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Coma Corrector Compariosn

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#1 pete strakey

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Posted 26 October 2016 - 09:25 PM

Just got the Skywatcher Quattro 4-element Aplanatic coma corrector and though I would compare it to my Baader MPCC Mark III. I was not thrilled with the MPCC, which is why I decided to go for the Skywatcher which set me back $285. The MPCC is a 2-element design that runs about $215 while the Skywatcher is a 4-element design made by GPU Optics in Hungary and also sold by TS. I used my Orion 8" f3.9 reflector with a full-spectrum modded EOS 500D and a Baader UV/IR cut filter. Done with my Celestron AVX mount, unguided 15 sec shots at ISO 1600 in the vicinity of Vega. I took great care to get the best collimation I could with a combination of laser, cheshire and DIY autocollimator. I achieved best focus for each test with BYEOS looking at Vega centered on the sensor minimizing FWHM and checking with a Bahtnikov mask.

 

The top row of images are crops roughly 160 pixels square from a region near the center of the senor.

center-crop.jpg

 

The bottom row are crops near the edge of the sensor (roughly 11 mm right of center at image plane).

edge-crop.jpg

 

My suspicions about the MPCC were confirmed that while it does significantly correct coma it does introduce substantial abberations over the entire field. From what I understand, these are spherical abberations. I did take great care to get the back spacing just right with the MPCC at 57.5 mm from the edge of the M48 mounting surface. The backspacing for the Skywatcher was at 54.3 mm (recommended is 53.66 mm for f=800 mm). Also note the double diffraction spikes from nearby Vega with the MPCC. I saw this type of thing (double or sometimes "fat" diffraction spikes) on several nights of imaging with the MPCC having tweaked collimation and coma corrector mounting each night so it seems this really is an artifact of the MPCC and not something specific to my setup on any particular night.

 

The Skywatcher produced nice stars across the field and doesn't seem to introduce any other abberations. There was some apparent elongation in the vertical direction near the edge of the field and this might be due to the camera/corrector not being perfectly square to the optical axis of the scope. I need to test this theory out.

 

I have been able to reproduce the MPCC results showing spherical abberation with OSLO using measured surface curvatures and lens separations. I'm a bit surprised to see all of the positive feedback on the MPCC here on CN, but maybe it works just fine for visual observing.

 

If anyone has the prescription data for the Skywatcher/GPU, I would love to plug this into OSLO as well. It would also be interesting to see a comparison of the Skywatcher with the Paracorr Type II.

 

Pete


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#2 Steve D.

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Posted 27 October 2016 - 04:59 AM

Great review.   Thanks for posting.



#3 happylimpet

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Posted 27 October 2016 - 05:42 AM

Interesting. Ive often wondered how the cheapy 0.9x skywatcher coma corrector I use with great success compares to the 'standard' paracorr.



#4 sixela

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Posted 27 October 2016 - 11:22 AM

That cheapie Skywatcher corrector ain't this one, and performs much like the MPCC.
 

but maybe it works just fine for visual observing.

At high powers it's far worse.

For photography, the MPCC is good enough or bad depending on your sensor's pixel pitch and how much the seeing blurs your stars anyway.

Edited by sixela, 27 October 2016 - 11:23 AM.


#5 billdan

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Posted 27 October 2016 - 07:26 PM

Its a pity we can't try out these things before we commit to buying.

 

I recently purchased a MPCC MKIII for my 8in F5 Newt and while I was very happy having round stars right to the edge of the field, I was a bit peeved that the stars in the centre were larger compared to no CC.

 

Bill

 

 

 

 



#6 sopticals

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Posted 27 October 2016 - 07:54 PM

Anyone done a general shootout between all (or most) of the currently available coma correctors??

 

Stephen.(45deg.S.)


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#7 Jeff Morgan

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Posted 27 October 2016 - 08:03 PM

That cheapie Skywatcher corrector ain't this one, and performs much like the MPCC.
 

but maybe it works just fine for visual observing.

At high powers it's far worse.

For photography, the MPCC is good enough or bad depending on your sensor's pixel pitch and how much the seeing blurs your stars anyway.

 

That's unfortunate. I picked up a 6" f/4 to use with my NV eyepiece and was considering the Baader since it does not change the effective focal ratio of the base telescope.



#8 happylimpet

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Posted 28 October 2016 - 04:02 AM

That cheapie Skywatcher corrector ain't this one, and performs much like the MPCC.
 

but maybe it works just fine for visual observing.

At high powers it's far worse.

For photography, the MPCC is good enough or bad depending on your sensor's pixel pitch and how much the seeing blurs your stars anyway.

 

Not so. I wouldnt necessarily trust it for planetary, but its certainly hugely better than the MPCC results shown here. When focusing using perhaps 10ms exposures on a bright star (1600MM-C advantages) I get pin sharp stars dancing around with the seeing in the centre (where I focus) and the resultant images show similar or identical FWHM/PSF across the field, at least to a slightly more than casual glance.  I am working at f5 though, or f4.5 after the 0.9x CC, which perhaps helps me out.

 

To be honest I find it slightly hard to believe that the MPCC performs as badly as shown here.


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#9 AlienRatDog

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Posted 28 October 2016 - 05:21 AM

How does this coma corrector change the focal ratio?

#10 Redbetter

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Posted 28 October 2016 - 06:10 AM

 

To be honest I find it slightly hard to believe that the MPCC performs as badly as shown here.

 

The MPCC certainly comes off poorly in this comparison and makes one wonder if it is of any real value.  I would rather have a sharper central field than suffer the damage shown here to modestly correct the outer field.  One can't help but wonder if it was a bad sample (especially due to the double image) or if the spacing was non-ideal despite the OP's attempts to account for this.  However, being only 2-element perhaps the problems are to be expected vs. 4-element designs.  Give a designer enough parameters to adjust and they should be able to outperform a more restricted designer of similar skill.  



#11 pbkoden

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Posted 28 October 2016 - 12:07 PM

Here is another comparison. The text isn't in English, but images are universal. It compares the MPCC, Paracorr, Skywatcher/GPU 4-element, and Wynn correctors.
 
The Skywatcher/GPU corrector fares very well in the test. It is nearly as good as the Paracorr, but cheaper and without the barlow effect.
 
An image from the article:

 

AcywTZD.jpg


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#12 sixela

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Posted 29 October 2016 - 05:21 PM

Not so. I wouldnt necessarily trust it for planetary, but its certainly hugely better than the MPCC results shown here.


But do you have an f/4 scope?

In an f/5 scope it will add a lot less spherical aberration.

#13 sixela

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Posted 29 October 2016 - 05:21 PM

How does this coma corrector change the focal ratio?

Not.

#14 sixela

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Posted 29 October 2016 - 05:24 PM

However, being only 2-element perhaps the problems are to be expected vs. 4-element designs.

Yes. None of the 4-lens (and 3-lens) correctors I've seen add spherical aberration like the Ross coma corrector-like ones (like the MPCC and the "simple" 2-lens Skywatcher).

That includes the GSO, the Skywatcher Quattro coma corrector (also sold by TS), the Baader RCC1 (which is fine but wants a whopping 90mm between its top and the focal plane!), the Explore Scientific and the Paracorr.

#15 pete strakey

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Posted 29 October 2016 - 06:19 PM

Thanks for the link pbkoden. I will have to translate it. A quick look and I saw a similar issue with the MPCC and fat diffraction spikes. I have read that the MPCC is exceptionally sensitive to tilt misalignment and maybe that could explain the results I and others have seen. I went through the most thorough collimating process I could so my take on that is if it is that sensitive to collimation then it is no good for my purposes.

#16 RAC

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Posted 30 October 2016 - 01:20 AM


My suspicions about the MPCC were confirmed that while it does significantly correct coma it does introduce substantial abberations over the entire field.

 

I found the exact same thing. The MPCC is just no good at f4 if you want nice tight small round stars. My MPCC mk3 was the cause of a lot of frustration.

 

14033952529_da3c71a20e_b.jpg

Corrector test by Raymond Collecutt, on Flickr


Edited by RAC, 30 October 2016 - 01:21 AM.

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#17 Redbetter

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Posted 30 October 2016 - 05:10 PM

The contrast in the fine structure of the dust lanes is lost in the MPCC image. 


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#18 happylimpet

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Posted 31 October 2016 - 10:43 AM

 

Not so. I wouldnt necessarily trust it for planetary, but its certainly hugely better than the MPCC results shown here.


But do you have an f/4 scope?

In an f/5 scope it will add a lot less spherical aberration.

 

Good point (currently f5) - though distressingly I will soon be using it in  an f4 scope so perhaps I have a nasty shock awaiting. At least I am forewarned! In fact I have suspected something that looks (in retrospect) like spherical abberation while running through focus....perhaps at f4 this will be a real issue. If so I will have to get a new coma corrector I suppose.



#19 GeneT

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Posted 31 October 2016 - 05:55 PM

Great review.   Thanks for posting.

Yes!



#20 Jeff Morgan

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Posted 31 October 2016 - 06:43 PM

How does this coma corrector change the focal ratio?

Note I said the effective focal ratio. The Paracorr has a 1.15 barlow effect.



#21 radioman67

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Posted 02 November 2016 - 12:22 PM

 

How does this coma corrector change the focal ratio?

Note I said the effective focal ratio. The Paracorr has a 1.15 barlow effect.

 

Hello everybody. I have a telescope Bresser NT203 200/1000 F5. I want to buy it for TS Quattro 2 "Coma Corrector for Newtonian Teleskopes. Tell me your opinion and what you think about this?



#22 sixela

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Posted 02 November 2016 - 04:00 PM

Do you have enough focuser in-travel available to bring a DSLR plus a T-ring in focus now? If not, then the Skywatcher Quattro corrector is not for you.

#23 alacant

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Posted 09 December 2016 - 04:35 PM

 

 

How does this coma corrector change the focal ratio?

Note I said the effective focal ratio. The Paracorr has a 1.15 barlow effect.

 

Hello everybody. I have a telescope Bresser NT203 200/1000 F5. I want to buy it for TS Quattro 2 "Coma Corrector for Newtonian Teleskopes. Tell me your opinion and what you think about this?

 

Hi. The ts-quattro brings the focus out from the telescope by about 8cm. You end up with your dslr hanging off the end of an unbalanced flex-rife setup . Here it is with my Bresser 150/750. Hopelessly unstable:

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#24 alacant

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Posted 09 December 2016 - 04:46 PM

Do you have enough focuser in-travel available to bring a DSLR plus a T-ring in focus now? If not, then the Skywatcher Quattro corrector is not for you.


Edited by alacant, 09 December 2016 - 04:48 PM.


#25 GlennLeDrew

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Posted 10 December 2016 - 06:43 AM

In the ATM forum Ed Jones points out that the MPCC III performs much better when the primary is a hyperboloid. Compared to a paraboloid, a hyperboloid is spherically overcorrected, and at the proper conic constant corrects for the MPCC's spherical under correction.

 

He goes a step further and provides a prescription whereby one of the lens surfaces is changed in radius. With an f/3 hyperboloid (of conic constant value I don't recall) the image is nearly diffraction limited over a 2 degree field! Pretty good for a simple 2-element optic.


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