Re: [Utah-astronomy] 13.1 inch Coulter Carcass, free
Hi Chuck The ladder at SPOC for the Ealing is good choice and takes up half the bay. I doubt there are to many portable ladders that are, Brent Watson had a tall ladder I think we where fortunate that there was no accidents. I remember his encouragement for people to look at bright objects with both eyes atop the ladder. Many struggled getting down ladder afterward. Yes, Chuck I am sure that your F5 gives good views, when you get into larger scopes, 16" and above, the eyepiece starts to get high of the ground. Bob pointed out how his 16 f 4.5 could be used without ladder. You do have get more expensive eyepieces and coma is present at edges and most objects do not require wide field. My f5 refractor also works well on deep sky and also has some coma at edges. The main advantage of f4 over f5 would be bigger exit pupil for same eyepiece. Yes, if you are going to figure your own mirror you are better off with f5 or f6, but if you are going to purchase a bigger commercially figured mirror consider faster than f5. There are well done fast mirrors available. SLAS gave an our John Dobson figured 18" f7 0r 8 to the Ogden Club because we had a hard time storing it and getting someone to use it. Perhaps we would not have had the problem at f5, but it would still be about 18" taller than an f4 My primary concern has been my own use ( I find myself unable to use most scopes I have not tailored to my own use), seems to me, the more convenient the scope is for yourself that it will also be convenient to others. Seems most oldtimers have sore necks and backs. 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: Wed, 7 Jan 2009 17:53:04 -0700 The wrong ladder in the dark can definitely be hazardous, but that's not exactly a problem with a long-focus telescope; it's the wrong choice of ladder. The right choice might not be easily transported, or even easily used at the eyepiece, so that's one of those choices people make. I think the original context of the question was short focal-length mirrors as a first effort at mirror grinding. They do take more work and can be more difficult to figure, but it's not out of reach of the average person, given the proper guidance. If, however, one is purchasing a commercially-made mirror, all the caveats of fabrication can be ignored. I don't agree that deep-sky observing is better at f/4, as a blanket statement. My little RFT is an f/5 and the coma is noticeably less than otherwise identical telescopes I built at f/4, yet the field isn't much smaller using the same eyepiece. I also don't weigh my equipment choices against their fit at a public star party. I'm kind of selfish with my astro-time- my personal enjoyment is my prime motivation. I'll blow-off being "star-party correct" in a heartbeat if I feel I can get some kind of advantage by doing so. On nights when I'm specifically doing the public outreach job, then convenience and safety will trump imagery out of necessity. The telescope must be designed for it's intended use. One person's trade-off list will probably not correspond one-for-one with someone elses. On Wed, Jan 7, 2009 at 1:59 PM, <zaurak@digis.net> wrote:
The narrow steps on a ladder can take a toll on your feet in the course of a night. The classic dobson is long focal length, but there are many companies that do a good job on fast optics in the 4f range. Deep sky observing is better with f 4's.
Tall ladders seem a liability for public star parties.
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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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