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Geometric calculation
Hi Chris,
If the image you posted is an uncropped APS C, then the defocussed star diameter is about 15.7mm depending upon the specifics of your APS C sensor and blurring at the edges. If the FSQ 85 is the model with the 450mm focal length, then without any reducer this is a simple geometric similar triangles calculation:-
Extension required is approximately
15.5mm x 450mm = ~82mm
........85mm
I would retake the image with the Estatto at mid-travel rather than at one end measure the out of focus image and repeat the calculation.
Cheers
Joe
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