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Posted by Daniel Johnson on November 14, 2002 01:59:56 UTC

Mind you, Televue makes some mighty fine eyepieces at short focal lengths (the Radian line), so I don't think the eyepiece or eyepiece/Barlow question is the main issue for high magnification. I agree that getting good high magnification for planets is harder at short focal ratios, especially since figuring an f/5 mirror is waaay harder than f/10. Also, short focal ratios FORCE you to use better (i.e. expensive) eyepieces, since anything cheap can't handle the sharply converging cone of light. But f/10 isn't much use for nebulae. I made the mistake of grinding my first mirror f/9 because I wanted high magnification (or so I thought), being naive at the time of the value of low-power, wide-field views. I've regretted the choice ever since. Mounting an f/10 tube is much harder--its tendency toward flexion goes up exponentially with length, as does wind vibration. See my comments on this at this link:
http://www.astronomy.net/forums/atm/messages/3211.shtml
So what do you want--an all-around scope? Then try f/7. Planetary? Then try f/9 or f/10. Deep-sky? Go for f/6 or f/5. I would go for f/7 myself.

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