Nick - Put that old Patrick Moore book back on the shelf , now !
Quote:
Originally Posted by Wavytone
- achieving high magnification with a short objective (f/4 or less) implies very short eyepieces that are less than satisfactory; a long focal ratio objective (f/15) coupled with a larger eyepiece will produce far better images.
- achieving very low magnifications with a long focal ratio objective (say f/15) implies using eyepieces that will give an exit pupil too large for your eye.
The optimum turns out to be f/7 where you can achieve a 10:1 range between lowest and highest magnification and still have an exit pupil less around 6mm at low power. But f/7 'scopes aren't small and pose real problems if you want an equatorial mount.
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