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Classical Cassegrains--do coma correctors work?

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#1 dougspeterson

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Posted 12 November 2019 - 06:42 AM

In particular the recent GSO sourced Astro Techs and and Orions--classical cassegrains have coma not dissimilar to newts. So do commercial paracorrs etc work? I can think of one reason they won't.


Edited by dougspeterson, 12 November 2019 - 06:43 AM.


#2 billdan

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Posted 12 November 2019 - 07:07 AM

Those GSO cass's have a focal ratio of F12 so there should not be any coma if collimated correctly.

Its unlikely a coma corrector would work anyway as they are designed only for the range of F3 to F6 focal ratios.



#3 dougspeterson

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Posted 12 November 2019 - 08:29 AM

Classical cassegrains are indeed quite comatic, Astronomics’ own description points that out. The coma is similar to a schmidt cassegrain. Please dont contaminate this discussion with assumptions and guesses, read Rutten and et all Telescopes Optics Design and Evaluation, Willman Bell. An eye opener. Hopefully we will hear from someone who actually tried a coma corrector, what did you see?


Edited by dougspeterson, 12 November 2019 - 08:40 AM.


#4 Terra Nova

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Posted 12 November 2019 - 02:26 PM

Those GSO cass's have a focal ratio of F12 so there should not be any coma if collimated correctly.

The system design may end up with a working focal length of F12 but that is a result of combining factors of primary and secondary figure, curvature, and spacing. The actual focal length of the Cassegrain primary is much shorter than the focal length of the typical Newtonian, hence the inherent coma in a CC telescope.


Edited by terraclarke, 12 November 2019 - 02:28 PM.

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#5 doug mc

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Posted 12 November 2019 - 06:45 PM

Please don't shoot me if I am wrong, but from memory, coma in a classic cassegrain is the same as a newtonian of the same focal length. That is, the GSO f/12s would show the same coma as a f/12 newt. 


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#6 Axunator

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Posted 13 November 2019 - 12:45 AM

Please don't shoot me if I am wrong, but from memory, coma in a classic cassegrain is the same as a newtonian of the same focal length. That is, the GSO f/12s would show the same coma as a f/12 newt.

That is certainly what the laws of optics tell us. There’s though more astigmatism and field curvature in CC compared to equal f-ratio & focal length Newt, both of which may also affect edge-of-field performance in CC.

If one doesn’t want to take my word on it, check e.g. telescope-optics.net for all the formulas, graphs and ray traces. ”Telescopes, Eyepieces & Astrographs” by Smith, Ceragioli and Berry (Willman-Bell) also has a nice treatise on the subject.

For subjective, personal experience, I invite people who have actually compared equal size, equal f-ratio CC and Newton to chime in. I certainly haven’t (although would love to! ;-)

Edited by Axunator, 13 November 2019 - 04:30 AM.


#7 doug mc

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Posted 13 November 2019 - 09:27 PM

Not too many f/12 newts out there. We could be waiting for a while.


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#8 TG

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Posted 13 November 2019 - 10:30 PM

I think the question would be more apt for D-Ks. CCs are actually pretty nice in the coma department, according to theory.



#9 Axunator

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Posted 14 November 2019 - 01:30 AM

Not too many f/12 newts out there. We could be waiting for a while.

grin.gif



#10 dougspeterson

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Posted 14 November 2019 - 07:12 AM

Amazing amount of rampant speculation and ill informed opinion that is a waste of everyone’s time. Why don’t we show some facts and then discuss the relative virtues of the different types of cassegrains. Here is an expert armed with facts:

 

http://www.rfroyce.com/cassegrains.htm


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#11 Terra Nova

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Posted 14 November 2019 - 12:58 PM

“Classical Cassegrain Aberrations”

A simple parabolic mirror, as in a Newtonian, suffers from coma.  Adding the hyperbolic secondary prevents spherical aberration in the system but does not remove the coma.”

 

https://starizona.co...segrain-design/


Edited by Terra Nova, 14 November 2019 - 02:05 PM.

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#12 DAVIDG

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Posted 14 November 2019 - 01:51 PM

 Here is a spot diagram for a 6" f/12 Classic Cass  ie parabolic primary, hyperbolic secondary and also a spot diagram for 6" f/12 Newtonian with a parabolic primary. Both are for a field of view of 1 degree.  The black circle is the size of the Airy disk. The Classic Cass has more coma then the Newtonian as the plots show. 

 

                              - Dave  

 

6classiccass.jpg

 

6f12newt.jpg


Edited by DAVIDG, 14 November 2019 - 01:56 PM.

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#13 Axunator

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Posted 14 November 2019 - 03:45 PM

Here is a spot diagram for a 6" f/12 Classic Cass ie parabolic primary, hyperbolic secondary and also a spot diagram for 6" f/12 Newtonian with a parabolic primary. Both are for a field of view of 1 degree. The black circle is the size of the Airy disk. The Classic Cass has more coma then the Newtonian as the plots show.

- Dave

6classiccass.jpg

6f12newt.jpg


But the total aberration shown in those plots comes from coma PLUS astigmatism and defocus induced by field curvature (both of which which are much more pronounced in CC than in Newt), right?
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#14 DAVIDG

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Posted 14 November 2019 - 05:16 PM

 My spot diagrams show what a star image would look like in both scopes both on axis and off. The major aberration is coma in both systems  and that is what is shown. The classic cass has more by a fair amount.  

 

                     - Dave



#15 dougspeterson

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Posted 14 November 2019 - 07:03 PM

This is why commercial coma correctors will have little effect on cassegrains. 

Also a lot of field curvature. In some ways the paraboloid is the cleanest system. 



#16 charlesgeiger

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Posted 14 November 2019 - 07:45 PM

So wouldn't a field flattner and coma corrector significantly help with a CC?  It seems like a test might be to use a cheap SCT unit and just see how it works.

Charlie



#17 Alterf

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Posted 14 November 2019 - 09:37 PM

Here's a corrector/reducer designed for a classical Cass:

 

https://telescopes.n...-corrector.html

 

Unfortunately, it's a little large for my scope.

 

Correctors are designed for specific systems.  Though I own both a classical Cassegrain and a Newtonian coma corrector, I think I'll wait and read someone else's experiment.

 

My scope is a 290mm f/13.5.  I don't notice the coma at all visually.  When I use the scope for imaging, I can tell coma might be a problem if my field were wider, but one images small targets with such a long focal length instrument.  I've never wanted a coma corrector for it.  I do use the Astro-Physics telecompressor to reduce the speed to f/8.1.

 

Val



#18 DAVIDG

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Posted 15 November 2019 - 09:51 AM

 Another thing to consider is that most coma correctors for Newtonians  also magnify a small amount  and add under corrected spherical aberration.  Most use a coma corrector on  a Newtonian for lower power wide field viewing since that is when coma is visible off axis so the slight increase in magnification is not an  issue.  With low power  the increase in spherical aberration is also not a problem because in many cases it doesn't show at low mag.

   Now if you use a commercial coma corrector with a Cass,  they will increase the focal length of the system by around 15% , so you have a longer focal length and narrower  field of view per given eyepiece,  for a scope that already has a long focal length and narrow field of view. Since they add  some amount of spherical aberration you don't want to use one for say planetary imaging or viewing.

 

                   - Dave  


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#19 TG

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Posted 15 November 2019 - 01:27 PM

 Here is a spot diagram for a 6" f/12 Classic Cass  ie parabolic primary, hyperbolic secondary and also a spot diagram for 6" f/12 Newtonian with a parabolic primary. Both are for a field of view of 1 degree.  The black circle is the size of the Airy disk. The Classic Cass has more coma then the Newtonian as the plots show. 

 

                              - Dave  

 

attachicon.gif 6classiccass.jpg

 

attachicon.gif 6f12newt.jpg

David, you did not factor in field curvature for the CC. Once you do this, you can see that the CC has much nicer spots. This leads me to say again that there isn't much coma in a CC when used visually. Note that the field size is two-lunar diameters, a size typically not used in CCs. It also vignettes since to get full illumination the CO would have to be huge.

 

SR125oth.jpg


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#20 TG

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Posted 15 November 2019 - 07:12 PM

I got a chance to compute star images at the edges of different field widths. As you can see a 6" f12 cassegrain at 0.75° field width is still very decent. At 0.5°, enough to encompass the full moon, it is almost indistinguishable from a coma-free scope.

 

IKR8E5Rl.png

In comparison, for a CCD at 1° field (i.e, if imaged on a plane), the star looks like this 

 

GBTuxwJt.png

 

 

 

Though, because of the low intensity of the comatic image and the small image scale at wide-field, nobody would actually be able to see or record the above. You would see the oval + maybe the y-wings, assuming you had an eyepiece corrected for astigmatism.

 

 

Tanveer.



#21 Stephen Kennedy

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Posted 15 November 2019 - 11:44 PM

My Newtonian reflector, aperture = 210 mm, focal length = 1,623 mm, is about an F/7.7.  It has no on-axis coma and essentially no off-axis coma whether it is used visually or as an astro-graph.  This does not surprise me since coma decreases as a function of the square of the focal ratio.  So why, even though they both have paraboloidal primary mirrors, is the F/12 CC showing coma when the F/7.7 Newtonian does not?  I believe is due to the fact that the CC's primary mirror is not F/12 or even F/7; it is actually about F/3 and the convex hyperparaboloidal secondary results in a system focal ratio of F/12.  The flat diagonal secondary in a Newtonian has no effect on focal ratio.

 

My solution to the coma problem in a Cassegrain was to buy the GSO RC6 instead of the CC6.  They both have approximately the same price. 



#22 coinboy1

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Posted 24 November 2019 - 08:10 AM

I think a coma corrector would tame the coma on these GSO F/12 Classical Cassegrains. 

 

Using the "paracorr-like" coma corrector on p. 389 Figure 14.9 of Telescopes, Eyepieces, Astrographs by Smith, Ceragioli, and Berry; I entered the parameters into OSLO (free version) and compared an 8" F/12 classical cassegrain with and without a coma corrector.

 

The coma corrector is a 2" diameter coma corrector similar to the Paracorr 2" version. The image size is for a 27mm diagonal or across an APS-C sensor. 

 

Here are the results:

Layout and PSF below.

Attached Thumbnails

  • layout.jpg
  • layout (1).jpg
  • PSF.jpg
  • psf (1).jpg


#23 coinboy1

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Posted 24 November 2019 - 08:10 AM

The spots are both across a 27mm diameter field but because the coma corrector has a ~1.11x amplification the field is smaller w/coma corrector. The circle in the spots indicate the airy disc.

 

And the spot diagrams with the same image scale:

Attached Thumbnails

  • spot.jpg
  • spot (1).jpg

Edited by coinboy1, 24 November 2019 - 08:14 AM.


#24 coinboy1

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Posted 24 November 2019 - 10:25 AM

Found the prescription to the Baader MPCCIII and entered it into OSLO:

 

So I would say yes coma correctors generally improve the performance of the Classical Cassegrain. 

Attached Thumbnails

  • layout (2).jpg
  • psf (2).jpg
  • spot (2).jpg

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#25 TG

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Posted 24 November 2019 - 05:35 PM

@coinboy1, do you have an OSLO .len file to share?




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