I guess that is great if one is searching for a coma corrector for an f/3 hyperboloid. But these are sold as "multi-purpose" coma correctors for paraboloids for which they appear to be ill-suited.

Coma Corrector Comparison
#26
Posted 10 December 2016 - 04:27 PM
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#27
Posted 11 December 2016 - 02:19 AM
Indeed, the realm of employment would be more "uni-purpose", that being prime focus imaging at not fast f/ratios on coarser pixel pitch sensors.
#28
Posted 05 March 2017 - 12:20 PM
Hi. I think the reason the Baader mpcc looks bad in the images is because it's not in focus. Check the soft stars and double spikes... Here it is on a f3.9 newtonian:
http://www.astrobin.com/281939/
HTH.
#29
Posted 06 March 2017 - 11:51 AM
I like the idea of comparing the images. But, the uncorrected image appears to show that the camera pixels are too large for this scope (camera is not properly matched to the scope). In cases where the CCD camera pixels are too big for the scope undersampling results in the image, and at its worst it can lead to square, blocky stars, or even stars that are missing completely. The uncorrected image seems to show square stars, a dead give away to the undersampling present in the image. Minor undersampling is not necessarily a bad thing though. I would love to see the same test where the CCD is properly matched to the scope. I am just not sure how much the correction results shown are influenced by the undersampling. I would think that if the image was correctly sampled, the Baader image stars would look more full and round. The Skywatcher images does seem to do a better job the with the undersampled image, but even there the stars still appear a little square. Just my two cents worth!
Kaos
#30
Posted 06 June 2018 - 06:07 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
Hi Bill,
I've been spending so much time trying to figure out what coma corrector works best for my 8" f/5 newtonian that I can get for under like $200-300. It would be preferable if it was a 1.0x so that I don't have to have a different FOV and effective focal length. I might be able to live with a 0.9x, but I was wondering if you could post a picture showing this effect where the stars in the center are slightly larger. I never heard of that happening.
Also, would you recommend for me the MPCC MKIII or the Skywatcher? This is for astrophotography, just to be clear.
#31
Posted 06 June 2018 - 07:56 PM
As I recall, typical photographic coma correctors tend to bloat the stars in the center while more effectively tightening star images at the edge.
I haven't heard of a specifically visually-oriented coma corrector that did not have magnification, however.
As for spherical correction on axis, it's not really fair to compare a simple lens like the MPCC to a 5 element design like the Paracorr.
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#32
Posted 06 June 2018 - 08:49 PM
Hi Dirac,
That quote I made was with an eyepiece, where the stars in the center were slightly larger than without the MPCC MKIII at best focus.
When imaging with the camera, the star size is variable depending on seeing blur, i.e. the longer the sub frames the fatter the stars can get.
The MPCC MKIII does not change the focal length like some other coma correctors, as this is the only one I have used I can't comment on the Skywatcher version. However I am happy with it for imaging purposes and it does its job well, I wish the back focus limitation of 55mm was a bit longer though.
If you use a mono camera with a filter wheel and an OAG, you have to ensure it fits into the 55mm back focus. Most CCD cameras have the sensor recessed 20mm from the front, the filter wheels are around 20mm wide, so that restricts the OAG size to less than 15mm wide. In my case I use the Orion thin OAG which is 10mm wide, but its not the best of OAG's.
Bill
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#33
Posted 07 June 2018 - 06:14 PM
As I recall, typical photographic coma correctors tend to bloat the stars in the center while more effectively tightening star images at the edge.
The simple ones convert coma into spherical and field curvature.
I haven't heard of a specifically visually-oriented coma corrector that did not have magnification, however.
These can be made, but the size of the scope has to be pretty big in order to fit the CC in the light path without vignetting the central light columns (15+ inches with a usefully large secondary) limiting demand.
#34
Posted 10 June 2018 - 06:42 AM
Hi Bill,
I've been spending so much time trying to figure out what coma corrector works best for my 8" f/5 newtonian that I can get for under like $200-300. It would be preferable if it was a 1.0x so that I don't have to have a different FOV and effective focal length. I might be able to live with a 0.9x, but I was wondering if you could post a picture showing this effect where the stars in the center are slightly larger. I never heard of that happening.
Also, would you recommend for me the MPCC MKIII or the Skywatcher? This is for astrophotography, just to be clear.
I bought 8" f/5 Newtonian and still in the box, looked at many reviews or comparisons, sounds ASA is almnost the best followed by Paracorr, and after that GPU f4 Aplanatic one is next, and that is Skywatcher one, i bought one and still didn't test it, but i have a big faith that it will be better than Baader MPCC MKIII, good luck!
#35
Posted 16 August 2018 - 09:33 AM
Hello,
Thanks to have initiated this thread.
I own a Baader MPCC first version mounted on a SW 200/800 with a camera QHY using a APSC sensor. I experienced all the problems mentioned here. The double spikes on the bright stars when in focus, quite spherical stars in center but mushroom or comets stars in the edge or the angles. Loss of details in the image. I have seen all of this.
Recently I exhumed a 30s jpeg image taken with my ancient SW150/750 and a 4/3 hybrid camera and I was stunned by the thin spikes (with viewable diffraction) of Alnitak and the roundness of the stars. And this without any coma corrector.
I never get this result with the MPCC Baader on the SW200/800. I guess I will have to make some tests, first without coma corrector, after with MPCC.
But I am quite sure that I will have to change the MPCC to a more performing device. But which one ?
The image of Alnitak : https://drive.google...8KsplaeUXTbNF_3
#36
Posted 18 August 2018 - 02:12 PM
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.
So if I'm grinding my own mirror, would it be easier to leave it spherical and then buy an MPCC to go with it? Would the spherical aberration be bad?
#37
Posted 26 August 2018 - 09:18 PM
Paracorr 2: perfect stars to f2.5, flattens field, 1.15x, $400 used
Paracorr 1: perfect stars to?f4?, Flattens field, 1.15x, $200 used
Skywatcher4: excellent stars at f4, flattens field?, 1x, $230 used
Cheap skywatcher: good stars at f5, 0.9x
Baader MPCCiii: Stars bad at f4 but good at f5. What about f4.5?, Flattens field?, 1x, $200 new?
GSO: designed for f4.5, flattens field? 1.1x, $115 new, $85 used, needs extension?
A few questions in addition to those above:
If the same CC is put in an f5 and then an f4, does the optimal distance from the focal plain change?
Does bringing it closer to the focal plain only reduce correction, reduce all effects, or increase aberrations?
Do the 2-lens coma correctors have better contrast at f5 than the 4-lens? (I know TV is 5)
Do the 1x flatten the field or at least not add curvature?
Do the 2 lens ones add noticeably more chromatic aberration than the others?
Thanks.
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#38
Posted 26 August 2018 - 10:06 PM
1.If the same CC is put in an f5 and then an f4, does the optimal distance from the focal plain change?
2.Does bringing it closer to the focal plain only reduce correction, reduce all effects, or increase aberrations?
3.Do the 2-lens coma correctors have better contrast at f5 than the 4-lens? (I know TV is 5)
4.Do the 1x flatten the field or at least not add curvature?
5.Do the 2 lens ones add noticeably more chromatic aberration than the others?
1.No.
2.reduces correction and may increase aberrations
3.No. I went from 4 element Paracorr 1 to 5 element Paracorr 2 at f/5 and saw improved correction and no loss of contrast.
4.Flattening can still take place, but the design may not do so. Many field flatteners are 1X. Many are not.
5.Not necessarily, but asking for good chromatic control, field flattening, no visible spherical aberration added (the bane of the 1x CC), and coma corrected to short f/ratios
is asking an awful lot for a simple corrector. Want all that at f/3.5, and it's going to be more complicated to achieve it.
Edited by Starman1, 26 August 2018 - 10:07 PM.
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#39
Posted 26 August 2018 - 11:02 PM
Add to that the question of using a coma corrector reducer, will it change things too? i mean using for example 0.73x reducer on f5, will the quality or degree of corrector be less or it exaggerates CA?
#40
Posted 27 August 2018 - 02:38 PM
Add to that the question of using a coma corrector reducer, will it change things too? i mean using for example 0.73x reducer on f5, will the quality or degree of corrector be less or it exaggerates CA?
First, I think you probably meant SA and not CA???
If the reducer you are talking about is something like the ASA Coma Corrector/Reducer from Telescope Service, I can tell you that it is quite outstanding. In my 6" f/4, it gives pinpoint stars across the field (f/2.8 with the corrector) and star test does not show any meaningful change in spherical aberration.
Now, that being said, this is not going to work with the vast majority of scopes out there because you really need about 90mm of back focus and even many imaging Newtonians don't have the necessary back focus to work with the ASA.
Also, a secondary that is sized for f/5 might not have a big enough secondary to give full illumination on an APS-C type sensor.
And of course when using the scope at f/4, one needs a big extension tube to reach focus.
I love my Boren Simon though.
In summary, the ASA reducer/corrector is absolutely excellent, but it is best applied to an optical tube that was optimized to the use of the corrector.
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#41
Posted 27 August 2018 - 08:29 PM
1.If the same CC is put in an f5 and then an f4, does the optimal distance from the focal plain change?
2.Does bringing it closer to the focal plain only reduce correction, reduce all effects, or increase aberrations?
3.Do the 2-lens coma correctors have better contrast at f5 than the 4-lens? (I know TV is 5)
4.Do the 1x flatten the field or at least not add curvature?
5.Do the 2 lens ones add noticeably more chromatic aberration than the others?
1.No.
2.reduces correction and may increase aberrations
3.No. I went from 4 element Paracorr 1 to 5 element Paracorr 2 at f/5 and saw improved correction and no loss of contrast.
4.Flattening can still take place, but the design may not do so. Many field flatteners are 1X. Many are not.
5.Not necessarily, but asking for good chromatic control, field flattening, no visible spherical aberration added (the bane of the 1x CC), and coma corrected to short f/ratios
is asking an awful lot for a simple corrector. Want all that at f/3.5, and it's going to be more complicated to achieve it.
Thanks Don.
I take it coma correctors can improve views in slow enough scopes, but even at f5, true pin points are only seen in a P2.
At f4 cheap correctors might not even be an improvement, and at f4.5 they improve but just not all the way.
I'll think about the P2.
#42
Posted 27 August 2018 - 09:52 PM
First, I think you probably meant SA and not CA???
If the reducer you are talking about is something like the ASA Coma Corrector/Reducer from Telescope Service, I can tell you that it is quite outstanding. In my 6" f/4, it gives pinpoint stars across the field (f/2.8 with the corrector) and star test does not show any meaningful change in spherical aberration.
Now, that being said, this is not going to work with the vast majority of scopes out there because you really need about 90mm of back focus and even many imaging Newtonians don't have the necessary back focus to work with the ASA.
Also, a secondary that is sized for f/5 might not have a big enough secondary to give full illumination on an APS-C type sensor.
And of course when using the scope at f/4, one needs a big extension tube to reach focus.
I love my Boren Simon though.
In summary, the ASA reducer/corrector is absolutely excellent, but it is best applied to an optical tube that was optimized to the use of the corrector.
I see, really don't know if it is SA or CA, but i forgot that it is a Newtonian so i deal with mirrors and not optics, thanks to correct me.
So i assume i can't get that ASA yet until i check out my Newtonian performance first, i thought this ASA reducer is fine with all or any Newtonian regardless of the design or quality, and didn't pay attention that big back focus distance, sounds i better buy another Newtonian then over an ASA reducer if it won't do good job and i know ASA corrector/reducer is an excellent one, saw tests and it is like even better than TV paracorr ones.
#43
Posted 28 August 2018 - 12:35 AM
Thanks Don.
I take it coma correctors can improve views in slow enough scopes, but even at f5, true pin points are only seen in a P2.
At f4 cheap correctors might not even be an improvement, and at f4.5 they improve but just not all the way.
I'll think about the P2.
Note that the coma-corrected field in the P2 at f/3.5 is the same size as a newtonian of f/13.2 without a coma corrector,
and the coma corrected fields of longer f/ratios than f/3.5 are larger still.
Ie. at f/3.5, a 40+mm field is free of coma.
At f/4 and f/4.5, the coma-corrected field is considerably larger than the largest eyepiece fields usable with the f/ratios.
It is only at f/3 that the coma-free field reduces to a smaller diameter (27mm, IIRC)
Compare that, however, to the no-coma-visible field without a coma corrector at f/4.5 of 1.62mm, and f/5 of 2.22mm, and f/6 of 3.84mm, and f/8 of 9.1mm.
You can see why I consider a newtonian a catadioptric scope design.
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#44
Posted 29 August 2018 - 12:10 PM
Dear all,
I was lost and didn't know what coma corrector I should purchase, so I bought many of them and made a worldwide coma correction exercise:
http://www.astrofotoblog.eu/?p=856
The study was done on TS carbon 10" f4 and MII G2 8300. CCD chip is micro 4/3, so for APS-C it would be even more pronounced.
Conclusion:
Explore Scientific coma corrector performs the best. It prolongs the focal length, but it's the price you pay.
It has acceptable vignetting and the stars are nice and round in every corner.
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#45
Posted 29 August 2018 - 01:27 PM
Too bad you couldn't afford the TeleVue Paracorr, but that's OK--your results are interesting.
Your sensors were 22.5mm on the diagonal. I wonder if the results would have been different with a 40-50mm sensor.
Rhetorical questions to the side, good report!
#46
Posted 29 August 2018 - 01:36 PM
One aside for visual observers about the Explore Scientific HRCC:
it requires a lot of in-travel compared to eyepieces. Be sure to have at least 1-1/4" of in-travel available
to achieve focus with most eyepieces.
#47
Posted 29 August 2018 - 05:41 PM
The P2 is so effective I doubt it will ever be upgraded except maybe with a better focuser or reduced need for barlow effect to appeal to some people, being marketed as remiving the need for a focuser uograde.
#48
Posted 30 August 2018 - 05:10 AM
Dear all,
I was lost and didn't know what coma corrector I should purchase, so I bought many of them and made a worldwide coma correction exercise:
http://www.astrofotoblog.eu/?p=856The study was done on TS carbon 10" f4 and MII G2 8300. CCD chip is micro 4/3, so for APS-C it would be even more pronounced.
Conclusion:
Explore Scientific coma corrector performs the best. It prolongs the focal length, but it's the price you pay.
It has acceptable vignetting and the stars are nice and round in every corner.
Fasinating reading, especially as an ASA 2korr owner! I have some issues with tilt, with one OTA more than another (the f4 is worse than the f5.....) but overall Im very happy. However, I think I may invest in one of the tilt adapters. And maybe even CCDInspector!
Note that while you found strong vignetting, the vignetting over the same TRUE field isnt so bad....remember the 0.73x. Flat fields are a must of course but work just fine.
Thanks again for this invaluable work! A great effort.
Also, for completeness, I'd like to see the skywatcher 0.9x included....if only as it gives a decent cheap option for some folks. I used one happily for a while before my ASA. But now Im just being picky!!!!!
Edited by happylimpet, 30 August 2018 - 05:11 AM.
#49
Posted 30 August 2018 - 07:44 AM
Paracorr 2: perfect stars to f2.5, flattens field, 1.15x, $400 used
Paracorr 1: perfect stars to?f4?, Flattens field, 1.15x, $200 used
Skywatcher4: excellent stars at f4, flattens field?, 1x, $230 used
Cheap skywatcher: good stars at f5, 0.9x
Baader MPCCiii: Stars bad at f4 but good at f5. What about f4.5?, Flattens field?, 1x, $200 new?
GSO: designed for f4.5, flattens field? 1.1x, $115 new, $85 used, needs extension?
A few questions in addition to those above:
If the same CC is put in an f5 and then an f4, does the optimal distance from the focal plain change?
Does bringing it closer to the focal plain only reduce correction, reduce all effects, or increase aberrations?
Do the 2-lens coma correctors have better contrast at f5 than the 4-lens? (I know TV is 5)
Do the 1x flatten the field or at least not add curvature?
Do the 2 lens ones add noticeably more chromatic aberration than the others?
Thanks.
How does the big 3 inch Paracorr compare for use with a 3 inch ES 30mm, for example?
#50
Posted 30 August 2018 - 08:44 AM
Here are the correction charts for the Paracorr II in 2":
http://televue.com/p...2_spotsizes.pdf
Mechanical drawing for 3" Paracorr:
http://televue.com/p... Big.pdf#page=2
In use in the field:
http://televue.com/n...r/#.W4fyg8BjOUk
It says chips as large as 52mm and scopes as fast as f/3.
If the parameters are the same as the 2", it will not 100% correct the 52mm field at f/3 but will at f/3.5.
That, though, is speculation on my part--you'd have to call TeleVue to find out.
It is not really designed for visual use--there is no 3" tunable top, for instance.
I think that TeleVue figured that conversion to 3" on dobsonians was unlikely. There are just too few scopes with a large enough aperture
that would benefit from it. Imagers, however, could benefit from it at much smaller apertures.