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10" f/3.5 Astrograph with Rosin focal reducer

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#26 Mike I. Jones

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Posted 21 November 2012 - 08:52 AM

Who knows? Only the ASA folks, and they ain't tellin'!

It could very well be similar to the ASA designs. I could probably get a good DL-ish design at f/2.8 over the same field and spectral band, but it would be a real bear to build and keep in alignment. f/3.5 is plenty fast for me.

Mike

#27 Loren Chang

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Posted 22 November 2012 - 11:27 AM

Hello Mike,

A beautiful design, again. I found similiar ideas in my database. Post one of them only here. Dose this Ross corrector variant works on newtonian?

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#28 MKV

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Posted 22 November 2012 - 04:22 PM

Dose this Ross corrector variant works on newtonian?

Loren, I hope you won't mind if I jump in on this. In order to get this level of correction, the primary mirorr must be hyperboloidal. A Ross corrector reduces coma in a Newtonian (i.e. paraboloidal primary), but at the cost of introducing spherical undercorrection - which is offset by hyperboloizing (overcorrecting) the primary. But then it's no longer a Newotnian.

Maybe modern glass melts offer a possibility of creating a Ross/Rosin type corrector that would correct for coma in a Newtonian without significant spherical residual. It may be worth looking into this. Mike, what say you?

Mladen

#29 Mike I. Jones

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Posted 22 November 2012 - 10:45 PM

I say, don't talk with your mouth full! Been enjoying family and food all day. I'll get back into CN maybe this weekend.
Gobble 'til ya wobble,
Mike

#30 Loren Chang

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Posted 23 November 2012 - 07:30 AM

In order to get this level of correction, the primary mirorr must be hyperboloidal. A Ross corrector reduces coma in a Newtonian (i.e. paraboloidal primary), but at the cost of introducing spherical undercorrection - which is offset by hyperboloizing (overcorrecting) the primary. But then it's no longer a Newotnian.
[/quote]

Hello Mladen,

Everyone is welcome. I understand what you said. For Newtonian, Wynne corrector gives excellent image but is diffcult to make. Ross corrector is simpler though causes spherical aberration. What I'm asking is will it be better to add one more element in Ross for newtonian?



Hello Mike,

Waiting for your comment! I'm playing a F/10 superachromat design in USP 2009/0296201. Take a look if you are interstring. I'll post it in another thread.

#31 MKV

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Posted 24 November 2012 - 07:07 AM

Hi Loren. My point was that if you add a third element to a paraboloidal mirror you end up with a Wynne-type corrector.
_____

Hey Mike, I hope you had a nice Thanksgiving. It appears that your corrector's first surface (#4) is the "hairline trigger" that must be spot on to a very high precision - I mean a radius of curvature accurate to a few ten thousands of an inch. Vow!

Radii of the real element (#8 and 9) have more generous tolerances and could be used to counterbalance #4 with much greater radius tolerance.

But that means the radius of #4 must be known to better than 0.05%. Seems like this would be the first surface to be finished and then all other sufraces would be built around that one - again to very high precision.

I also find that perhaps thickness/spacings of the correctors elements must be in the range of ±0.0001 inch, and that the diffraction limited focal range is is in the order of only ±0.0005 inches.

As I said , this is just a preliminary look, using a slider wheel tool. Curious to see what your tolerance data will tell us.

Mladen

#32 Mike I. Jones

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Posted 24 November 2012 - 09:51 AM

Just checking in briefly. Mladen, when you varied your slider wheels, what compensators did you use? You need to have compensators that can be easily changed during tolerancing, such as refocus, airspaces, etc. Otherwise you get just what you found, outrageously tight tolerances, when they needn't be. In this application, the BFL could vary a millimeter or so and have no impact on system utility. The BFL, airspaces and distance from the diagonal can all vary a small amount by re-optimizing. I will use all those values when I get back and study the tolerancing.
Mike

#33 MKV

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Posted 24 November 2012 - 10:15 AM

Mike, you're so right. I overlooked the refocusing. My bad. Indeed, once you compensate, the tolerance range becomes more realistic.

Mladen

#34 ed_turco

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Posted 24 November 2012 - 11:40 AM

May I say something a little off topic? Robert Lurie in his 1976 paper proposed a system with a full lens corrector (Asymmetrical in form and two BK7 elements in contact) with a hyperbolic mirror; it was a dandy. I know that the term "Too much glass" is bandied about but the system was not only coma corrected, but anastigmatic, meaning no astig either. I wish I had built it. Mike, any comments? I think I might have the numbers somewhere...

#35 Mike I. Jones

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Posted 24 November 2012 - 04:56 PM

Hi Ed,
We're still out on the road for holidays. If you'll post up Lurie's prescription, we can give it a look in ZEMAX and OSLO.
Mike

#36 kfrederick

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Posted 24 November 2012 - 05:54 PM

Mike how big can this design go ? Thanks

#37 MKV

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Posted 24 November 2012 - 09:09 PM

Ed, Mike: Lurie's 1975 work gave G-sum equations for a corrector to be used in conjunction with a parabolic (not hyperbolic) mirror. It was intended to serve as a means of converting ordinary Newtonians into anastigmatic cameras. But his principle can be applied to an entire family of conic mirrors, not just paraboloids.

I discussed his design and one special case he intended for amateurs (using an ellipisoidal mirror and a plano-convex, plano-concave corrector) in The Best of Amateur Telescope Making Journal, Volume 2, 17-18.6 "Lurie Anastigmats", pp. 375-392, Willmann-Bell, 2003.

I have his 1975 JOSA article and will gladly post the equations - unless Mike thinks it's off topic.

Mladen

#38 wh48gs

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Posted 25 November 2012 - 12:53 PM

Loren,

That's interesting arrangement, positive element in front. still only four glass-to-air surfaces. When scaled up to 250mm f/3.5 it is not quite as good as Mike's color-wise, but that wouldn't cause any difference in practical terms. But why use FPL51, if FPL53 is (I think) cheaper? Here's what it looks like.

Vla

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#39 cjc

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Posted 26 November 2012 - 03:25 AM

...
Speaking of tinkering on those cloudy nights, it would be interesting to do a direct theoretical comparison of this Mike I. Jones-Rosin corrected system as opposed to a 10-inch f/3.5 Newtonian with a Paracorr and a 10-inch f/3.5 Newotnian using a Wynne-type corrector (both of which are commercially available).
...
Mladen


These are not quite what you want, but there is a coma corrector design with two cemented doublets in the Telescopes, Eyepieces, and Astrographs by Smith, Ceragioli and Berry. Orion Optics have spots for a set of F3.8 Newtonians with an Optimised Wynne corrector with the 10" spots here:

http://www.orionopti.../ag10spots.html

and the others being accessible from here (follow the red links in the table):

http://www.orionopti...eand(ota)t.html

#40 MKV

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Posted 26 November 2012 - 05:34 AM

Thanks cjc. Interesting "hybrid" corrector. Bear in mind that the Rosin works only with a hyperbolic mirror, which means the corrector must be used all the time, whether observing visually or photographically. The others, the Ross, Wynne, Paracorr, and the Orion's version work on your ordinary Newtonians with parabolic mirrors which can be used with or without the corrector. Of course, if you have an f/3.5 Newotnian you'd be using the corrector all the time! :)

The design and the results of all of these are either public knowledge or can be obtained by raytracing, but not of Paracorr. My understanding is that it consists of two cemented achroamts. It would be interesting to see how Paracorr would compare in an f/3.5 Newtonian, one degree off-axis, against the other correctors.

Mladen

#41 Loren Chang

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Posted 26 November 2012 - 01:12 PM

Hi Loren. My point was that if you add a third element to a paraboloidal mirror you end up with a Wynne-type corrector.


Hello Mladen,

You mean finally I'll get a Wynne corrector? :question:

#42 MKV

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Posted 26 November 2012 - 04:22 PM

Hi Loren. What I meant to say is that, for a parabololoidal mirror, a high quality corrector at such fast focal ratios will most likely resemble the Wynne configration.

Paracorr's design is not public knowledge, but it is my understanding that it does not resemble the Wynne configuration. Rather I read somewhere that it consists of two sets of cemented achromats. Regardless, since it's a family secret of sorts, it is impossible to say how good it is on f/3 to f/3.5 paraboloids.

So far, all high performance correctors for very fast Newtonians resemble the Wynne corrector, having either three or four elements.

Mladen

#43 MKV

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Posted 26 November 2012 - 09:04 PM

Loren, I just found this site, which discusses the performance of three commercial Newotnian coma correctors, a Wynne, Paracorr, and RCC (Celestron's) brand.

The article is in French (thank you, Lord, for Google Tranalate!). The author presents imaging evidence of the performance and concludes that the Wynne and Paracorr are abot the same for f/3 to f/3.5, but that the Wynne has less vignetting and covers larger image frames. At the same time, he concludes that Paracorr is the most cost-effective product.

The plot thickens.

PS I would also like to add that if the Paracorr is indeed a set of two achromatic doublets then it beats any other corrector thus far in manufacturing as well. The Wynne requires very thin, deeply curved lenses that look more like potato chips! Not an easy thing for an amateur.

The Rosin requires a pretty strong hyperbolic primary and a couple of meniscus lenses, with slightly more agreeable thickness, but still n ot easy. One must never forget that an f/3 or f/3.5 hyperboloid is not an easy thing to make especially because many of the available tests presnet a problem with full illumination of optics - and the steeper the curves the tighter the tolerances!

So, at least one, rather well documented test seems to favor Paracorr as the corrector of choice for Newtonians.

Hopefully we didn't drift too much off topic. :)

Mladen

#44 Ed Jones

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Posted 26 November 2012 - 09:38 PM

Most ATMs aren't into lens making. 8ere's an 8 inch F/4 hyperbolic using off-the-shelf lenses.

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#45 Loren Chang

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Posted 27 November 2012 - 12:35 AM

But why use FPL51, if FPL53 is (I think) cheaper? Here's what it looks like.


Hello Vlad,

Why FPL51? The relative price of fpl51 to BSL7 is cheaper.

OHARA Glass

I do have several FPL53 designs. And you can even use ordinary glass. Here is OSLO file.

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#46 cjc

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Posted 27 November 2012 - 03:10 AM

As to Paracorr design, this is from the Optcorp page:

Paracorr (parabola corrector) uses 2 multi-coated, high index doublets, is completely color-free, center and edge, and installs like a Barlow. Coma is corrected so well, the diffraction limited field area of an f/4.5 Dob/Newt is increased 36 times!" -- Al Nagler

For spot sizes see:

http://www.televue.c...orr_2_chart.jpg

#47 MKV

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Posted 27 November 2012 - 07:23 AM

Thanks Chris. I guess that settles it. The phogoraphic evience I referenced to Loren seems to agree with Nagler's data. At under $500, as a simple add-on, it is both cheaper and more efficient than other correctors - and you can keep your existing parabolic mirror! There is no way a Rosin-type astrograph, or a Wynne-type corrector, can be manufactured at a comparable price.

As for Paracorr's vignetting, most folks with imaging devices today have digital processors of such size that worrying about vignetting hardly seems relevant.

Mladen

#48 wh48gs

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Posted 27 November 2012 - 08:04 AM

Loren,

The relative price of fpl51 to BSL7 is cheaper.



You are right. Not long ago I saw the pressing price table, and only remembered FPL53 looking relatively cheap (obviously, processing adds quite a bit).

Yes, common glasses can be used (in the arrangement with the cemented positive doublet in front as well), but they always have the problem of not been able to correct for longitudinal and lateral color at the same time. When scaled to 10-inch f/3.5, and with minimized longitudinal aberration, the 406-8300nm spot at 1 degree off axis is spread over more than three green Airy disc diameters, laterally (not much less at 0.7 degrees). It is still within 0.025mm circle, but would probably result in elongated star shapes. The far red end is not that important, since CCD sensitivity drops toward it, but it is still there.

Vla

#49 MKV

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Posted 27 November 2012 - 08:17 AM

Ed, how can you have BK7 next to BK7, and shouldn't all the thicknesses be negative? Could you present this design in a standard tabular form, i.e. roc, space, medium, conic? Thanks.

Mladen

#50 Mike I. Jones

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Posted 27 November 2012 - 09:31 AM

Here's a 2-doublet coma corrector solution I messed around with over the weekend that probably works about as well as the Televue Paracorr, at least for f/4.5 mirrors and slower. I didn't try it with faster mirror focal ratios, but this at least shows it can be done with two doublets, for a usable field of view and over a wide spectral band.

Might be a good project to work further on with lower focal ratios. We could call it the Cloudynights Collective Coma Corrector!

Mike

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