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!"
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.
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.
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
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