Folded Refracting Telescope?

[mjperini]

Bob,
As a Boy I was an Amateur telescope builder / assembler, and have always had an interest, although I've done almost nothing with it for many years.
I did purchase a little 80mm f/7.5  with a 2" focuser for my house in Montauk pt NY which can be fun.
But I do occasionally read stuff with the vague Idea of someday upgrading to something better -but not crazy maybe a 4" Apo (Which seems to be a bit loosly used by some manufacturers)
Then I came across this  http://www.zerochromat.net/features.html
They claim true Apochromatic performance from an 8" / 203mm f/12  double convex singlet corrected by mirrors and a rear lens system.
Not that I'm thinking of buying one, but is this a scientifically viable design ? They claim a Strehl ratio of 0.995.
?

[Bob Atkins]

I don't see why it's not a viable design, though they don't give details of the precise optical configuration. The singlet objective would have chromatic aberration, but that could presumably be corrected by the rear lens group. Both the mirror(s) and the rear lens group could correct the Seidel aberrations.

Seems like it would make a very nice planetary telescope. At 2400mm and f12 it's a little slow for photography and wide field views might be tricky. Sounds pretty nice though and the price is certainly well below what you'd pay for an 8" refractor with a conventional doublet or triplet objective using exotic glass or fluorite.

I have a 4" f5 (500mm fl) Genesis APO (with a fluorite corrector element)  which is a nice compact scope and works well for photography and wider field viewing. It's still reasonably portable too.

[klindup]

The other way that seems to. work well is to use  ELD glass in the doublet at the front and then to mount  two more elements at the rear to flatten the field and to  reduce the focal length.  This gives a nice 540mm f 5.4 that is aberration free.  It has a wide field  of view and also makes a very sharp long focus lens for terrestrial photography.  It is bigger and heavier than a Canon equivalent lens, especially with its tripod and is of fixed aperture.   If you can work with these limitations you have a superb telescope and  camera lens in one package.  If you go to the Televue website you can see examples of some of the results people have achieved with one brand of such a telescope.

Ken Lindup

[Bob Atkins]

That's the classic Petzvel lens design which was developed in the mid 1800s. It's what Televue use on their short refractors like mine 500mm f5 and the newer 540mm f5.4. They also use a fluorite element in the correction doublet to better suppress chromatic aberration.

The objective is actually a long focus doublet and the rear doublet not onlt coorects aberrations but shortens the final focal length.

It does make a very fine camera lens, but, as I can tell you from carrying one out to Yellowstone many years ago, it's very long and rather heavy for a 500mm lens.

[mjperini]

TeleVue makes great stuff.
So in a Petval view camera lens we two doublets -one on either side of the iris, in the TeleVue , no iris bit the doublets are much further apart (correct?)
The rear doublet in the TeleVue is is the second half of the Petzval Objective and  different than a "field flattener"  TeleVue brand name Parracor I think-- is that correct?
Is the only purpose of the field flattener to make the image plane coplanar with the sensor? or is it for visual observation too.
And is the field flattener different yet again from the focal reducers I see for SCT's ?

I own a 24" f/11 Apo Artar which is also a 4 element design, is it also Petzval derived?

And finally speaking of high quality four element systems, I seem to remember Leica having some very simple (as in few elements) telephotos, An early Telyt or maybe Tele Elmar??  I can't find it at the moment but I thought it had only 4 elements  would that have been a petzval derived design?

Thank you for this education ; -))

[klindup]

The Parracor is a doublet that can be used with a Newtonian telescope to correct for coma.  In the current Televue  Petzval design the rear doublet is an integral part of the telescope and cannot be removed.  I believe that they used to sell another model (NP102)  where the rear doublet was an optional extra.   However as Bob says, one of these scopes weighs around 10 lbs and is 26 inches long so is not really portable.  And that goes without the bulk and weight of the tripod which is even heavier.
I am curious about one thing though.  Canon use more elements in their 500mm.  OK it is f4 and much shorter and double the cost.  How does the Canon offering compare in image quality with a Televue such as the NP101?
Ken

[Bob Atkins]

For Photography, the Canon 500mm lens is very similar to the Televue Genesis. I've tested the two side by side. However the Canon wins out in terms of size, weight and convenience (as well as having a variable aperture, autofocus and image stabilization!). I didn't test field flatness, just resolution in the central region of the image.

However in an absolute resolution contest (e.g. splitting close double stars) I suspect that the Televue would win. A lens really doesn't need to be (and rarely is) diffraction limited wide open. The limitations of film and digital sensors mean they don't need 1/10th wave optics.

I do have a way of putting an eyepiece on a FD Canon 500/4.5L converted to an EF mount (see http://www.bobatkins.com/photography/technical/Lens-scope-adapter.html) that I have and it makes an excellent telescope. However it's limited to 0.965" eyepieces because there's not enough back focus distance to get a 1.25" diagonal and eyepiece in there. I don't have any high quality short focal length 0.965" eyepieces to do resolution tests with the lens.

Field flatteners are for photographic use, often when you want to shoot with a full frame 35mm or Medium format camera. They are often used with telescopes which just have a doublet or triplet objective. I think the Genesis is pretty flat field without a flattener, because the second doublet flattens the field as well as shortens the objective focal length (i.e. act as a reducer for the ~1000mm fl of the objective doublet) and compensates for chromatic and seidel aberrations. I'm not sure if there's a field stop between the front doublet and the rear doublet in the Genesis, but I haven't ever looked at it closely.