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Another thing - f/7 or 8 is IMHO about optimal in a couple of respects:
- it will accommodate a 40mm eyepiece at the low power scale, yet for high power it could use a 4mm on the best of nights - a 10:1 range of magnification.
- the secondary will be small enough not to degrade the image significantly.
Admittedly given the aperture of your scope, its more likely to have a 1.25" focusser and your lowest power eyepiece will be 25mm - it won't fill the field of view of a big 40mm widefield eyepiece, let alone a 2" focusser.
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