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Level of optical correction of several prominent designs and corrected aplanatic Gregorian

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#1 Lucullus

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Posted 26 June 2021 - 10:01 AM

I recently read in another thread about the best design for astrophotography, that concerning off-axis correction, probably the best native(!, so including [uncorrected] RCs [I assume corrected RC are again better than CDKs]) optical design of them all is the CDK (which I consider a native system, because a CDK without it's integrally designed corrector is absolutely worthless, so inferior and not even equal to a native Dall-Kirkham).

I wonder if it is possible to design a Gregorian telescope as fast as f/5? Whether this is possible with the same level of correction as a CDK and if this would require two or even three corrective lenses?

 

Just have found this interesting website http://ganymede.nmsu...tes/node16.html. It says that there are two classes of two-mirror telescopes: classical Cassegrain and classical Gregorian. The classical Cass has a parabolic primary mirror and a hyperbolic secondary. The classical Gregorian has a parabolic primary and ellipsoidal secondary. The aplanatic (spherical aberration and coma corrected) versions of the classical Cassegrain have a hyperbolic primary and hyperbolic secondary, better known as Ritchey-Chrétien. The aplanatic Gregorians have an ellipsoidal primary and ellipsoidal secondary.

The website states concerning optical quality:


[...] The image quality is clearly better for the aplanatic designs than for the classical designs, as expected because coma dominates off-axis in the classical design. In the aplanatic design, the Gregorian is slightly better. However, when considerations other than just optical quality are considered, the Cassegrain usually is favored : [...]

Is the following ranking (increasing towards perfection) correct concerning optical off-axis quality: CC, RC, CDK, corrected RC? Where in this ranking would the classic Gregorian, aplanatic Gregorian, and two/three lenses corrected aplanatic Gregorian be ranked? Mostly interested in the questions before, but for the sake of completion: what about those Harmer-Wynne and Riccardi-Honders products offered by several manufacturers in the suggested ranking?


Edited by Lucullus, 26 June 2021 - 03:16 PM.


#2 Lucullus

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Posted 26 June 2021 - 10:42 AM

telescope-optics.net has further information concerning my suggested ranking with

 

[...] Again, for an equal set of parameters, aplanatic Cassegrain [aka Ritchey-Chrétien] has somewhat stronger best field curvature than aplanatic Gregorian. [...]

This would confine the aplanatic Gregorian design at least beyond the RC in the red design spectrum as follows:

CC, RC, CDK, corrected RC. But where exactly? Moreover, I also wonder about the corrected aplanatic Gregorian placement in this ranking. Any knowledge in theoretical telescope optics design here?

 

Chapter 4.2.5., Fig. 127 shows an interesting spot comparison including the Dall-Kirkham being worse than the classical Cassegrain, unfortunately not including the CDK, and corrected RC. The depicted focal ratios are not all equal, but it is revelatory to bear in mind that the respective f/8 CC and CG (classical Gregorian) would have at least noticably larger spot sizes than their f/8 RC and aplanatic Gregorian relatives. So:

DK, CC, RC, CDK, corrected RC.


Edited by Lucullus, 26 June 2021 - 03:23 PM.


#3 MitchAlsup

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Posted 26 June 2021 - 10:55 AM

Is the following ranking (increasing towards perfection) correct concerning optical off-axis quality: CC, RC, CDK, corrected RC? Where in this ranking would the classic Gregorian, aplanatic Gregorian, and two/three lenses corrected aplanatic Gregorian be ranked? Mostly interested in the questions before, but for the sake of completion: what about those Harmer-Wynne and Riccardi-Honders products offered by several manufacturers in the suggested ranking?

I think it has become clear that the correctors for a DK are easier to do and achieve at least as good a field correction as correctors for RC (at amateur scales); that CDKs are the easier and better design point--so long as you never want to use the scope without the correctors--like deep into UV or IR.]

 

Overall, the Wynne style correctors can correct pretty much any reasonable optical system--especially if the reflecting optics and the refracting optics are designed together.

 

But there comes a point of "why correct at all"--there are Three Mirror Anastigmats where all of the Sidel terms are identically zero (0.00000) ! These just don't fit into "amateur" bank accounts.


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#4 BGRE

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Posted 26 June 2021 - 06:28 PM

It would be helpful to tabulate aberration correction, ease of testing/making the various optical components, examples of each type etc.
Are you limiting this to 2 powered mirrors? Otherwise there are some interesting 3 and 4 powered mirror telescope designs.
At least one 4 mirror design by Korsch allows a 1m OTA to fit within a ~1m cube.

The usable field of an RC is limited by astigmatism whereas astigmatism can be corrected in a well designed CDK.

The VATT is one example of an aplanatic Gregorian.
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#5 BGRE

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Posted 26 June 2021 - 09:04 PM

CDK example: 1m McLellan telescope installed in the 1980's.
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#6 luxo II

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Posted 27 June 2021 - 04:54 AM

What aperture ? A gregorian at f/5 could be made if you’re prepared to consider a parabolic primary at f/2 and a central obstruction around 40-45%.

If you’re going to include catadioptrics with correctors - I’ll suggest you also include the Rumak and Simak designs. Intes and intes-Micro offered some at f/6 upto 40cm aperture though how many were ever built no idea. There is also a 50cm f/8.5 rumak for sale (you’d need deep pockets for that).

And possibly some of the imaging designs from Maksutov’s original paper. Some of those appear feasible with a modern camera at the primary focus, equipped with a flattener; could be as fast as f/2.

NB Ottiche Zen offer maksutov cassegrains at f/5.6.

Edited by luxo II, 27 June 2021 - 06:01 AM.

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#7 TOMDEY

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Posted 27 June 2021 - 06:46 AM

Hi Lou,

 

Your request, as presented, is a tall order, so I'll just summarize my recollections that relate >>>

 

Several decades ago I audited Daniel Malacara's "Astronomical Optics" course at the University of Rochester's (NY) Institute of Optics. He was on sabbatical from the U of Mexico. I recall that his course was excellent, covering theory, applications, and especially optical testing. I also recall that the young matriculated undergraduate students were profoundly lazy. They were delinquent with homeworks and miffed that Malacara included midterm questions that extended beyond things covered in class. They felt that was somehow unfair; theirs was a sophomoric, entitled mentality. What a bunch of slackers!

 

One of our homework assignments was to evaluate telescope designs relative to performance. To make the shoot-out fair, Dr. Malacara set all at 8-inch F/8 50mm diameter field... right in the amateur astronomy ballpark. I think I was one of the only students who actually completed the assignment. We became friends and later collaborated on and presented an informal white-paper "Telescope Quality - How good is good enough?"

 

Anyway, from our design evaluations, it became obvious that the humble Newtonian and Ritchey Chretien were head and shoulders above all others; if you add Coma Corrector to the Newt and two-element Field Group to the Ritchey, they become magnificent. Malacara also forced us to address practicalities of execution. Fabrication and testing of the elements, materials, alignments, baffling, field curvature and how to address it in practice. If you vacuum-bend your film to the curved field, achieves diffraction-limited performance over gargantuan field. I later worked Hubble-Class Ritcheys and indeed carpeting the gargantuan curved field with CCDs is the only sensible way to fly. For flat field, the three-mirror anastigmat is wonderful.Most of our subsequent production aerospace telescopes are of that configuration.

 

I know you asked for design-only optima... but Malacara wouldn't let us get away with only that. If you design perfection but are unwilling to then build and use it, the entire exercise is meh. At work, we were constantly receiving high-performance computer designs, gratuitously offered up from otherwise disinterested programmers. Nearly all suffered completely-debilitating practical flaws >>> unobtainable materials, no way to fabricate, endless list of deficiencies. Even more so today, with powerful optimization and evaluation software... it's created a generation of pipe-dream designers. That's the easy part. Building it is where 99% of the action is.     Tom


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#8 Lucullus

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Posted 27 June 2021 - 10:56 AM

It would be helpful to tabulate aberration correction, ease of testing/making the various optical components, examples of each type etc.
Are you limiting this to 2 powered mirrors? Otherwise there are some interesting 3 and 4 powered mirror telescope designs.
At least one 4 mirror design by Korsch allows a 1m OTA to fit within a ~1m cube.

The usable field of an RC is limited by astigmatism whereas astigmatism can be corrected in a well designed CDK.

The VATT is one example of an aplanatic Gregorian.

I only have 2 mirror (+ refractive correctors) in mind. But the 3 and 4 mirror designs sound very interesting for a dedicated topic!
 



#9 Lucullus

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Posted 27 June 2021 - 11:52 AM

Hi Lou,

 

Your request, as presented, is a tall order, so I'll just summarize my recollections that relate >>>

 

Several decades ago I audited Daniel Malacara's "Astronomical Optics" course at the University of Rochester's (NY) Institute of Optics. He was on sabbatical from the U of Mexico. I recall that his course was excellent, covering theory, applications, and especially optical testing. I also recall that the young matriculated undergraduate students were profoundly lazy. They were delinquent with homeworks and miffed that Malacara included midterm questions that extended beyond things covered in class. They felt that was somehow unfair; theirs was a sophomoric, entitled mentality. What a bunch of slackers!

 

One of our homework assignments was to evaluate telescope designs relative to performance. To make the shoot-out fair, Dr. Malacara set all at 8-inch F/8 50mm diameter field... right in the amateur astronomy ballpark. I think I was one of the only students who actually completed the assignment. We became friends and later collaborated on and presented an informal white-paper "Telescope Quality - How good is good enough?"

 

Anyway, from our design evaluations, it became obvious that the humble Newtonian and Ritchey Chretien were head and shoulders above all others; if you add Coma Corrector to the Newt and two-element Field Group to the Ritchey, they become magnificent. Malacara also forced us to address practicalities of execution. Fabrication and testing of the elements, materials, alignments, baffling, field curvature and how to address it in practice. If you vacuum-bend your film to the curved field, achieves diffraction-limited performance over gargantuan field. I later worked Hubble-Class Ritcheys and indeed carpeting the gargantuan curved field with CCDs is the only sensible way to fly. For flat field, the three-mirror anastigmat is wonderful.Most of our subsequent production aerospace telescopes are of that configuration.

 

I know you asked for design-only optima... but Malacara wouldn't let us get away with only that. If you design perfection but are unwilling to then build and use it, the entire exercise is meh. At work, we were constantly receiving high-performance computer designs, gratuitously offered up from otherwise disinterested programmers. Nearly all suffered completely-debilitating practical flaws >>> unobtainable materials, no way to fabricate, endless list of deficiencies. Even more so today, with powerful optimization and evaluation software... it's created a generation of pipe-dream designers. That's the easy part. Building it is where 99% of the action is.     Tom

I completely agree with you IF one's requirement is to realise a theoretical design in the end.

But there's nothing wrong per se with a design-only ranking if you keep it restricted to theory only and are interested in the aberration theory of telescope designs.

Yet, it is hard to find direct telescope optics design comparisons, and those which do still don't cover all of the designs they themselves talk about in their own texts. It seems impossible to present an enlightening collection of an equal aperture and f-ratio, on- and off-axis spot size comparison of all the telescope designs talked about. Such a thorough reference of telescope optics designs could reasonably be limited to designs having their own "name" like Newtonian, Maksutov-Cassegrain, Schmidt-Newtonian, Schmidt-Cassegrain, Schmidt, Cassegrain, Ritchey-Chrétien and it's corrected version, Dall Kirkham, Corrected Dall-Kirkham, Mersenne, Schwarzschild, (aplanatic) Gregorian and corrected version thereof... Such a reference would surely be a treasury for the theoretically curious. I have to say I ordered the book "Telescopes, Eyepieces, Astrographs" with hopes of such a reference and was a bit disappointed because it wasn't what I had hoped for. But after such a reference would exist, the realists can then still wave aside any realisation questions with legitimate objections such as prohibiting seeing for quotes above e.g. 6". And those determined to nevertheless realise a specific design would know their designs theoretical potential.


Edited by Lucullus, 27 June 2021 - 02:23 PM.


#10 MKV

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Posted 27 June 2021 - 03:26 PM

Lucullus, 

  • An f/5 or even f/8 Gregorian with an f/1 primary will have severely curved image field which can be corrected with additional refractive optics. In the "old" days (before digital photography), a simple plano-concave or plano-convex lens (depending on the nature of the field curvature) of proper radius of curvature would be placed close to the film emulsion to achieve that. The curved face of the field-flattening (FF) lens would have a radius of curvature RFF = RF(n - 1)/n, where RF is the field radius of curvature, and n is the refraction index of the substrate used at the design wavelength. Thus, for ordinary crown lenses with n ≈ 1.52, RFF is very nearly RF/3.
  • With digital cameras it's difficult to place an FF lens close to the image sensor, which calls for a different approach to correct field curvature. One is simple and expensive method involves a properly curved image sensor, while the other requires two or three refracting elements placed some distance inside the paraxial focus. Large observatory telescopes (which are mostly of the RC configuration) use the former method. Here's an example used on the Kepler telescope with a composite curved image sensor: https://www.cnet.com...-to-the-future/
  • I agree with Tom that designing a telescope on "paper" is the easy part and that building one is 99% of the work needed. However, I agree with you that the only way one can directly compare different design configurations is by using purely theoretical (ideal) layouts. Theoretical configurations give us an idea of the potential correction, but they don't tell us if such solutions are actually doable. And the only telescope worth anything is one that actually works! :o) So, in the end, reality rules. But to study potential benefits of different designs, the theory is the only approach.
  • Perfect multi-mirror solutions have been known for more than a century. The reason one doesn't see them very often is because they're very difficult if not impossible to realize. Telescope optics, especially professional grade optics, have to made to specified tolerances or better. That is very easily satisfied with a push of a button and an optimization routine in a split second on modern computers and software. Often times the ideal substrates suggested by optimization don't exist or have other limitations (such as brittleness, discoloration, poor homogeneity, price, etc.) that make theoretical solutions insolvable. Then there's a plethora of obstacles. Simply put, easier said than done.

On a personal note, I think your topic is too broad and non-specific. In order to get your answers, I suggest that you decide which instrument types you're interested in, and specify the intended purpose and scope of work. You have to decide on the exact aperture diameter and focal ratio for all of them, as well as the working field of view, and specify if the system is to be aplanatic or anastigmatic, flat field, or curved field, etc. and acceptable tolerance range. Respondents can then work along those guidelines and someone can tabulate all the results for a direct comparison. Otherwise,   honestly imo, this isn't going anywhere.

 

Mladen

 

____

 

edit:ed: formatting;typos


Edited by MKV, 27 June 2021 - 08:38 PM.

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#11 TOMDEY

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Posted 27 June 2021 - 03:43 PM

I completely agree with you IF one's requirement is to realise a theoretical design in the end.

But there's nothing wrong per se with a design-only ranking if you keep it restricted to theory only and are interested in the aberration theory of telescope designs.

Yet, it is hard to find direct telescope optics design comparisons, and those which do still don't cover all of the designs they themselves talk about in their own texts. It seems impossible to present an enlightening collection of an equal aperture and f-ratio, on- and off-axis spot size comparison of all the telescope designs talked about. Such a thorough reference of telescope optics designs could reasonably be limited to designs having their own "name" like Newtonian, Maksutov-Cassegrain, Schmidt-Newtonian, Schmidt-Cassegrain, Schmidt, Cassegrain, Ritchey-Chrétien and it's corrected version, Dall Kirkham, Corrected Dall-Kirkham, Mersenne, Schwarzschild, (aplanatic) Gregorian and corrected version thereof... Such a reference would surely be a treasury for the theoretically curious. I have to say I ordered the book "Telescopes, Eyepieces, Astrographs" with hopes of such a reference and was a bit disappointed because it wasn't what I had hoped for. But after such a reference would exist, the realists can then still wave aside any realisation questions with legitimate objections such as prohibiting seeing for quotes above e.g. 6". And those determined to nevertheless realise a specific design would know their designs theoretical potential.

Yes, I completely agree! Design is of great value as brain-teaser. The Optical Society of America (and also some locals) ran annual design challenges (think I got that right) to members. Wonderful crazy requirements, cost and practicality mattered not. The crazier the concepts the better. One was to design a broadband camera that would resolve an arc-sec over the widest possible field. I finished my submission in half an hour... It was the simplest and least practical. It was a plain vanilla Pinhole Camera with the "pinhole" a foot across and the ~focal sphere~ 100 miles in radius and centered on that pinhole. It resolves an arc-sec over a generous 120-degree field. It can be made 10x more compact by placing a 62 micro-diopter singlet over the hole. 

 

Advantages:

 

>pinhole is the only optical element

>pinhole mass is zero, wavefront is perfect

 

Disadvantage:

 

>long exposure times operating at F/half-a-million

 

Anyway... stuff like that!    Tom


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