Quote:
Originally Posted by Kal
think of a SCT - if you add an external focuser your focus control is very slow compared to how much focus change you get from moving the primary mirror. I don't have the exact math, but a change of seperation of the two mirrors by 45 microns will certainly not equate to a change of 45 microns at the focal plane - because of the secondary magnification it will change the focus point by 5x
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More like 26x (secondary magnification squared, plus one). That is why SCTs have such a huge focusing range for relatively small movement of primary !