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Folded Newtonian Telescope
johanneswilm writes "Michael Fallwell has figured out a way to overcome many of the problems of traditional telescope construction - making it way more compact and economical. And the whole thing is completely portable and achieves accuracy down to one or two millionths of an inch across an 18 inch surface!"
a question to those who've built their own newtonians:
how is this fundamentally different?
(to me, the picture looks basically like a standard newtonian)
Only for the extreme hobbyist and universities.
Probably hell of a lot cheaper than University telescopes!
"And the whole thing is completely portable and achieves accuracy down to one or two millionth of an inch across an 18 inch surface!"
Wow, that sounds really cool and accurate. But current telescopes are obviously also very very accurate too. So this doesn't really say much. I wonder how good this one is compared to current ones, made for the same purpose. Is there a noticeable loss of quality?
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The figure itself is stabilized by a trick developed years ago for stabilizing glass lasers eliminitaing any need for Pyrex at least for mirrors of this size.
The reason for using Pyrex is thermal stability (ordinary glass expands, changing the carefully-worked shape in the process). What's this trick then? How does it work? Being able to use plate glass effectively would make amateur telescope making much easier, yet I've never heard of this method. Some references would be nice.
Would you trust a computer review which said something like "this machine is cooled using a trick developed years ago for cooling nuclear reactors, eleminating the need for fans for a processor of this speed" without some kind of additional information? That's what this article sounds like to me.
These kind of statements and the lack of, say, an optical diagram, make it very hard to judge the article. Theres a photo of a guy with a telescope, so I guess he built it, but I'd prefer to see some more concrete information and proper test results (diffraction rings, spot diagrams, whatever).
The price list is strange - an encoder? There's no drive on that thing. A $4500 Schmidt camera? that has nothing to do with this telescope (its a kind of telescope in itself, used for very wide fields). 40" mirror grinder? 16" mirror? The article talks about an 18" mirror telscope. The only thing I can think of is that this an attempt at a price comparison with other technology.
In short, interesting, but strange.
He's using a truss tube design similar to that used by many makers of large commercial Dobsonian telescopes. In addition to being very lightweight, it's easily broken-down for transport. The triangulation makes it extremely strong and rigid.
It may look spindly, but it's a good design.
The folded Newtonian is nothing new, though the design described here is a bit odd, to say the least. Here's an example of a similarly designed scope with much better construction.
There are more ways to fold telescope optical paths than most people imagine as shown here.
That said, the referenced article is filled with inaccuracies and I almost wonder if it's intended as some kind of practical joke. For example, it describes the "tracking accuracy" of Schmidt Cassegrains, Newtonians, and Folded Newtonians as "poor", "poor", and "very high" respectively. That's bunk. The tracking accuracy is determined by the mounting and drive. In the case of his scope, it's on an altazimuth (Dobsonian, to be specific) mount with no apparent drive at all, so it doesn't track anything! The author mispells Cassegrain repeatedly throughout the article, which I would hardly expect from someone knowledgeable about telescope optics. He describes the mount of a conventional Newtonian primary mirror as "fussy" while describing the mount of the primary in the folded Newtonian as "robust." There is no difference. The folding of the light path at the other end of the tube has nothing to do with how the primary is mounted. He describes the "weight" of Cassegrains and Newtonians as "heavy" and classifies the Folded Newtonian as "Very Light", yet there is no evidence that his folded Newtonian is any lighter than a conventional Newtonian -- and it's probably heavier due to the larger secondary, larger secondary mount, and the baffled tube that holds the focuser. He says that the "Field Width" of Schmidt Cassegrains, Newtonians, and his Folded Newtonian are "Narrow", "Wide", and "Very Wide" respectively. That's simply wrong and illogical -- as anyone with a reasonable knowledge of telescope optics can tell you. The tilt of the secondary mirror has no effect on real or apparent field width. In fact, because he is advocating a longer focal ratio (f8), he will have a narrower real field of view than a typical Dobsonian Newtonian (typically f4-f6) with the same eyepiece.
He makes absurd claims like "So the only real advantage of a small diagonal in a large telescope is a tiny improvement in contrast/resolution that can easily be recaptured with image processing." Anyone who knows anything about telescope construction can tell you that the secondary obstruction causes light loss and that's a serious concern. Also, image processing implies astrophotography. Astrophotography implies long exposure times and that necessitates an equatorially mounted telescope -- which his is not.
I don't find the article to be at all credible.
For those of us who haven't been watching the stars all night and just woke up, could we have a few jargon definitions here? ATM got explained, but what's a DOB?
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This has been done to death. I can probably locate a 50+ year old article by an amateur astronomer that did this at one point. What's so great about a "folded" (no, it isn't really) 18" f/8 newt? Great, less coma! Long fl so I can get up close and personal with Jupiter or Mars or Saturn... Oh, but there's a 33% obstruction. There goes contrast. And you don't need all that much light gathering for the planets, they're pretty bright already. You need good contrast to see detail.
Hmm, so it's good for wide field views then! Oh, but its long fl makes it less attractive for that, unless I use very low power, very expensive 2" eyepieces. And the obstruction sorta leaves visual viewing less than impressive. I don't want to spend $20,000 on a 2 ton steel and concrete mount to do good astrophotography with this, either.
Tell ya what, I'll put my home-made f/3 primary, f/18 Gregorian system, 19% central obstruction 12.5" telescope on a G-11 up against his 18" on planetary or galaxy viewing any day. And my 12" f/5 is nothing to sneeze at either on the DSOs.
If you grind your own mirror, you can make an 18" scope for under $2200. And an amateur can grid an excellent mirror - it doesn't take exotic equipment to do it.
A quality 18" dobsonian telescope like a Starmaster is going to run you $6,400 without any options
I'm surprized that they are now over $1K more than an Obsession Telescope.
I'll put my refractor up against this guys mirrors any day! ;)
Even an 8" AP refractor is toast against a 18" dob with a very good mirror on most objects. The slight advantage on planets is demolished by the dobs better reach on Deep Space Objects. And how much for a 206 Starfire EDF, with mount? $25K to $50K? (used, of course).
-MDL
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Actually, the amount of light let in has nothing to do with the f ratio. It's wholly a function of the clear diameter of the mirror or lens. In this case the smaller central obstruction means that his longer focal length design will let in more light, albeit probably not enough to be very significant.
We're accustomed to think of large f numbers as "slow" because of cameras. I don't know much about cameras, but I suppose because there is a fixed area in the focal plane you are exposing; this translates to different clear diameters through which the film is "looking". You could have a long focal length lens with a huge diameter, but since the image it would create would be bigger than the film there's no point: you're stuck with smaller useful aperatures for longer f numbers. With telescopes the area of the focal plane you are examining is dependent upon the eyepiece you use: its focal length and FOV. That is to say that if I choose eyepieces to provide the same magnification, objects should look equally bright in two scopes with the same aperature and different focal lengths.
Generally speaking, things get optically better the longer the f number you choose, but mechanically worse. For example, any eyepiece will work quite well in an f6 scope, but for f4 you need a pretty good eyepiece to get a good view. An f15 reflector mirror could be figured spherically and perform well, but an f5 must be parabolic. An f15 refractor's objective lens focuses all colors in the same place, but an f4 requries exotic materials to get close to the same peformance. The list goes on and on. If convenience is no object, then longer is better.
The problem with long focal length scopes is that, in conventional designs, they are mechanically impractical. Eyepieces have to go to two inches, then higher for adequate fields of view; mounts have to become larger, and heavier; you have to climb ladders to look through them etc.
What this guy is doing is exploiting another optical advantage of long focal lengths to mitigate their mechanical inconveniences. To wit: he's exploiting off axis performance to acheive a comfortable viewing position. In a fast (f4.5) scope, the stars in the center are sharp, but at the edge they tend to be spread out like a comet. This effect is not noticeable in long focal lengths like f8. By folding the optical path, the observer can stand on the ground and look through the eyepiece; he is viewing the entire image off axis, but it probably is not too bad given the relatively long focal length. Where the sweet spot is is probably hard to say. He could have gone with a 12" f12 and had an optically superior system with the same mechanical advantages, but I suspect you aren't going to gain much ATM mojo with anything smaller than 18". Over 18" then you're back to the stepladder.
This guy is not the first person to think of this. I've seen references to this approach in ATM books, and I think I even remember an ad for a commercially produced scope, of smaller aperature.It's probably not popular because it doesn't meet most people's needs. A scope involves so man tradeoffs between optical performacnes under certain conditions and convenience, that what makes a scope "good" is surprisingly subjective. For example, this design is not going to be good for astrophotography; it might not work well with wide field eyepieces; it may be great for planetary work; who knows?
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Interesting...very interesting. As someone who has built their own newtonian, I feel obliged to comment...
One thing that's important to realize is that any telescope is a compromise. However, this design makes some compromises that I don't know I would be willing to make.
The obvious benefit of such a design is to get a large aperture and a long focal-length without having to balance on a ladder. In general, if you want an 18" newtonian scope, you'd have to go down to a focal ratio of 4.25 or less to stay on the ground (that corresponds to a focal length of (4.25 x 18") = 76.5"). The problem with short focal length scopes is that they have to be much more accurate for their aperture...basically, it's easier to get a really good figure for a long focal length mirror. Long focal length scopes also have less coma (a certain kind of aberration), so kudos to him for this design with a focal ratio of 8.
However, I see three serious problems with this design:
1) Secondary size. In order to pack a greater percentage of that long focal length into the beam after reflection from the secondary, you have to make your secondary significantly bigger. This, to me, is unacceptable. He's using a 6" secondary, which is covering fully 33% of the main mirror's aperture. Not only does this cut down on the total light you see, but also reduces the minimum angular resolution...as long focal length scopes excel at high-res viewing, you're essentially shooting yourself in the foot right after you bought a really excellent foot. To give you a basis for comparison, my scope has only 21.6% of the primary covered by the secondary (mine also has a focal ratio of 7.5).
2) That 15 degree angle has got to be killer. When constructing scopes, it's plenty easy (er, well, easier, anyway) to make a perpendicular angle from your secondary. It seems like lining up that 15 degree angle correctly (known as collimation) every time you set up the scope is going to be difficult at best, especially when you have to line the "eyepiece tube" up at a 30 degree angle every time, as well. A couple degrees off and you're already introducing significant aberration.
3) Viewing angle. How do I look through an eyepiece that's only 30 degrees off the optical axis? With difficulty, at best. One of the main purposes of the scope - viewing comfort - is compromised by this. The obvious solution is to use a mirror diagonal, but that, again, is then only cutting more into the amount of light you see (no surface reflects 100% of the light), as well as presenting the potential for more surface defects.