Slashdot Mirror
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!"
So now instead of using my telescope to look at my neighbor's 20 year old daughter, I can look at THEIR neighbor's cuter 20 year old daughter! Yay! +5 Hormonal
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)
A job well done -- always nice to see someone revist and improve on past research rather than blindly following the current trends.
The only problem with my knowledge of Newton these days is that 70% of it involves Waterhouse and other Neal Stephenson creations.
I mean I could regale you with tales of Newton deforming his eye with a knitting needle but it could be complete fiction...
--
Rare Window - free your photos
Only for the extreme hobbyist and universities.
Probably hell of a lot cheaper than University telescopes!
Bino Viewer $250
Focusers
1.25" $49
2" $69
4" $140
Equatorial Table $275
Mirror Grinder
10" $250
20" $475
40" $1800
80mm Binocular Holder $250
16" DOB $1800
18" DOB $2200
16" Mirror $900
2" Adapter $30
Led Colimator $30
Silvering Kit $50
Encoder $30
Guide Scope $40
Interferometer 10" $160
10" Folded Schmidt Camera $4500
I wouldn't call $13298 Very Low Cost...
(Assuming all of this stuff is needed of course.)
"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?
Beware: In C++, your friends can see your privates!
# 30% reduction in tube length
# 50% reduction in eye piece height
# 4x more back focus
Compared to, of course, smaller secondaries...
It's a semi interesting page but frankly doesn't do it for me. Great for telescope geeks no doubt. But the key question he surely missed... if you point it at some interesting bedroom or bathroom windows... is it able to see better through slightly opaque curtains?
Conversion Rate Optimisation French / English consultant
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18" f: 8 Folded Newtonian
Weight 70lb
Eyepiece Ht. at Zenith 5'4"
33% Obstruction
3 Min: Setup - Ultra Portable
I assume the guy doesn't live in an appartment. My 12' Schmidt Cassegrain telescope however, while less interesting that this project, can be moved with a bicycle trailer to go stargazing on the high hill near my place, and doubles as a handy tool to watch my neighbours' boobs in the appt complex down the street.
"A door is what a dog is perpetually on the wrong side of" - Ogden Nash
Talk about culture clash. Imagine putting in an order ofr an `ultra portable' laptop and getting back something that size.
_O_
.|< The named which can be named is not the true named
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.
Usually, the secondary mirror is elliptical and at a 45 degrees angle. In this case, it's a circular mirror at a 15 degrees angle. This puts the eyepiece closer to the main mirror, making it easier to mount a long focal distance telescope. Notice the eyepiece position spec. A circular secondary mirror is easier to make than the usual elliptical that's required if you mount it at 45 degrees. A larger secondary mirror has a lot of advantages (listed in the article) at the cost of more obstruction.
The world will beat a path to the door of the man who builds a better mousetrap
Now it should read: Slashdot will burn a hole in the server of the man who builds a better telescopeMy Aurora : http://www.youtube.com/watch?v=o91ZsGwJYyg
FB : https://www.facebook.com/TanveersPhotography
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.
Flatness measurements, often represented as fractions of the height of a lightwave, smaller fractions are better) for hand-figured mirrors from amateur telescope makers are about as reliable as performance gains claimed by enthusiastic overclockers. Large doses of salt required unless verified by a reputable third party.
As homebrew telescopes go, this one isn't terribly refined. It uses a unique optical arrangement, but not all that unique. Check out this folded refractor, or this set of 22-inch newtonian binoculars for some real jaw-droppers. (Also check out that last guy's all-metal 14-1/2" Alt-Az telescope... truly a beautiful instrument, even if it's a conventional design.)
There are a ton of exotic telescope designs out there being crafted by enthusiastic hobbyists, many of them on-par with deleriously expensive research-grade instruments. Most of them aren't made out of cheap plywood and bed rails. (I plan on building a 12" off-axis newtonian this summer.)
SoupIsGood Food
I would imagine that it must have been a bitch to figure (shape) the mirror - it's not a simple parabola, and would require much more effort than a conventional mirror the same diameter. Kudos to Mike Fallwell for doing something different!
-MDL
Happy meals fund terrorism
This looks like low quality or poorly made stuff. Basically, in Astronomy, you get what you pay for. Quality is going to be LOW at these prices.
;)
$250 for a binoviewer will get you crap! The good ones are about $900 - $1500 for a Denkmeier or for a Baader Planetarium model.
Focusers for $49... to $140 for a 4" model? puh-lease! Superb Feathertouch focusers are going to run you $300 at least for a 2" model. Clement Focusers are going to be around $400. AP focusers are going to be $400 - $700.
And the biggest problem of all. 16" mirror for $900?? 18" DOB for $2200?? Go fish! Some crackpipe dreams here. Superbly figured mirrors, which focus light superbly well, in well built dob structures, are going to run you into bucks. A quality 18" dobsonian telescope like a Starmaster is going to run you $6,400 without any options, a far cry above $2200.
I'll put my refractor up against this guys mirrors any day!
Check out this telescope. It weighs a total of 52 pounds (40 was the target) instead of the article's 70 pounder, and has an f5 aperature instead of f8 so it lets in more light. Very similar construction, but this one was made 6 years ago.
Want to improve your Karma? Instead of "Post Anonymously", try the "Post Humously" option.
Wow! Can you imagine a Optical Telescope Arrays by Amateur Astronomers of these!
Ho! Haha! Guard! Turn! Parry! Dodge! Spin! Ha! Thrust!
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.
IAAATM (I am an amature telescope maker, working on my third design)
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There isn't much to see here. This is an old concept, one with advantages and disadvantages.
The main issue is that a folded design allows for a lower eyepiece height when you have a long focal length.
A long focal length mirror is faster to make (less grinding) and easier to figure (making a high quality mirror is easier when it is shallower).
The problem with a long focal length is you end up needing a ladder. You also lose the ability to get the brightest images (exit pupils of 7mm) when you go over an f/6.
The folding also introduces loss of contrast... from both the big secondary and the MAJOR baffling problem. You run the risk of extra star light entering the eyepiece and washing out the image when the eyepiece is pointed up.
So this design is nothing more than what this designer wanted for trade offs. There is no major design advances that lets an ATM do something they couldn't do before.
For more designs, check out:
http://members.efn.org/~mbartels/tm/ul-dobs
(scroll to the bottom)
and specifically another folded design...
http://www.irony.com/Ed/astro/18inch/
-Jeff
Your reference is a bit simplistic since it focussus
on consumer products. Maks and
Schmidts are not subtypes, they are are Cadioptics based on the Cassegrain design which have
a corrector plate at the front. A cassegrain
(note no prefix) has no corrector plate. Spoken
as one who has worked with several cassegrain telescopes.
if he needs a 'scope THIS big to see them nekked
And the people shall be oppressed, every one by another, and every one by his neighbour Isaiah 3:5
In other words, one of those pictures that show where the light rays go? Those generally tend to do a better job of explaining the setup than a picture of the scope and a long description.
http://www.heffernans.org/gifs/scope6.jpg
It's a bit slow to load the whole page, and the picture is the last one on the page. I'm sure his bandwidth will thank you.
#naabhaprzrag, #sverubfr-000, #agi-fcbafberq, negvpyr[pynff*=' negvpyr-ary-'] { qvfcynl: abar !vzcbegnag; }
Sorry, but as others have pointed out, this is a non-story. It's a variation of a Dobsonian design. A real story is the story of John Dobson (short bio here), a monk from San Francisco who designed and built the original Dobsonian telescopes and got people interested in astronomy by taking his telescopes to the streets. Being a monk, he lived in poverty and built his telescope as cheaply as possible. Because he had to continue living in poverty, he was unable to sell them and become rich, so now the bigger telescope makers are making money off of his design.
You wanna run a story about amateur telescopes, that's a good one. Or I could point you to the story of the three guys who ground their own 30" mirror and built a telescope from that. There's a lot of cool stuff being done by amateurs. Sorry, but this isn't that cool compared to most of it.
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?
#naabhaprzrag, #sverubfr-000, #agi-fcbafberq, negvpyr[pynff*=' negvpyr-ary-'] { qvfcynl: abar !vzcbegnag; }
Let me check the trace from my crackpot-o-meter:
+2: frequent spelling mistakes
+4: train of thought derailments
+5: extraneous figures
+7: grandious claims not backed up by facts
+6: derision towards common theories/devices
-5: cool picture of finished product
crack-point total: 19
crackpot category: eccentric
Re-running the scan after callibrating for your input, we get:
+2: frequent spelling mistakes
+4: train of thought derailments
+5: extraneous figures
+7: grandious claims not backed up by facts
+6: derision towards common theories/devices
-5: cool picture of finished product
+10: ignorance of standard knowledge in the field
+5: misspelling common technical terms
+15: claiming common techniques as unique innovations
crack-point total: 39
crackpot category: wingnut
Thanks for helping tune my crackpot-o-meter for astronomy.
I have been an amatuer telescope nut (ATN)for quite a few years. Main reason to build a telescope yourself (other than the self satisfying geek factor) is that it is possible to build a very high quality scope (better than any consumer grade scope) for cheap.
It is quite within the range of any reasonbly competent person to grind and figuer a telescope mirror so that it is diffraction limited. This means that the surface has been shaped close enough to the ideal parabola (for a newtonian design) that the limiting factor for resolution is the wavelength of light you are interested in. In other words, the mirror is opticaly perfect.
The larger you go, the more difficult it is to do this, however.
unfortunatly, the design presented in the article seems to have been optimized solely for a lower eyepiece height, and all the rest of the choices made will result in a less than optimal image. eyepiece height.
the plate glass will make in very sensitive to temperature changes.
the large central obstruction will reduce contrast
the spray on mirror coating will almost certainly change the effective shape of the figure.
I could go on. There is not much to recommend this design other than eyepiece height.
But hey, the guy built his own telescope, and it works!
Light coming in from far enough off axis won't reflect into the focal plane. In this regard, most "light shrouds" actually make things worse by providing a surface for off-axis light to reflect off of on its way to the focal plane. No shroud = no reflecting surface.
A newtonian can be easily tested by looking into the focuser w/out an eyepiece. If all you see is black and the mirror, the scope is adequately baffled, and no shroud or tube will improve it.
A well-crafted lie appears unquestionable - Dama Mahaleo
Here's a design for a Newtonian reflector that uses the building as the tube. It works well conceptually. The eyepiece(s) are in the middle of the tube itself, but because the mirror is 18feet in diameter, and it focuses to infinity, there is not that much image degradation. The site is www.digitaldarc.net goto 'projects' - 'observatory' Mike
The site is slashdotted, so I figured someone would have posted a duplicate link.
I really should have known not to search the comments of this story for "mirror".
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.