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The coma free zone is pretty small and outside this zone the coma gradually gets larger in a linear fashion.
The formula is --- Coma Free Zone = 0.022mm x FR³
So for your scope its
F5.6 = 0.022 x 5.6³ = 3.86 mm
You could make an off-axis mask, say a 4 inch hole fitted between the spider vanes. At your focal length of 56 inches with a 4 inch aperture would give you a F14 scope, pretty much coma free. With no diffraction spikes will give refractor type performance.
Last edited by billdan; 14-05-2018 at 03:44 PM.
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