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#1 Jeronimo Cruz

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Posted 21 February 2010 - 09:10 PM

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.

Any thoughts?

:jump:

#2 Alan French

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Posted 21 February 2010 - 09:21 PM

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.

Any thoughts?

:jump:


Some ATMs have made folded refractors. R.E. Brandt made some larger ones commercially, at least up to a 10" f/16. AP had a folded 6" f/12 apochromat at Astrofest one year, and I believe made a larger version for a customer.

I think there are several reasons they never became popular. One of the advantages of a refractor is that it tends to maintain collimation and rarely, if ever, needs collimation. Another is the lack of reflective surfaces, which scatter more light than lens surfaces. Finally, having a entrance pupil well above ground and away from the observer. All of these advantages disappear or diminish when you fold the instrument.

One approach that I always thought looked interesting was recommended by Henry Paul as a way to mount a larger lens. He recommended folding it "Newtonian style," with a large flat at the bottom reflecting the light up to a small flat near the objective and opposite the focuser.

Clear skies, Alan

#3 KP3FT

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Posted 21 February 2010 - 09:43 PM

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

#4 Alan French

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Posted 21 February 2010 - 09:46 PM

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

#5 Alan French

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Posted 21 February 2010 - 09:48 PM

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


I see some folks who took Paul's suggestion, or came up with idea independently.

Clear skies, Alan

#6 CharlesStG

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Posted 21 February 2010 - 09:58 PM

I remember an article in Sky and Telescope from back in the 1990s -- it was a very nice 6" f/15 folded refractor which used two mirrors. The first mirror was about a 4" at about the 35" mark and it gently reflected the light to the far altitude pivot point (Alt-Az mounted) where it was reflected again through the altitude axis to the eyepiece mounted at the other end of the altitude axis -- this way, the scope is pretty small, and the eyepiece doesn't change position when you move the scope in altitude -- the eyepiece does move in a circle when you change the azimuth of the scope, but it's a nice design trade-off. At least you can stay seated at the same height always looking straight into the eyepiece. The first reflection is the most potentially damaging but because the first angle is so shallow, little damage is done and its about the same for the second reflection. This is about the sweetest, to my mind, folded refractor design.

#7 magic612

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Posted 21 February 2010 - 10:28 PM

Attached as a PDF to this post is the idea I've been toying with for a folded refractor design for a while now. Basically I figured that if I made the folded scope as a circle, it could then rest in a curved base that matches it, perhaps with some Teflon or UHMW tape on it. Then the base could be set on a mount that also swivels in azimuth, and I'd have a small, compact scope that could be set on a table like a small Dob, but be an unobstructed refractor. The diameter across that 5" scope I have drawn in CAD is only 19.5" in diameter. Cool thing about it is that it could even be made to have small storage compartments for eyepieces or barlows.

Alas, I've had so many other irons in the fire, this one is still sort of a, "Well, SOME day..." kind of thing. But it's definitely intriguing, and I've wondered why more manufacturers haven't done the same things.

Seems to me that a long focal length achromat and a few high quality mirrors would be less expensive than the glass required for an apochromat, and ought to be a great planetary performer. But... maybe it's just destined to be an "ATM-only" kind of thing, too.

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#8 KP3FT

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Posted 21 February 2010 - 11:03 PM

Do you know of any sources for quality flats? I originally wanted to make a "two-fold"
design, but the first larger mirror flat was prohibitively expensive, so I figured I would go with a "single-fold" like this (orange scope at bottom):
link
The cheapest way I've found so far is to use high-reflective (96%) quality Newtonian elliptical flats from Protostar or Antares. Surplus Shed has flats cheaper, but after websearching the original manufacturer part numbers, I found they have around 87% reflectivity; not so good especially when using more than one?

#9 Astrojensen

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Posted 22 February 2010 - 07:31 AM

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



Fun fact: The guy in the second picture, holding the 6" f/15 folded Zeiss refractor, is my friend and mentor Per Darnell, who passed away some years ago. The lens in the folded refractor was the objective of a WW2 german submarine periscope. Darnell knew both Sorgenfrey and Nemec personally and visited Nemec in Switzerland in the 1960'ies.

Clear skies!
Thomas, Denmark

#10 magic612

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Posted 24 February 2010 - 09:00 AM

Do you know of any sources for quality flats?


I don't other than what you mentioned. I was planning on doing a prototype using some Surplus Shed stuff, and then when I'd worked out the kinks, make a "nice" one with some good, high quality flats.

But that's also a project for down-the-road when I've got more time and money to put into it. I've got so many other basic projects right now, I've not gone beyond the basic design stage yet. At the moment, tripods, tube rings and telescope tubes have taken precedence.

#11 astroneil

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Posted 24 February 2010 - 09:57 AM

Hi,

Personally I'd love to see a commercially viable folded refractor design. I seem to recall a phone conversation I had with Barry Greiner of D&G Optical who said he made a few over the years.
Don't know if they look as cool as the long pencils pointed toward the sky though :lol:

#12 CounterWeight

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Posted 24 February 2010 - 11:14 AM

A while back there was this thread, don't know what happened other than maybe the general economic downturn affected plans...

#13 Ziggy943

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Posted 24 February 2010 - 01:08 PM

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


My thought exactly for a 6" F/8 also. The tube on a 6" F/8 is not all that long.

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.

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!

#14 magic612

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Posted 24 February 2010 - 01:45 PM

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.


A shorter moment arm, and often a much higher eyepiece placement, are the benefits. Though I agree with both you and Alan that there's not much point on a 3" instrument. :)

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!


Volume isn't as much an issue as the moment arm. Increasing the moment arm makes it more difficult to keep the tube steady (i.e. - vibrations, susceptible to even light winds, etc.). A shorter tube - even if greater in diameter (or volume) at similar weight/mass is going to be easier to keep steady on a mount.

And again, there's the issue of eyepiece placement. I have a 5" f/9 refractor - not quite as long as the 6" f/8 you mentioned, but darn close. I have a tripod and mount that has the altitude bearings at about the height of my eyes (nearly 6 feet) so that the focuser is about 3.5" feet off the ground when the scope is pointed at the zenith - and that's using counterweights on the eyepiece end to actually make the eyepiece height higher (by shifting the balance more towards the focuser end instead of the objective end). That still requires sitting in a chair and bending over to see in the eyepiece at that position.

The longer the tube, the higher the tripod must be to put the eyepiece at a comfortable level. Then the materials for the tripod must be longer, and stronger (and heavier). The folded design eliminates more than one problem, and introduces several benefits. Yes, mirror reflectivity is an issue, but reflectors have similar issues already, plus they deal with diffraction as well.

There are many trade offs to be had, and no perfect telescope exists. But in my mind, the folded refractor is a nice option that combines the best of several designs, and minimizes the drawbacks of others. :)

#15 Jared

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Posted 24 February 2010 - 02:35 PM

I believe Dietmar Hagar has a 9" folded f/9 TMB. You can find information about it on his website. He has certainly produced some superb results using this scope. I don't know whether TMB/APM made more than the one copy, though, or whether you could still order it.

#16 Ziggy943

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Posted 24 February 2010 - 03:45 PM

Fair points all.

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? For a longer f/ratio I think concept is more practical but it still wouldn't be for me.

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. Folding it would only cut the swing down to about 1', more or less.

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?

There probably be no savings in OTA weight. Sitting in an observing chair to look through the eyepiece is a good thing. An eyepiece 3½ feet off the ground is very comfortable.

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.

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?

#17 magic612

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Posted 24 February 2010 - 05:02 PM

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?


I mentioned these already - lower tripod required, less massive mount required, better eyepiece height, etc.

For a longer f/ratio I think concept is more practical but it still wouldn't be for me.


Fair enough - to each their own. :)

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.


Depends on how adjustable your chair is. Or how comfortable you are on a step-ladder, if the zenith eyepiece height is at a level higher than, say, 3.5" feet. (The other possibility is digging a hole or laying on the ground, but most people don't find this terribly comfortable or practical.)

There's more than one reason why shorter f/l refractors are more popular than longer ones.

Folding it would only cut the swing down to about 1', more or less.


For some people, that's a significant benefit. They don't need an adjustable chair with a minimum 2 foot adjustable height range. :)

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? :)

The swing distance of my 8" SCT was about 9"-10", I think. That was a huge benefit. Yes, other scopes have swing/height issues, but that's less of a factor for reflectors, given that we tend to want to view objects that are higher in the sky, and that places the eyepiece at a more advantageous position. With a standard refractor, the opposite is true.

So I certainly would consider doing that with a 6" f/8. I planned on doing that with my 5" f/9. Just haven't gotten around to it yet. ;)

There probably be no savings in OTA weight.


I don't think I said there was.

Sitting in an observing chair to look through the eyepiece is a good thing. An eyepiece 3½ feet off the ground is very comfortable.


Again, I agree. But a tripod 6 feet off the ground is not as stable as one 3.5 feet off the ground (unless it is more massive / heavier), wouldn't you agree? Doesn't the additional weight required for the longer scope become a disadvantage at some point? A folded refractor can get around this problem very nicely, so that 3.5 feet height never changes, and the mount is more stable at a lighter weight.

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.


Becoming a "sail" is exactly the problem, and one that is minimized with a folded design.

That said, a nice long refractor definitely looks freaking cool! :cool:

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?


I'd think the additional cost of a mount on which to adequately place a, say, 6" f/12 refractor is FAR more than the cost of two good quality flats, not to mention the smaller space required to store it and the lower cost of the smaller mount required to use it. :)

Cost is indeed a benefit, depending on what's being done. For a 5" f/9 or 6" f/8, perhaps not. But for larger - or at a very minimum, longer - scopes? Most definitely. :)

They're also easier to move outside without banging up the scope or one's residence! :grin:

#18 atelierbks

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Posted 24 February 2010 - 09:29 PM

A gentleman on http://telesc0pe.com/t2130a.htm has “refractified” a Meade 130mm reflector into a 127mm f/9 refractor which has interesting possibilities. This “upside-down Newt” configuration could be reworked into a Springfield fixed-focus mount of the type Russell Porter describes starting p.333 of Vol.2 in the ATM series. If, say, 8 to 10 inches were lopped off the tube at the top, the light cone could be directed via a larger Newt secondary into the polar axis, where a regular refractor diagonal would redirect the light to a focus. The observer sits down and doesn’t need to chase a tube all over the sky. The lopped tube would make a nice dewcap. One can readily see mods to Porter’s design that would lighten the assembly considerably. Additionally, the Dob-mounted folded refractor on the bottom of p.10 on the http://www.privatste...Faltrefraktoren weblink could be similarly modified to direct the light cone out the dec axis. Yahoo forum web page

#19 PJ Anway

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Posted 24 February 2010 - 09:35 PM

There is a Zeiss 200mm folded refractor for sale on eBay right now, if you care to bid: :smirk:

http://cgi.ebay.de/Z...=item2a04e6b3b8

#20 Jeff Morgan

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Posted 24 February 2010 - 10:48 PM

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? :)



If anyone is interested in a little first-hand experience ...

I became curious on the possibilities, so I acquired a 4" achromat with the goal of building a folded refractor. While advertised as f/14, it was actually f/13. I used the two mirror figure-4 layout to minimize reflections, avoid reversed images for star-hoping, and obtain a constant eyepiece height. Yes, there is a lot to be said for this as it is very comfortable to observe this way. (A Springfield would be even better.) There are some drawbacks too. Here are my observations:

1) I would not do it again with anything shorter than f/15. At f/18 the case for folding would be very strong.

2) The light loss was noticeable compared side-by-side to my 80mm APO. The 4" objective gave a brighter image, but not by much. And I was using very high quality flats (ProtoStar quartz with 96% coatings, brand new). I would most certainly avoid anything that utilized three reflections (remember, the star diagonal counts).

3) Speaking of flats, for significant shortening the first flat is going to be near the midpoint of the light path (otherwise, why bother?). This means that the first flat will need to be just slightly larger than 1/2 the diameter of your objective lens. This will not be inexpensive. A shorter APO or larger German equatorial mount (to handle the long "classic" tube) need to be considered.

4) While I could have optimized my geometry a bit better (which would have required larger flats), I still needed significant ballast in the rear of the OTA to make the balance where I wanted it. The set-up was manageable, but not grab-n-go. With refinement I could have made it somewhat lighter, but you can't make f/15 achro as light as f/7 apo.

All in all it was a fun project, but in the end it made little sense when I had an easy to use 80mm APO and an easy to use 12.5" Dob. If I suffered a back injury I would do it again. If I somehow came across an f/18 lens of significant aperture, I would probably do it again.

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#21 astroneil

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Posted 25 February 2010 - 05:41 AM

Hi Jeff,

Thanks for sharing your experiences of your folded refractor. I totally agree, not much point in doing it at less than F/15 with small apertures (<4").

Neil.

#22 magic612

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Posted 25 February 2010 - 09:10 AM

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.


Jeff, great thoughts, thanks for sharing! Great thoughts indeed, especially the disadvantages that I hadn't considered.

I can understand about the counterweight issue, even with the folded scope. My present 127mm f/9 scope utilizes 2 five pound counterweights near the focuser end just to keep the "swing" of the focuser height minimized to a 24 inch swing.

I also agree that these are likely better suited to longer scopes in higher f/ratios. To keep CA to a minimum, using the "3x the aperture in inches" rule-of-thumb applies. So for a 6" scope, that would require an f/18 scope - no short tube at 9 feet in length! Yes, 4 feet(ish) for a 4" f/13, 5" f/9 or 6" f/8 is manageable, but any longer and tripods and mounts get unwieldy for sure. (And only the 4" f/13 is within the 3x rule, too.)

That said, I'd still consider the design I posted earlier for my 5" f/9, though I wouldn't choose a 3 mirror design; I could live with two, minimizing the loss. Though for me, a lot of it has to do with me enjoying the process of building / designing / tinkering, so perhaps my strong arguments "for" should be taken with that in mind.

#23 dougspeterson

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Posted 25 February 2010 - 10:04 AM

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?

#24 Ziggy943

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Posted 25 February 2010 - 10:09 AM

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? :)


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.

It's also a different type of fold than the 3" Unitron example used above.

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.

#25 Ziggy943

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Posted 25 February 2010 - 10:24 AM

Nice job on that and thanks for sharing the experience.






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