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Folded Newtonian Telescope

Posted by timothy on from the carry-on-luggage dept.

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!"

10 of 164 comments (clear)

  1. Re:Prices by brad-d · · Score: 5, Funny
    Bino Viewer $250
    Focusers
    1.25" $49
    2" $69
    4" $140

    I wouldn't call $13298 Very Low Cost...
    ... and so on

    You must be new around here. The correct take off of a Ma$tercard add is:

    18" DOB $2200
    Mirror Grinder 40" $1800
    10" Folded Schmidt Camera $4500

    Finding out you spent ALL your money on glass and plastic and are getting kicked out of your apartment. Priceless.

    For some things in life there is Slashdot, for everything else there is Google.

    --
    -Brad
  2. Not much information by Xolotl · · Score: 5, Interesting
    There's actually very little concrete information in the article, just some very general and often confusing statements, like:

    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.

    1. Re:Not much information by Lips · · Score: 5, Informative

      You don't need a drive to use of an encoder. There are hand controllers which take input from encoders, but provide instructions, left/right/up/down x units, so that a human user can point the thing in the right direction. Something like this: http://www.wildcard-innovations.com.au/

      Last year I finished my first, an 8" f6.6 and the figuring was rather hard. I think my next attempt should be better, but something like this at f8 is much easier to figure!

      I'll show the article to my ATM mentor and see what he thinks about it. Especially some of those contentious sentences.

  3. Secondary mirror angle by mangu · · Score: 5, Informative
    how is this fundamentally different?


    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.

  4. It's a normal construction method by fmaxwell · · Score: 5, Interesting

    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.

  5. Re:different? by donheff · · Score: 5, Interesting

    I built a six inch DOB with my daughter see photo at bottom of page but we were not confident going for a 12 to 18 incher because of the size, weight, and overall difficulty. This design looks different than any I saw when my daughter and I researched DOBs. The larger, circular secondary picks up a lot more light from the primary and the extreme angle reflects the light back down the structure to a lower eyepiece increasing the length of the light path without increasing the length of the scope. This is the "folding" that allows the scope to be much shorter and lighter than is the case with a standard model. I don't know anything about the silvering approach he mentions, but it sounds interesting. Grinding and silvering the primary is a big deal with mirrors 12 inches and up. 18 inches (his scope) is quite difficult.

  6. My favorite Homebrew 'Scopes by SoupIsGood+Food · · Score: 5, Informative

    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

  7. Huge central obstruction by MonkeyDluffy · · Score: 5, Informative
    The secondary mirror is a huge 33% (by diameter) - usually, for a telescope like this you would try for around 20% or so. The larger the central obstruction, the lower the contrast. The upside is that it is a F8 (focal length/diameter ratio) scope, so that it is easier to collumnate (keep the mirrors in proper alignment) and will have less coma (stars near the outer edge are more eliptical, instead of circular points).

    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
  8. Folded Newtonians are nothing new. by fmaxwell · · Score: 5, Interesting

    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.

  9. Not much to see here... by jalbro · · Score: 5, Informative

    IAAATM (I am an amature telescope maker, working on my third design)

    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. html
    (scroll to the bottom)

    and specifically another folded design...
    http://www.irony.com/Ed/astro/18inch/

    -Jeff