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~63 microns expansion for the tube in a 20 degree temp drop Fred..
Expansion is expansion, if your mirror spacing does not change, the only thing you have to calculate is how far the sensor is moving with regards to the focal plane in a given temperature drop.. 63 microns would be well within the CFZ of your scope, hence, as you mentioned earlier, you get 0 focus shift night after night with your scope...
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