Erik, I understand what you are saying completely. The disadvantage of getting old is a shrinking exit pupil. Mine never gets larger than about 5.5mm anymore, even in absolute darkness. I have trouble with wide-field eypieces; the "kidney bean" effect sometimes blacks-out large portions of the field of view. 80 degrees is about the max I can use comfortably, and only if the eye-relief is decent. The exit angle of the light cone can't be too steep or the eyepiece is just a paperweight to me. There is no substitute for youth! There comes a point in life where age or infirmity requires certain trade-offs at the eyepiece. Those trade-offs are what allow us to continue to use a telescope, in may instances. I'm building a 17.5" f/4.5 because of the exact reasons listed. I won't need a ladder to reach the eyepiece when pointed at the zenith. My balance is terrible in the dark, and I don't want to haul a ladder around with me. When I was 18, I wouldn't think twice about scrambling up a tall ladder in the dark. Now, no way! But at the same time, I'm not going to stand on a soapbox and say that my f/4.5 yields imagery as good as an f/8 just to feel better about my choice...and I've seen the mirror on the test-stand. It has an excellent figure. F/4.5 is a compromise. If I had my 'druthers, it would have been f/5.5. But...no ladders- I'm not 7 feet tall, and I own a coma corrector optimized for that exact mirror. I'm not condemning fast mirrors, they have their place. The discussion started with the idea of a beginner grinding their own first mirror. An f/4 is a tough hurdle right out of the gate, and a beginner isn't likely to start with a 16" or larger. And a long-focus mirror will ALWAYS give a higher quality image given the same eyepiece. A 16" f/4 with a decent figure will give good planetary views because of the resolving power of a 16" mirror. Aperture will win over a large coma-free field. That planetary disk is occupying the sweet spot of the focal plane, most of the time. But a long-focus 16" would definitely give a better planetary image, especially if the planet gets away from the center of the field. And, you can use longer-focal-length eyepieces to achieve the same magnification (read: easier to use). Yes, some beginners have tackled large, fast mirrors as a first effort- some right here on this list- but it's not typical, and they had excellent instruction along the way. So, I restate: To each, his or her own. My reasons are probably not the same as others. On Thu, Jan 8, 2009 at 10:11 AM, <zaurak@digis.net> wrote:
The main advantage of f4 over f5 would be bigger exit pupil for same eyepiece.