Quote:Why aren't folded refractors more common among amatuer astronomers? Why do I not know of any commercially available folded refractors?
Has anyone tried to use a 150mm f/8 lens as part of a folded refractor? How about a D&G lens?
It seems to me that the idea of constantly fiddling with collimation may be warding off potential users. But anyone that owns a Newt. is already used to that.
Quote:Unitron used to make folded refractors commercially. I'm personally fascinated by the idea myself. I'm really close to starting a "single-fold" 6" f/15 so it uses a minimum of light-robbing optics...less is more as they say. Here's a link to a page with a lot of folded designs. It's in German I think, but is still understandable. It's a 3 mb PDF file. Folded refractors
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Quote: Here's a link to a page with a lot of folded designs. It's in German I think, but is still understandable. It's a 3 mb PDF file. Folded refractors
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Quote:Do you know of any sources for quality flats?
Clear skies - Jim Crazy PNW imager !
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Quote:Ah yes, the Unitrons. A friend had one of the 3" folded Unitrons. My thought was always "Why bother?" It seems a concept that is a better fit for larger instruments.Clear skies, Alan
Quote:Besides, it's not as though you just shorten the tube, you create a nearly twice as thick shorter tube. I don't see the benefit.
Quote:If you go from a 7" tube to a 10" tube (10" might be tight") for the folded portion, you are actually increasing the volume of the tube. The folded portion has to be longer than the optical path it folds. It's a bigger tube!
- Jared Willson
Quote:It was the Op's original question that first mentioned a 6" F/8, which I think is pretty short to start with. For a 6" F/8 what are the gains?
Quote:For a longer f/ratio I think concept is more practical but it still wouldn't be for me.
Quote:A 48" long telescope balanced in the middle would only have about a 2' swing from horizontal to vertical (a little more or less). That's not much.
Quote:Folding it would only cut the swing down to about 1', more or less.
Quote:Swing distance is something every telescope has, unless mounted on a Springfield type mount, and just needs to be dealt with. Would you think of doing this with a 6" F/8 reflector?
Quote:There probably be no savings in OTA weight.
Quote:Sitting in an observing chair to look through the eyepiece is a good thing. An eyepiece 3½ feet off the ground is very comfortable.
Quote:Momentum arm is a legitimate concern although I personally prefer the longer arm. Leverage works both ways. The longer arm makes it easier to move the telescope against the set resistance of the mount's friction clutches. The trade-off for me is almost an offset. In the shorter telescope you tend to loosen the clutches more to make moving the scope easier. That reduces the resistance against wind. In the longer scope you tighten the clutches more. Moving resistance is about the same then but it does become more of a sail.
Quote:Then there is the cost. Good 4" and 3" flats will add cost, the mounts and controls for these mirrors will add weight and expense. Are you overbuilding this telescope?
--------------------Clear Skies, PJ Refl.- 203/1420; Refr.- 110/616, 75/500; Bino.- 15X60, 10X50, 8X30 __________________________________________________
Quote:Actually, some of the folded refractors get around this by having the bearing and focuser be in the same place. The eyepiece height never changes, no matter where the telescope is pointed. I agree - 3.5 feet IS comfortable, and if a person keeps their eyepiece height there the whole time, that IS an advatange, is it not?
Jeff Morgan - Wile E. Coyote School of Telescope Making
Quote:If anyone is interested in a little first-hand experience ... If I somehow came across an f/18 lens of significant aperture, I would probably do it again.
Quote:Quote:Swing distance is something every telescope has, unless mounted on a Springfield type mount, and just needs to be dealt with. Would you think of doing this with a 6" F/8 reflector?Actually, some of the folded refractors get around this by having the bearing and focuser be in the same place. The eyepiece height never changes, no matter where the telescope is pointed. I agree - 3.5 feet IS comfortable, and if a person keeps their eyepiece height there the whole time, that IS an advatange, is it not?
Quote:There does seem to be a lot of opinion on this thread, so those of us who have actually built them should speak up. None of the hypothetical issues people bring up are a problem in real life. Good elliptical flats bigger than the beam are available cheap, scatter is not a problem, and a nice newtonian style form factor is easy to use on a smaller mount than the scope would otherwise require.I have built a 6"F20 D&G (currently in the hands of Starlight instruments' Detlef Schmidt) and a 4"F15 Edmund, both folded in half. The trifold option is indeed more complicated. Both instruments maintained the superb optical quality of the objectives and were simply a pleasure to use. How about a 6"F20 refractor on a GP mount?
Quote:Then you're talking about a Coude' mount which is a more complicated mount than a GEM, especially for a relatively fast scope, and, I think, a consideration beyond the Ops question. That's introducing an entirely new concept.
Quote:It's also a different type of fold than the 3" Unitron example used above.
Quote:Still, for a 6" F/8 it seems like a lot of hoops to jump through. But if your heart is set on it, go for it.
Quote: Good elliptical flats bigger than the beam are available cheap, scatter is not a problem, and a nice newtonian style form factor is easy to use on a smaller mount than the scope would otherwise require.
Quote:I'm talking about the same kind of mount that Jeff Morgan showed in his picture. If that's a Coude, that doesn't look all that difficult to build, and the eyepiece remains at the same height. Sure, he mentioned disadvantages; that's why having this kind of discussion is beneficial.
Dave Celestron C102 4" achromat, Antares 1529 6" achromat, CGEM mount, Eyepieces: Proxima 31, Televue 20 Plossl, Celestron Ultima 12.5 & 7.5, TMB 5 & 4