Chuck, Well, at least some of best magnifications have a 4-5 mm exit pupil. In the big picture a shrinking exit pupil is not the worse thing about getting old. I would agree that in smaller apatures f5 or f6 is better, mainly to avoid crouching too much at the eyepiece, to reduce effects to the aging process that causes. Age and infirmity also seem to reduce the size of telescopes, the professionally figured f4, I observed with was well done and had minimal coma with Naglers. I would say if you plan to use a coma corrector place your focuser with that in mind when you build your dobson. I have been less affected by shrinking exit pupil, one of the few bright spots in my aging process. I am due to see the ophthalmologists though, it has been about 5 years since my last visit. I am more like 6 mm, my 7x42 binocs still work well for me, as for my refractor, at widest fields I do have to move my eye around to see full view. I will have bring my exit pupil gauge to a few star parties. I remember passing it around at the Sun Tunnels one evening, a few years ago, and a lot of people were around the 5 mm range. BTW, I would not want to forget Jerry Footes' ATM escapades, although he is closer to the professional side now. I also think, it is telling that Riverside is now called the "Astronomy Expo" and not the "Telescope Makers Conference". Erik --- chuck.hards@gmail.com wrote: From: "Chuck Hards" <chuck.hards@gmail.com> To: "Utah Astronomy" <utah-astronomy@mailman.xmission.com> Subject: Re: [Utah-astronomy] 13.1 inch Coulter Carcass, free Date: Thu, 8 Jan 2009 11:05:29 -0700 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.

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