
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Join Date: Nov 2008
Location: Shebbear, N Devon
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Do some very simplistic sums; most scopes have aluminium tubes, aluminium has a coefficient of linear expansion of 0.000022 per degree. If that tube heats up from say 10 degrees to 30 degrees and that tube is nominally 400mm long (not a bad starting guess) then it will increase in length by .0000022 x 20 (degrees) x 400 (mm) which comes to 0.176 millimetres. That is how much it grows in length.

Now, an assumption that is not correct but again it's a starting point. If you regard the eyebell as fixed, this means that the objective glass has moved 0.176mm further away. This is exactly what happens when you focus a front-focussing scope, for shorter distances you "unscrew" the AO part and move the front lens outwards.

So in truth, it's hard to imagine the complexity of designing a scope that caters for all of this, as the growth in the length of the tube is going to happen, regardless.
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