179 replies to this topic

[Starman1]

If the question is actually, "does a Celestron SCT have coma," and we can't come to a solid answer here, I don't know what else can be said.

The fact that SCTs have off axis coma is common knowledge and for those that use the scopes.

It is a know, accepted, and well documented fact by SCT imagers and those doing photometry with moderate sized detectors on SCT telescopes. It is known and documented by manufacturers of the scopes and by manufacturers of correctors for SCTs.

Page four of the Celestron edge hd white papers shows spot diagrams for the standard sct, Meade acf, and edge hd telescopes. The standard sct spot diagrams can be verified with a single exposure image using a full frame camera in a sct is representative if what I have experienced with the many years imaging and doing photometry with a c925.

If this can not be agreed upon I'm not sure what can be...

Screenshot_20230924-083544.png

Anyone who claims that standard SCTs are free of coma is someone who has never looked through an SCT or is simply unable to see it.
I've owned several SCTs over the years, and they all had coma--not bad, like an f/6 newtonian, approximately. And using a focal reducer/field flattener only makes it more obvious. Kudos to Celestron for the Edge design, designed, of course, to correct the edge, LOL.

Edited by Starman1, 24 September 2023 - 11:31 AM.

[davidc135]

So, in the end, raytracing solves the problem. No surprise there. The whole discussion could have been finished on the first page. Yet, earlier I was asked "Why do you need ray tracing? All this talk just camouflages very simple [sic] things." 

 

Ray tracing didn't solve the problem. It had to be solved by the simple means of inspecting images or accepting the results of tests which were based both on sound principles as well as data from respected authors.

 

Because some thought that classic sct photographic images don't suffer from coma, the next step was to test the 2ndry mirror which,in the absence of the simplest test of all, contact inferometry, involved the tests that I've set out.

 

Ray tracing or Sigler's equations can't tell you the design of the sct that you have in your hand but practical experiment can, if interpreted sensibly.

 

David

Edited by davidc135, 24 September 2023 - 04:08 PM.

[Matthew Paul]

PS Meade's spot diagram's symmetrical blurs suggest uncompensated field curvature but otherwise a well corrected coma. 

Like you said, the raytrace shows, and contrary to popular belief; the Meade ACF is not a flat field scope. This is why generic field flattening focal reducers for refractors typically give good results with the acf scopes. But, we don't need another thread about that. 

Edited by Matthew Paul, 24 September 2023 - 01:35 PM.

[MKV]

Ray tracing didn't solve the problem. It had to be solved by the simple means of inspecting images or accepting the results of tests which were based both on sound principles as well as data from respected authors.

 

Because some thought that classic sct photographic images don't suffer from coma, the next step was to test the 2ndry mirror which,in the absence of the simplest test of all, contact inferometry, involved the tests that I've set out.

 

Ray tracing or Sigler's equations can't tell you the design of the sct that you have in your hand but practical experiment can, if interpreted sensibly.

 

Raytracing solves the problem of needless marathon back-and-forth guessing by showing that a SCT with two spherical mirrors will not be coma-free.  

Raytracing can also prove that only a concentric SCT can be fully anastigmatic with spherical mirrors.

 

 

Because some thought that classic sct photographic images don't suffer from coma, the next step was to test the 2ndry mirror which,in the absence of the simplest test of all, contact inferometry, involved the tests that I've set out.

 

You can see coma in a C8 using ordinary eyepieces and especially photographically. The contact interference test (which isn't simple, as nothing really is in compound optical  configurations) would be a proof positive whether the secondary is a conic or not, and by how much.

[davidc135]

Raytracing solves the problem of needless marathon back-and-forth guessing by showing that a SCT with two spherical mirrors will not be coma-free.  

Raytracing can also prove that only a concentric SCT can be fully anastigmatic with spherical mirrors.

 

 

 

Yes, indeed.

 

It's not that raytracing isn't valuable or even essential. It's just that we don't need to be concerned with it here as its work is already done and we already know that there is a family of solutions for our compact sct of which two are of particular interest as candidates: Firstly, the all spherical design for which Vla Sacek gives the key figure of 0.712 for corrector g and secondly, the completely aplanatic design where Rutten and van Venrooij give the 2ndry conic as 0.88 and g as 0.83. These figures would change somewhat with small changes in the scope parameters.

 

The two design types as well as those in between will each have (at least) three signatures that can be examined, namely the figure(s) of the surfaces of the corrector plate, the figure of the secondary and the off axis images.

 

Of the three, the corrector plate was avoided as impractical whilst I chose the 2ndry figure as most definite and easy to illustrate.

 

Using the figures above by respected authorities and knowing the SA of the primary alone and also knowing that the spherical contributions from the thee components add up to zero, then it's simple and commonsensical to deduce the 2ndry figure for the aplanat and practical ways of identifying it. It's not speculative theorising.

 

As the 2ndry figure was examined in three different ways and each gave the same clear answer and that taken with off axis coma seen both visually and with the camera I hardly think that the test plate interferogram will show any surprises.

 

But I'm repeating myself too much and everyone will have made up their own minds.

 

A last little bit extra on the astigmatism test using a small diameter collimated beam transmitted through the spare Celestron sct corrector, fwiw.

 

The beam, whose diameter was reduced to 11mm at f102 and which was calculated to have 0.44 waves astig also showed slight but noticeable clocking of the Ronchi lines. Substituting an eyepiece, the diffraction ring surrounding the Airy disc was just broken into 4 sections, less noticeable than I would have thought, as 0.44 waves is still quite bit.

 

David

Edited by davidc135, 25 September 2023 - 07:39 AM.