Quote:
Originally Posted by AlexN
A mate of mine and I were discussing this just last night, the thermal expansion/contraction of an aluminium tube the size that we would guestimate the GSO RC10 has would be, Given a 20°c temperature change, the worst case scenario (assuming very poor quality Aluminium) was a contraction 45 microns.
The F/8 optics of the GSO RC10 give a CFZ (Critical Focus Zone) of 140 microns. Hence, given you focus well at the start of the imaging run, and don't experience more than a 20°c drop in temperature during the imaging run, your focus would not noticeably drift over the course of the run... Its a different story when you're imaging with a fast system, Ie, an F/4 scope with a CFZ of 35 microns.. but with the slower ratio's, your focus would very rarely shift.. not before you hit the meridian and have to flip the mount anyway, at which point, how hard would it be to just touch up the focus before recommencing the run after the flip?
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That contracton of 45 microns will be far more pronounced in a two mirror telescope. It is not a simple 45 microns at the focal plane, it is a difference in seperation of the two mirrors by 45 microns, which will shift the focal point by a much larger number (unless your calculations were far more complex than I give you credit for!!!)