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Space Science Technology

Folded Newtonian Telescope 164

Posted by timothy
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!"
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Folded Newtonian Telescope

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  • Woohoo! (Score:3, Funny)

    by graveyardduckx (735761) on Tuesday February 03, 2004 @06:46AM (#8167624)
    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
  • different? (Score:4, Interesting)

    by grosa (648390) on Tuesday February 03, 2004 @06:51AM (#8167634)
    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)
    • by mangu (126918) on Tuesday February 03, 2004 @07:19AM (#8167699)
      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.

    • Re:different? (Score:5, Interesting)

      by donheff (110809) <donheffernan&yahoo,com> on Tuesday February 03, 2004 @07:40AM (#8167741)
      I built a six inch DOB with my daughter see photo at bottom of page [heffernans.org] 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.
      • I built a six inch DOB with my daughter see photo at bottom of page

        I gotta say though; she doesn't look too happy in the picture. Maybe you should have gone for the 18 inch DOB after all? ;-)

        Seriously though, nice pictures.

      • Faster link (Score:3, Informative)

        by Megane (129182)
        Those of you who just want to see the pic of the telescope itself, go here:

        http://www.heffernans.org/gifs/scope6.jpg [heffernans.org]

        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.

    • Re:different? (Score:4, Interesting)

      by OneOver137 (674481) on Tuesday February 03, 2004 @10:12AM (#8168336) Journal
      This design has been around for quite some time and telescope makers have been folding light paths of reflectors since their inception. Check out these designs [seds.org], which not only fold the light path but make it ubobstructed as well. HIs telescope, while nice for viewing deep sky objects, will likely produce low-contrast planetary images due to the large central obstruction. This project is really about optimizing a design around his viewing habits rather than anything revolutionary.
    • Re:different? (Score:3, Informative)

      by mwood (25379)
      Yeah, a Newtonian already has a folded path. I was wondering what was going on. This is a slightly different use of the word "folded", I think. The text is not precisely clear and the photo, while rather neat, didn't help as much as I expected.

      Instead of going straight out the side of the tube (normal to the tube's surface, that is), the path from the secondary mirror goes back in the general direction of the primary, but canted a bit so that you don't need a hole in the primary. The idea is to bring t
  • by schmack (32384) on Tuesday February 03, 2004 @06:52AM (#8167636)
    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...
  • by Anonymous Coward on Tuesday February 03, 2004 @06:53AM (#8167637)
    Only for the extreme hobbyist and universities.
    Probably hell of a lot cheaper than University telescopes!
    • by kfg (145172) on Tuesday February 03, 2004 @07:54AM (#8167783)
      True, but much of the point of the article is that it is within the reach of the extreme hobbiest to build a scope of this size.

      And I know any number of hobbiests who spent far more on bicycle racing, or a kart, or golf, or their stamp collection, or modifying their car, etc. Fairly lower middle class income types among them.

      Not to mention what the computer geeks/gamers I know have spent. The flight sim folks can get downright silly at times. God bless 'em; and I'd love a full motion cockpit myself.

      Call it passionate hobbiest rather than extreme, and I'd say anybody setting out to grind their own mirrors rather than drive to the mall and pick up a Meade is, by definition, passionate about telescopes.

      KFG
    • Which for the rest of the world, by today's exchange rates, is around the same cost as a loaf of bread.
    • You can build a traditional 18" newt for half that price.
  • Prices (Score:4, Informative)

    by imbaczek (690596) <imbaczek@@@poczta...fm> on Tuesday February 03, 2004 @06:53AM (#8167642) Journal
    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.)
    • Re:Prices (Score:5, Funny)

      by brad-d (30038) on Tuesday February 03, 2004 @07:03AM (#8167659)
      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.

    • Re:Prices (Score:2, Funny)

      by asifyoucare (302582)
      Bino Viewer $250
      Focusers
      1.25" $49
      2" $69
      4" $140
      etc.


      Quality nude pics of the neighbor's wife : Priceless
    • Compared to the 4 million GBP bid we're about to put in for a telescope, it's quite cheap. Mind you, I don't know what an 16" optical telescope should cost.
    • by deathcow (455995) on Tuesday February 03, 2004 @08:00AM (#8167800)
      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 [denkmeier.com] or for a Baader Planetarium [astro-physics.com] model.

      Focusers for $49... to $140 for a 4" model? puh-lease! Superb Feathertouch focusers [buytelescopes.com] are going to run you $300 at least for a 2" model. Clement Focusers [clementfocuser.com] are going to be around $400. AP [astro-physics.com] 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 [starmastertelescopes.com] 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! ;)
      • by MonkeyDluffy (577002) on Tuesday February 03, 2004 @10:22AM (#8168389)
        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.

        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

      • Walk the barrel my man, walk the barrel. Buy a lathe and milling machine. There is NO reason you have to spend that lind of bucks - do the labor yourself!

    • Re:Prices (Score:4, Informative)

      by cbmeeks (708172) on Tuesday February 03, 2004 @09:38AM (#8168140) Homepage
      >I wouldn't call $13298 Very Low Cost...

      You don't add all of that together. If you did, you would have about 3 full telescopes and another 16" mirror for a 4th. Plus, you would have mirror making equipment. It is quite possible to build that 18" scope you see for probably $1100 to $1500. With about 90% of the cost always being the mirror (pre-made).

      cb
    • by peu (163472)
      you don't have to add all the prices, just pick wich mirror/lens you want and discard the other sizes...
  • In comparison to...? (Score:3, Interesting)

    by Jugalator (259273) on Tuesday February 03, 2004 @06:55AM (#8167644) Journal
    "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?
  • Great idea and all, but his construction looks a little "mickey mouse". I hope that his telescope doesn't Fall apart to well. I think i'll stick to my nice shiny telescope in a big white tuben.
  • by fruey (563914) on Tuesday February 03, 2004 @06:59AM (#8167654) Homepage Journal
    Talking about his
    # 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?

    • ...if you point it at some interesting bedroom or bathroom windows... is it able to see better through slightly opaque curtains?

      Doubtfully, but if you are going to spy on people don't you think something less conspicuous then an 18" f/8 might be in order?

      • After "Can you see the rings around uranus", "Can you look at the neighbours?" is the oldest and lamest joke astronomers need to put up with. Come on guys, try something better! Ask us where the Eludium Q-36 Explosive Space Modulator goes!
        -aiabx
  • by FreeMath (230584)
    This really isn't much more than a truss tube dobsonain. Nicely done, but not that different.
  • Does this qualify as an invention? Is it patentable?

    If so, I hope a big corporation doesn't manage to scoop it behind his back. Any time you radically reduce the cost of something there's a big risk of that.

    • Re:I wonder... (Score:2, Offtopic)

      by orthogonal (588627)
      Does this qualify as an invention? Is it patentable?

      If so, I hope a big corporation doesn't manage to scoop it behind his back. Any time you radically reduce the cost of something there's a big risk of that.


      We look through the telescope and we wonder...
      why Darl McBride is staring back.

      Isn't it great the way modern patent and copyright law frees us all to concentrate on creativity and innovation, rather than legal minutia and protecting our work from overbroad and stealth patents?
    • Does this qualify as an invention? Is it patentable?

      Prior Art - I know Captain Ahab had a folding telescope when he was looking for Moby Dick. Unfortunately he was looking straight down when from the depths of hell he stabbed at it...

  • by Rosco P. Coltrane (209368) on Tuesday February 03, 2004 @07:11AM (#8167676)
    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.
  • by R.Caley (126968) on Tuesday February 03, 2004 @07:12AM (#8167679)
    Talk about culture clash. Imagine putting in an order ofr an `ultra portable' laptop and getting back something that size.
  • Not much information (Score:5, Interesting)

    by Xolotl (675282) on Tuesday February 03, 2004 @07:16AM (#8167690) Journal
    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.

    • by Lips (26363) on Tuesday February 03, 2004 @07:37AM (#8167731) Homepage Journal
      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.
  • by tanveer1979 (530624) on Tuesday February 03, 2004 @07:22AM (#8167706) Homepage Journal

    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 telescope
    • AFAIK, the box it's running on is a four-CPU solaris box with a 1Gbits/s pipe. My old homepage is on the same server. But then, it could be interesting to see if it can withstand /.ing when everybody loads the pics.
  • by SoupIsGood Food (1179) on Tuesday February 03, 2004 @07:43AM (#8167748)
    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 [aol.com], or this set of 22-inch newtonian binoculars [foothill.net] 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
  • by MonkeyDluffy (577002) on Tuesday February 03, 2004 @07:47AM (#8167759)
    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

  • 16" f5 (Score:5, Informative)

    by nacturation (646836) <nacturation.gmail@com> on Tuesday February 03, 2004 @08:05AM (#8167811) Journal
    Check out this telescope [sympatico.ca]. 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.
    • Re:16" f5 (Score:5, Informative)

      by nacturation (646836) <nacturation.gmail@com> on Tuesday February 03, 2004 @08:19AM (#8167846) Journal
      Oh, and not to totally karma whore, but from his main page there's a link to his ultra-portable 10" f5 [sympatico.ca]. Click and drool.
    • Re:16" f5 (Score:5, Interesting)

      by hey! (33014) on Tuesday February 03, 2004 @10:25AM (#8168418) Homepage Journal
      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?
      • 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".

        Yes, I was thinking about cameras and assumed it's similar for telescopes. In cameras, the f-stop number determines the amount of light let in, controlled (usually) by an iris aperature. The lower the f number, the larger the aperature of
  • But the Hubble Space Telescope and Chandra more or less eliminated my interest in building or buying a telescope of my own. No cold nights. No light pollution. No Setup problems, No searching the night sky for hours to discover I'm an incompetent fool and looking in the wrong place! The faulty optics were repaired. Those big images look great on my 23 inch cinema display. I really hope the JWT is not watered down to the point where it is not that useful. And I'm really enjoying the images from Mars...
  • by bakes (87194) on Tuesday February 03, 2004 @08:15AM (#8167835) Journal
    Wow! Can you imagine a Optical Telescope Arrays by Amateur Astronomers [slashdot.org] of these!
  • by fmaxwell (249001) on Tuesday February 03, 2004 @08:20AM (#8167851) Homepage Journal
    The folded Newtonian [oldham-optical.co.uk] 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 [irony.com].

    There are more ways to fold telescope optical paths than most people imagine as shown here [k12.wv.us].

    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.
    • by Anonymous Coward
      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 towar
      • Congrats, you win the prize for best /. post.

        I gotta get me one of those crackpot-o-meters. Does James Randi have the local distributorship for this octant of the galaxy?
    • Fmaxwell in general I agree with you that there is not much to see here. The central obstruction is on a par with a schmidt-cassegrain (SCT). For optical use (i.e. sticking your eye at the eyepiece) this will matter alot especially on
      targets like the moon, planets, or double stars where contrast matters. It shouldn't be too bad on faint fuzzies (galaxies, nebulae etc) and that 18" aperature doesn't hurt.

      He also complains that collimation in on an SCT
      is critical. It is critical but not particularly hard t
      • Thanks for your reply. A few points to consider:

        For optical use (i.e. sticking your eye at the eyepiece) this will matter alot especially on
        targets like the moon, planets, or double stars where contrast matters. It shouldn't be too bad on faint fuzzies (galaxies, nebulae etc) and that 18" aperature doesn't hurt.


        That's somewhat contrary to my experience. A target like a nebula is much more subtle in contrast than, say, craters in shadow relief on the moon. Deep space objects are low-brightness and low
  • by jalbro (82805) on Tuesday February 03, 2004 @08:24AM (#8167871)
    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
  • emplies (Score:1, Informative)

    by Anonymous Coward
    The article "emplies" something; could an English native tell me if that is slang, or... what?
  • by CProgrammer98 (240351) on Tuesday February 03, 2004 @09:03AM (#8167948) Homepage
    if he needs a 'scope THIS big to see them nekked
  • by p_trekkie (597206) on Tuesday February 03, 2004 @09:12AM (#8167990) Homepage
    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.
  • The scene: First meeting on an OSDN personals date...

    Her: "Is that a Folded Newtonian Telescope in your pocket, or are you just pleased to see me?"
  • Non-story (Score:5, Informative)

    by Pedrito (94783) on Tuesday February 03, 2004 @09:39AM (#8168148) Homepage
    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 [nasa.gov]), 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.
    • to point out that Dobson is a Buddist monk, not the Catholic sort.

      Not that this has any impact on his telescopes or importance (I've built one of his scopes myself) but for years I had a rather incorrect mental image of what he looked like. :-P
  • TLAs? (Score:3, Interesting)

    by Megane (129182) on Tuesday February 03, 2004 @09:51AM (#8168204) Homepage
    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?
  • Did anyone else see the photo and think Junkyard Wars: Race to Space?
  • by Anonymous Coward
    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 fie
  • I'm a telescopic layman, and I admit I only know peripherally about telescope design, but doesn't having an open telescope like this allow light to bleed onto the mirror? Doesn't he need to cover it up with a tube from the lens to the mirror, or at least wrap a tarp around this? How can this thing work if the street light down the road is blurring my view of Jupiter?
    • Correct. If you're using a telescope like this in a light-polluted area, one typically wraps it in a shroud to prevent stray light reflecting into the light path. Out under dark skies, you leave the tube open to allow air to circulate and bring the scope to thermal equilibrium quicker.
      -aiabx
    • by Boulder Geek (137307) <archer@goldenagewireless.net> on Tuesday February 03, 2004 @01:14PM (#8170521)
      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.

  • by Sir_Kurt (92864) on Tuesday February 03, 2004 @10:30AM (#8168473)
    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!
  • Shheezzz, as others have said, this concept is perhaps a hundred years old.
  • by mwnuk (748765)
    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
  • Bad Idea (Score:3, Funny)

    by DotWarner (56614) on Tuesday February 03, 2004 @03:30PM (#8172435)
    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".
  • by PassiveLurker (205754) on Tuesday February 03, 2004 @07:11PM (#8175039) Homepage
    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.

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