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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!"
Quite true. A particularly nice example exists on the old 74" telescope at the David Dunlap Observatory. The encoders feed into a computer which displays not the absolute position but the difference between the current position and where you want to point to. All you have to do is move the telescope until the display shows zero. (The 1920's design of the telescope makes it impractical to fully automate, large movements are done by hand, once the telescope has been roughly pointed the automatic guiding system takes over. The offset encoder system is very accurate though.)
My original point was that the price list seems to have little relation to the telescope itself. Neither the photos nor the list of dissasembled parts shows an encoder, hand controller or the like. I'd be interested to read your ATM mentors comments, perhaps you could post them up here as reply when you have them.
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
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.
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!
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 to do as there is only one variable to play with (as opposed to two in a newtonian). Once collimated most SCT's hold collimation well. Because his design gets reassembled every time he's going top probably tweak collimation every time he sets up.
One point where I disagree is on your statement about astrophotography. Film Astrophotography requires long exposures (and a well aligned equatorial mount). CCD astrophotography has some folks that use lots of short (order of a couple seconds) exposures that are then derotated and and aligned and then digitally composited. I'm still not sure this scope would be a winner for that as it might tend to shake especially if there's wind and once you have all the heavy imaging hardware at the focal point.
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-contrast compared to stars, planets, the moon, etc. Sadly, by going with such a large central obstruction, he loses a significant amount of the light that he's gathered, though.
I agree with your comments on the collimation of SCTs. I have three Celestrons and they simply never even need touch-ups.
CCD astrophotography has some folks that use lots of short (order of a couple seconds) exposures that are then derotated and and aligned and then digitally composited.
Although such imaging is sometimes done, it's normally done with a driven scope (although not necessarily equatorially mounted). Without a driven scope, the CCD-captured images must not only be derotated, but they also must be shifted. So the only real way to do it is with lots of images is to reposition the scope manually between sets of exposures. The truss tube design will tend to lessen wind-driven shaking, but you're right about the weight issue. Dobsonians need some kind of adjustable counterweight system (or sliding saddle) in order to handle heavy accessories -- something which his scope lacks.
I could have gone on further, but resisted the temptation to write a book. In short, I find no advantage to his design that's not bettered by the traditional folded Newtonian. In fact, the thought of craning my neck skywards to see the eyepiece makes me crince. I'd much rather have a traditional eyepiece mounted perpendicular to the tube.
If none of his claims were meant as tongue-in-cheek, then I suggest that he do some more studying of telescope optics, mounts, astrophotography, and amateur astronomy in general. That said, the construction of any telescope, especially one of that aperture, is a major undertaking and I wish him the best for his efforts.
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