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coma-correcting a Maksutov-Cassegrain

Maksutov Accessories
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#1 Nicole Sharp

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Posted 24 November 2021 - 02:37 AM

What happens if you use a 0.63X SCT reducer-corrector (focal reducer plus coma corrector) on a Maksutov-Cassegrain telescope such as the Sky-Watcher Skymax 1540/127 MCT (which has SCT threads to accept SCT reducers)?

 

https://www.skywatch...her-skymax-127/

 

with reducer-corrector

 

https://www.starizon...coma-corrector/

 

https://www.celestro...ucer-corrector/

 

versus reducer

 

https://www.optecinc.../catalog/lepus/


Edited by Nicole Sharp, 24 November 2021 - 02:39 AM.


#2 Nicole Sharp

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Posted 24 November 2021 - 02:50 AM

Aberrations are additive, end-to-end through an optical system. The Mak should have negligible coma to start with, so by adding a device that attempts to correct for coma, the result probably won't be nice.

 

What you want is a reducer without the coma correction.

 

I have this reducer, it works fine on my 6" and 10" maks (both f/12) with an ASI533 camera:

https://www.teleskop...ter-thread.html

 

TS have a large range to choose from including 0.8X 0.63X and 0.5X.

 

I figure that the reducer without coma correction is superior.  However, I have read about a number of people on Cloudy Nights using 0.63X SCT reducer-correctors on Maksutov-Cassegrain telescopes.  Even Sky-Watcher recommended on their YouTube Channel to use the Celestron 0.63X SCT reducer-corrector for their Skymax telescopes (the Skymax 127, 150, and 180 all have SCT threads for this apparently).  I would be curious to see a photo of what that might look like.

 

Would the coma corrector add coma if the telescope is already coma-free?  And if so, would the severity of the aberration be proportional to the angular distance from the optical axis (like coma is)?


Edited by Nicole Sharp, 24 November 2021 - 02:53 AM.


#3 luxo II

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Posted 24 November 2021 - 02:54 AM

I figure that the reducer without coma correction is superior.  However, I have read about a number of people on Cloudy Nights using 0.63X SCT reducer-correctors on Maksutov-Cassegrain telescopes.  Even Sky-Watcher recommended using the Celestron 0.63X SCT reducer-corrector on their YouTube Channel.  I would be curious to see a photo of what that might look like.

 

Would the coma corrector add coma if the telescope is already coma-free?  And if so, would the severity of the aberration be proportional to the angular distance from the optical axis (like coma is)?

Hi Nicole, 

 

The point about aberrations being additive is this... The aberrations of the complete assembly are the sum of the contributions from each optical element. They can add positive, or negative amounts.

 

Most reducers are designed to match a specific type of scope - small APO's (which have field curvature but not coma), newtonians (which have coma and field curvature) or SCT's (variable combinations depending on classic SCT, ACF or Edge).

 

Since your gregory-mak already has:

Spherical - zero

Astigmatism - zero

Coma - very low essentially zero

Field curvature - slight, concave like a refractor or SCT

Distortion - very low, essentially zero

 

what you should look for is a plain reducer, no field flattening and no coma correction. While a little field-flattening might be desirable, bear in mind most of the flatteners are designed to suit small refractors with FAR WORSE field curvature than your scope has, so the net result may be worse, not better.

 

One for an SCT should work OK. And so will others. Yet others for Newtonians or small refractors will be terrible.

 

NB a reducer designed to suit a small newtonian will have  a lot of coma correction AND a strong field curvature of the wrong sign for a mak, and the result won't be pretty. A Parracor is one example, there are others. 

 

The downside with trying to find the right one to match a specific scope is that it does involve some trial-and-error, unfortunately; many resellers do not state what sort of scope the reducer is designed to suit. Some of the better ones do (Stellarvue, Astrophysics, Celestron) but many do not. You may be lucky and make a good choice, maybe you will have to try two or three before you find one that suits.


Edited by luxo II, 24 November 2021 - 03:19 AM.


#4 maroubra_boy

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Posted 24 November 2021 - 06:18 AM

Like Luxo said, correctors for SCT's are designed to correct for the aberrations inherent to the SCT, which Maks either do not show or nothing as significant as SCT's (depending on the aberration type).  So putting an SCT corrector on a Mak will introduce a set of aberrations that it is trying to correct for that don't exist.

 

Reducer/field flatteners for refractors work the same way.  They are designed for a specific scope and f/ratio and cannot be successfully interchanged with other f/ratios of other fracs, and certainly not with SCT's or Maks or Newts.

 

Intes at one time did make a reducer for their Rumak Maksutovs, but these are no longer made new, and to find one second hand is like finding a diamond unicorn...  I know of only one having come up for sale in the last few years and this was recently.

 

Cheap generic focal reducers will also introduce a bucket load of aberrations as these reducers are not designed for any specific scope/aberrations set.  Sure they reduce focal length but at a steep cost of aberrations off axis, especially with Newts.

 

If this is for photo, best use a shorter focal length scope than try for a focal reducer for a Mak.  If this is for visual, then use longer focal length eyepieces, in particular if your Mak takes a 2" diagonal.



#5 jan.vantomme

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Posted 24 November 2021 - 08:40 AM

On long focus classic cassegrains, a 50mmx200 achromat (binocular objective) can sometimes be used as FR.

Long ago (20 years ;o) we had quite good results with a Edmund 50x200 (BaK4/Sf10) lens as main element in a reducer we made for a 620mm f/15 classical cass, reducing it to f/8 for the small field on a KAF1600 chip.

 

I made a quick test in OSLO based on a classical cass scaled down to 127/F12.1, ( = NOT a mak ! ) and put the lens in in a position to get 0,63 reduction, it gave a quite acceptable field of up to 1°.

(It is just a test to see if it works, not a practical design as the lens is now in front of the main mirror; i have to change the cass configuration to get focus further back behind the mirror)

 

If someone has the exact specs of the scope, entering it in OSLO, and trying some off-the-shelff achromats, may come up with a workable combination.

Attached Thumbnails

  • spot cass met FR.jpg

Edited by jan.vantomme, 24 November 2021 - 09:12 AM.


#6 Jon Isaacs

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Posted 24 November 2021 - 10:35 AM

NB a reducer designed to suit a small newtonian will have  a lot of coma correction AND a strong field curvature of the wrong sign for a mak, and the result won't be pretty. A Parracor is one example, there are others.

 

 

Just to be clear:

 

The Paracorr is not a reducer, it has a slight Barlow effect.

 

It also has very little correction of field curvature, it's arguable if it has any. Newtonians have very little field curvature. I've seen it suggested it's designed around a radius of curvature of about 1500 mm. 

 

Focal reducers for Newtonians are quite rare because Newtonians are not limited to slower focal ratios like most other designs. The challenge is correcting the coma. 

 

Jon



#7 Nicole Sharp

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Posted 24 November 2021 - 06:09 PM

I am guessing if you have both SCTs and MCTs, and only want one focal reducer, then it would be better to use a non-coma-correcting reducer on the SCT than to use a coma-correcting reducer on the MCT?

 

I'd still be curious to find out what the coma corrector looks like on the MCT, since apparently a lot of people are doing this.  If the coma corrector applies a "negative coma" then I would guess that maybe the MCT will look like an SCT, but with the coma reversed?  Would the comatic aberration on the MCT then be proportional in severity to the distance from the optical axis?

 

BTW, I talked to Sky-Watcher, and apparently there was some misinformation on their YouTube Channel.  The Skymax 127 is not SCT-threaded.  So I don't think you could use an SCT reducer-corrector in it anyway.  But I have read about people using a 0.63X reducer-corrector in an Orion Apex 127.


Edited by Nicole Sharp, 24 November 2021 - 06:12 PM.


#8 luxo II

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Posted 24 November 2021 - 07:05 PM

If someone has the exact specs of the scope, entering it in OSLO, and trying some off-the-shelff achromats, may come up with a workable combination.

That plot was for a cassegrain - not a gregory maksutov-cassegrain - different beast....and the other issue is the reducer, without the full specs of that (ie glass types, radii and spacings) it's just a guess anyway.

 

Nicole - given the cost of these things they're like eyepieces - trial and error. The annoying thing is some cost as much as your scope, and they're probably nothing special.


Edited by luxo II, 24 November 2021 - 07:09 PM.



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