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OK, I've decided to put the effort in and do the math open to the public so if I get it wrong feel free to correct me. I will have to make a few assumptions, since I don't know the full specifications of the GSO 10" RC
Calculating Expansion of the tube - which effects mirror seperation
Expansion = z * L * T
Where:
z = Coefficient of linear expansion of the material
L = Initial length (metres)
T = Temperature Change (celcius)
For aluminium, z = 0.0000231 per degree
I'll assume a tube length of 40cm (I don't have measurement)
I'll calculate for temperature change of 20 degrees
Therefore expansion (which is a contraction since the temperature change is a negative value) = .0001848 metres = approx 185 microns
Calculating focus shift
Focus shift at focal plane = -Am²/[1-A(m-1)B]
Where:
A = change in mirror separation
m = secondary magnification
B = secondary mirror to primary focus separation, negative in sign for a cassegrain telescope (the RC layout is a cassegrain configuration)
A will be 185 microns
m I will assume to be around 4 (the scope is F8, so maybe F2 primary and 4x magnification secondary?)
B I'll assume to be about the tube length I used earlier, 40cm.
Therefore focus shift works out as 0.002955 metres, or nearly 3mm for a 20 degree drop in temperature !
Last edited by Kal; 11-12-2009 at 07:23 PM.
Reason: Clarified expansion since it was a contraction
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