astrokido
sage
Reged: 06/09/08
Posts: 244
Loc: Phoenix, AZ
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Off-axis primary mirrors exist in some special-purpose research scopes. It seems like a lot could be gained by using an off-axis primary in Newts. What's so difficult about making an off-axis primary? Any idea about collimating such a beast?
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- Gill C. - All opinions subject to a speed limit of 299,792,458 m/s unless noted otherwise.
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mconnelley
super member
Reged: 03/14/06
Posts: 182
Loc: Fremont, CA
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Hello:
From what I've read and heard, figuring an off-axis paraboloid (OAP) is really hard. OAPs are often made by figuring a large parent mirror then cutting the OAPs out of them. For example, an 8" f/10 OAP might be cut from a 20" f/4 parent mirror. With this method, you can get 3 or 4 OAPs from one parent mirror. It is possible to figure a single 8" mirror to be an OAP, either through stress polishing or just using the appropriate polishing strokes, but it'll still be very difficult. There's a good article in "The Best of Amateur Telescope Making" where a guy uses the stress polishing technique. Collimating an OAP is also a real pain, or so I've heard from professional optical engineers. In research scopes, an OAP is usually only used as a primary mirror if high contrast is essential. For the kinds of things that amateurs do, I think very little could be gained by using an OAP primary mirror versus a well designed conventional Newtonian. The easiest way to get a OAP as a primary mirror is to get a big dob and use an off-axis aperture mask. Having done that, you can then add a central obstruction and see if the difference is noticeable.
Cheers
Mike Connelley
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 1937
Loc: Boechout, Belgium
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Appart from an obstructionfree scope, why would someone build a scope with coma in the centre of the FOV...?
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Chris
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Mark Harry
Carpal Tunnel
Reged: 09/05/05
Posts: 2470
Loc: Northeast
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I made a couple of mirrors with same-size respective blanks. It's not as simple as a conventional mirror, and you also need a precise understanding of exactly what's happening while figuring. Then, it's pretty straightforward, but time consuming. Most of these mirrors that are accurate enough will be rather long focus affairs (F/12 & up). It's possible to get a shorter focus mirror with the method described by Mike above, but it usually won't be below F/10~, on account of the larger mirror's focal ratio becoming quite short for its aperture.
One other thing to realise- to figure a larger mirror and cut a smaller segment out of it, is throwing a much larger aperture away, essentially. Not too wise, IMO.
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To have a reasonable aperture size, and focal length without using anything else but a standard size blank, you could check out Ed Jones' "Chiefspiegler", which is a really good example of an unobstructed scope in its better form. I think he really came up with something worth considering, though I haven't had the chance to study it in detail. I think he has the info in a Yahoo group, and there may be an address for it here in CN somewhere.
Regards, Mark
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Scopes in the works-
Too many for putting down here! Favorites- 8" F/6, 8" F/4.72, 4.5" F/5.4, 14" F/4.455, all completed.
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kfrederick
sage
Reged: 02/01/08
Posts: 212
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http://tech.groups.yahoo.com/group/spiderless/ this is ed jones/s group
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walt r
Carpal Tunnel
Reged: 02/13/07
Posts: 2390
Loc: Doylestown, PA
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The biggest problem with an offset parabolic mirror is there is all the coma of a much faster mirror that is in the center of the FOV.
I also believe that the image degradation of a central obstruction is way over rated. Even on an SCT (that is properly collimated and cooled) with a 34% CO the images are very good. The key is collimated and cooled.....without which the image will suffer.
I've made and evaluated on many nights an offset mask for my 18" Newtonian (6.5 inch, f/12). Except the effect of dimming bright double stars the see the split easier (which an ND filter also does but with a higher resolution) there has never been an improvement in the image quality.
If you want an unobstructed scope buy or build a refractor, or another design that does work (sorry guys but I'm not up on all the names of these great scope designs). An off-set parabolic is not an advantage.
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Walt
Obsession 18" f/4.45 #1370 AN/SC
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Gene Hunter
super member
Reged: 12/29/07
Posts: 194
Loc: SC
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I agree on that. I have done the same tests on a 10" and 12" newtonian and I did not see what all the fuss was about. you still end up with a long focal length, hard to collimate scope that does not perform as well as the original you cut it out of. Just my two cents.
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Clear Skies
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jg3
sage
Reged: 05/27/07
Posts: 207
Loc: near Auburn, CA
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In the center of field, an off-axis Newtonian has as much coma as the "parent" axial mirror from which it was cut: zero. Off-center coma is around a third of the parent mirror's, if the focal plane is slightly tilted.
Collimation, including the finding of best-focus tilt, is by star-test. Tricky.
If you want more contrast, resolution, and brightness, the simplest and cheapest solution is a larger standard Newtonian. More aperture will make up for the central obstruction. A longer focal ratio reduces coma.
Where some tilted-component reflector telescopes (TCTs) beat them all is coma. Well-designed and built TCTs offer much less coma than all but the most absurdly long standard or off-axis Newtonians. Collimation is even more complex, however.
Here's a site full of TCT examples: http://bhs.broo.k12.wv.us/homepage/alumni/dstevick/weird.htm
Off-axis Newtonians occupy a somewhat narrow niche, crowded between large-aperture Newtonians and more complex tilted-component reflectors (and, arguably, expensive premium refractors). They are primarily lunar-planetary scopes, since the longer focal ratio imposes limits between exit pupil and field of view. (High-magnification contrast is what gains from going unobstructed.)
DGM Optics (in the CN sponsor list) sells off-axis primaries, presumably cut from larger parent mirrors, alone or in assembled telescopes. They cost quite a bit more than comparable-size standard mirrors.
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Wes James
Carpal Tunnel
Reged: 04/12/06
Posts: 2418
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That TCT link is no good. Additionally, DGM Optics is not producing any off-axis primaries currently- his supplier has quit producing his mirrors. I tried to buy from him- he has none to sell. Failed to reply to my emails- or didn't provide the reply he promised to my phone call when I finally got in touch with him where he works ("I might still have one set.. I'll have to see if I can find it/had someone else who was interested in it").
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Wes
Atlantic Beach, FL
Some bino’s from Miyauchi 5x32 Binon's up through Garrett 20x110 Signature's,
Some telescopes from a Stellarvue 80mm NHNG up through a couple of 8” reflectors…
And a wonderful 4.25" Delmarva Shiefspiegler!
Some good friends, made here on C/N.
Oh- several cats and a wonderful wife!
Anyone want a cat???? :-O
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jg3
sage
Reged: 05/27/07
Posts: 207
Loc: near Auburn, CA
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Try the "Wierd Telescopes" TCT link later. I pasted it straight from the address bar to my post while viewing it.
Bummer about DGM and their mirrors. Their niche is a narrow but good one. Maybe if three or four folks would order the same size at the same time, they could get a good pie to slice from somewhere else. I fear all the good mirror makers are feeling the squeeze between the big-pyrex shortage and the economic calamities.
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Jeff Morgan
Carpal Tunnel
Reged: 09/28/03
Posts: 1509
Loc: Prescott, AZ
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The two approaches I have read about were stressing the blank during manufacture, or stressing a regular paraboloid during use.
A more approachable method may be one introduced by Jose Sasian about 20 years ago. He tilted a regular paraboloid and figured a torodial secondary. There were two or three articles about it Sky & Tel. The author of the second article was a noob ATMer, Sasian coached him through the fabrication.
What you end up with is an 6" unobstructed Newtonian about f/9. The fold in the light path was across the tube, so the overall dimensions were similar (perhaps shorter) than a conventional 6" f/8 Newtonian. When I lived in the Chicago area I had a chance to look through both scopes at Astrofest. As I recall, both were excellent performers on Saturn. These days I am fairly happy to purchase my optics, but if I grind another mirror, this will be #1 on my project list.
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Jeff Morgan
Prescott, AZ
Wile E. Coyote School of Telescope Making
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Wes James
Carpal Tunnel
Reged: 04/12/06
Posts: 2418
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I'm definitely interested in these type of unobstructed scopes. I was hooked immediately upon first light of my 4.25" DelMarva Schief. Anyone who questions the advantages needs to have a look through one. The combination of no color correction issues (refractor) and contrast (newtonian) are both obvious and amazing.
Wes
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gregj888
sage
Reged: 03/26/06
Posts: 209
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Another option is to use spherical optics. As an example, an off axis Houghton. You have to create a set of lenses of known wedge, but that's fairly easy. The lens cell might be a little tricky...
For long fl, of course the schiefspiegler.
Greg
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mconnelley
super member
Reged: 03/14/06
Posts: 182
Loc: Fremont, CA
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Hello:
Similarly, you could make an off-axis Maksutov-Newtonian. Again, you "just" need to make the corrector with the proper wedge and align it properly. The stout of heart could also try to figure an off-axis Schmidt corrector plate. I've never heard of anyone trying to do that. If I ever get back into glass pushing to make a planetary scope, I'll just do a Schupmann. You get nice, high contrast, color-free images without funky off-axis figures or wedged lenses.
Cheers
Mike Connelley
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Mark Harry
Carpal Tunnel
Reged: 09/05/05
Posts: 2470
Loc: Northeast
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"The stout of heart could also try to figure an off-axis Schmidt corrector plate. I've never heard of anyone trying to do that. "
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Hmmm, interesting idea. Very similar to what I did to make my Herschelian mirrors, but with the opposite curve....
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Does anyone else have issues about logging on to the "spiderless" group in Yahoo? I get all set up with the login procedure, and wind up waiting for the moderator to approve; and I get a "no-show". Maybe I'm thick, but is that a rejection???
Mark
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Scopes in the works-
Too many for putting down here! Favorites- 8" F/6, 8" F/4.72, 4.5" F/5.4, 14" F/4.455, all completed.
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1081
Loc: Sin-sin-atti
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Mark,
I'm the spiderless moderator and approve all memberships as soon as I can. 
An off axis Schmidt would way too much work compared to using lenses.
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Ed Jones
Edited by Ed Jones (08/21/08 08:29 AM)
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