Coma problems on images - Forums

Hi,

After few nights trying to adjust the correct distance for the coma corrector (the Baader MPCC MKIII) i always see some stars on the border that are not circle. Could be my collimation that is not perfect? This is a 3min test no dark/noflat etc.
The optic train is CAMERA->The 2 included extenders ->M42-M48adapter->Coma corrector (55mm backfocus with the T-kit) 1st image

No, it is that they lied about that performance. The corrected image circle is approximatly 16mm in diameter. And it puts a lot of spherical into it, witness the massive PSF you get on axis. Do like I did and return it. And get a TS/Sharpstar corrector (Maxfield at least).

John:
i use a gpu coma corrector,they are much better.
little tip https://cdn.shopify.com/s/files/1/1594/4815/files/Back_Focus_Spacing_Guide_1024x1024.jpg?v=1606528627

So is too close...maybe the included spacer of 0.1 and 0.2 mm can do the trick? If so i'm gonna test with one or the other to see which one works for me

Manuel Aprile:
John:
i use a gpu coma corrector,they are much better.
little tip https://cdn.shopify.com/s/files/1/1594/4815/files/Back_Focus_Spacing_Guide_1024x1024.jpg?v=1606528627

So is too close...maybe the included spacer of 0.1 and 0.2 mm can do the trick? If so i'm gonna test with one or the other to see which one works for me

Only if you have coma on axis. Besides I've already tried and doesn't work for me. Best of luck.

I have been having the same problem with my mpcc mark iii for about three years. I have tried it with my QHY163M + QHYCFW2 and QHY168C. I have the spacing right at 57.5 mm from the sensor to the M48 side, but I still get bad coma on the corners.

Everyone on the forums says the sky-watcher quattro is better but I really haven't wanted to blow $375 on a new corrector.

^this is how my stars typically look in the corners when I have over-stretched it to make it more apparent.

If you ever get yours working message me with the fix, good luck!

Here is the performance of the Sharpstar 0.95x coma corrector (which is identical to the TS Maxfield). APS-C sensor on a F/4 newton (~ 5min integration around Polaris). It is only £45 more than a"naked" MPCIII. Slight tilt NE to SW of the camera:

andrea tasselli:
Here is the performance of the Sharpstar 0.95x coma corrector (which is identical to the TS Maxfield). APS-C sensor on a F/4 newton (~ 5min integration around Polaris). It is only £45 more than a"naked" MPCIII. Slight tilt NE to SW of the camera:

I don't think it could get much better than that. I may get a sharpstar coma corrector for myself. I'd never heard of that brand until now 👍

Edit: I just ordered a sharpstar corrector from agenaastro. It's on backorder but I will post my results here whenever it comes and the sky is clear.

Thanks guys. Before i send it back i will try to add some spacers to see if maybe it will work for me. If not i will send it back and buy another. Thanks again!

You already made your choice but I could have only supported what Was already said.

I used the MPCC from day one and tried to get round stars to the corners for a big portion of the clear nights after I got my 10" F4 Newtonian but even with more spacers it never came to a satsisfactional result. Therefore, after spending hours and hours without success I went for the TS GPU corrector.

What can I say, in direct comparison the MPCC only wins in build length, the TS GPU presents great results already with a very rough collimation and overall the stars are more pinpoint and smaller throughout the while FOV.

The first images made me feel like I bought a new scope.

Looking forward to hear about your experience with the sharpstar.

I've done some test adding 0.6mm and 0.5mm this is the result(1st and 2nd image). But i notics that sometimes the stars on 0.6mm are more uniform than 0.5mm in different corners like the images(3rd and 4th image) What can cause this? Also...what is the better distance between the two? or should i try adding more

Usually is due to a slight tilt of camera + CC. I wouldn't worry too much. Choose the one that gives the tightest stars on average.

I've done several tests and it looks like that 0.6mm is the one that makes the majority of stars round. Should i be happy with that or return it and buy a more efficient one?

If you feel you can live with it then by all means keep it. If not, then return it.

I spent a serious amount of time trying to correct my 8” F/4 with an MPCC. The best I could get on a CCDinspector test was slight over correction on axis and slight under in the corners. I swapped it out for a GPU unit and got near perfect correction across the field. More significant was the reduction in bloating. My FWHM average reduced by about 25% !

Ryan Jones:
More significant was the reduction in bloating. My FWHM average reduced by about 25% !

Exactly this was my main observation as well when I changed to the GPU. I don't miss the MPCC, not saying that it wasn't sufficient for a while but with F4 and a larger sensor it wasn't able to catch up, no matter how hard I tried (loosing rare clear nights).

I've decided to return the Baader MPCC...i've done so many tests and nothing was working right... i was deciding between these two :

1. TS-Optics 0,95 Maxfield
2.TS-Optics 1.0 TSGPU Superflat

If u have others let me know ....and thanks for the Help guys!

I am using the latter one, the TSGPU is a perfect fit for the TS Newtonian I am using.

The TSGPU is certainly going to thick all the boxes except for price (if you can find it). It is also considerably longer and it may protude in the light path in some configurations. The Maxfield is considerably shorter but does cover only up to APS-C format although is considerably cheaper and might be easier to source in some markets.

andrea tasselli:
The TSGPU is certainly going to thick all the boxes except for price (if you can find it). It is also considerably longer and it may protude in the light path in some configurations. The Maxfield is considerably shorter but does cover only up to APS-C format although is considerably cheaper and might be easier to source in some markets.

Both of them are good though. If that's the case ill go with the Maxfield...it's cheaper...in the near future i don't see me buying and APS-C format camera..i'll stick with my newton 150+294mc pro for a long time..so i think it will be a good substitute of the Baader MPCC

Thanks again for the feedback

How fast is your newtonian? The Maxfield corrector will introduce slight spherical aberration at f/4, a GPU corrector will give tighter spots on axis at those f-ratios.

I didn't see on your post what camera you are using. The 55mm back focus will include the gap between the sensor and the mount on the camera (in my case that is 6.5mm), so spacers for me are 48.5mm. Sorry if I am stating the obvious.

Hi folks,

I am following this discussion with a huge interest. Personally I use an Explore Scientific HR coma corrector. You are talking about using spacers in the range of 0.5mm or close to that. But how do you deal with filters? Do they effect the star shapes? I heard, that they may increase the backfocus distance a bit. If so, do the different filter types influence this distance, too?

Thank you for your help…

CS

Christian

Christian Großmann:
But how do you deal with filters? Do they effect the star shapes? I heard, that they may increase the backfocus distance

You should add to the rated backfocus of your corrector one third of the filter thickness.

Most filters brands are 2mm thick so you should add 0.67mm. Chromas and Astrodons are 3mm thick so with them you should add 1mm of backfocus.

Because of this it is not a good idea to mix filters of different thicknesses in the filter wheel.