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!"

Comments

Not much information

[Xolotl]

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.

It's a normal construction method

[fmaxwell]

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.

Re:different?

[donheff]

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.

Folded Newtonians are nothing new.

[fmaxwell]

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.