HELP! Bad coma with 8 in newt and baader mpcc mk III - Forums

Topic starter

Feb 1, 2022

Am I doing something wrong? I keep getting bad coma at the edges. Has anyone had better luck with this CC?

Ive tried to make the imaging train as "flat" with the focuser, as adjusting how the train sits in the focuser has improved the coma significantly. Despite being as straight in the focuser (I spent over 2 hours ensuring the imaging train was flat in the focuser. The coma appears to be more significant at the top right of the image. I have made sure the OAG is secured well and "flush". I notice the coma is worse with the Lum filter, which is a baader UV-IR cut filter. I dont believe collimation is the issue... Tracking was okay, 0.60 arc sec of error, I didnt notice star trailing in my other images taken last night.

Any ideas? Too much spacing? Too little? Why is the coma more significant on the right side?

Should I throw everything out and give up? lol

Aastro123

Topic starter

Feb 1, 2022

Here is another photo. Its 120 2 min photos with the lum filter. I think it shows how bad the coma is...

Guillermo de Miranda

Feb 1, 2022

I'm having the same issue with a newt (130mm f/5) and the MPCC MkIII. I personally think it's a spacing issue; it's not exactly 55mm but I think a little more. I have yet to put my theory to the test due to lack of spacers ( I think it's a matter of millimeters).

Aastro123

Topic starter

Feb 1, 2022

Could possibly be the focuser? I think it looks like the imaging train is tilted, and maybe the focuser isnt holding the cc as "snugly" as it should

andrea tasselli

Frequent contributor

Feb 1, 2022

I don't think is an issue with the CC. It's more likely a tilt issue within the imaging train. I can't see much other than flaring on one side of the image and nearly nil on the other.

D. Jung

Feb 1, 2022

The reviews that i have seen with this corrector were not convincing...

https://www.astrofotoblog.eu/?p=856

andrea tasselli

Frequent contributor

Feb 1, 2022

I tested it and found it wanting at f/4. At f/5 it should not be an issue as even the old MPCII was doing a decent job.

mastermerlin

Feb 1, 2022

I had similar issues and can say that the aligning the MPCC is a bit of a tedious process and it is easy to have a tilting issue or the distance being off.

From looking at your image train it seems you are missing the original distance ring on the MPCC which will already produce a coma similar to your images. I made the same mistake coming from an EOS OAG and forgot to put it back to where it belongs. Maybe you can check that first and verify that the distance from the position on the top of the MPCC to the camera sensor you have 55mm (better 55.5 to 56 mm from my experience ).

Then try to find a way to not tilting whilst tightening the MPCC in the focuser.

Then realize that it might be easier to go with another corrector, as I went for the TSGPU, and never look back to the MPCC again, as it just works without this hazzle with way better FWHM.

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Ryan Jones

Feb 1, 2022

If you look at the illumination in your first image, the brightest area is off center. It’s to the left and down. That means your optical alignment isn’t 100%. That might be tilt in the focuser or collimation. Either way I think if you address that it will fix the problem. I know the baader MPCC III is ok at f/5 but from experience it cannot correct f/4 properly and as has been said before, isn’t as sharp as a GPU corrector…….but it is half the price.

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Haakon Rasmussen

Feb 1, 2022

Hallo Aastro123,

i have had similar problems with my 8" Newton. It was due to not sufficiently good enough collimation. I can see from your photo that your field of illumination ist no quite symmetric. How do you collimate your newt? I struggled for about a year to get my newt collimated properly.

CS
Haakon

Concise

Aastro123

Topic starter

Feb 2, 2022

I had similar issues and can say that the aligning the MPCC is a bit of a tedious process and it is easy to have a tilting issue or the distance being off.

From looking at your image train it seems you are missing the original distance ring on the MPCC which will already produce a coma similar to your images. I made the same mistake coming from an EOS OAG and forgot to put it back to where it belongs. Maybe you can check that first and verify that the distance from the position on the top of the MPCC to the camera sensor you have 55mm (better 55.5 to 56 mm from my experience ).

Then try to find a way to not tilting whilst tightening the MPCC in the focuser.

Then realize that it might be easier to go with another corrector, as I went for the TSGPU, and never look back to the MPCC again, as it just works without this hazzle with way better FWHM.

Yea i have that ring still. Ill try putting it back on!

Aastro123

Topic starter

Feb 2, 2022

Ryan Jones:
If you look at the illumination in your first image, the brightest area is off center. It’s to the left and down. That means your optical alignment isn’t 100%. That might be tilt in the focuser or collimation. Either way I think if you address that it will fix the problem. I know the baader MPCC III is ok at f/5 but from experience it cannot correct f/4 properly and as has been said before, isn’t as sharp as a GPU corrector.......but it is half the price.

this scope is f5. I think you are right, the imaging train is probably tilted. I may try to add a 3rd screw to the focuser?... Might be worth a shot, its not an expensive focuser (which also may be part of the problem)

Aastro123

Topic starter

Feb 2, 2022

Haakon Rasmussen:
Hallo Aastro123,

i have had similar problems with my 8" Newton. It was due to not sufficiently good enough collimation. I can see from your photo that your field of illumination ist no quite symmetric. How do you collimate your newt? I struggled for about a year to get my newt collimated properly.

CS
Haakon

I use a cheshire eyepiece. I believe the illumination is off because the frame is uncalibrated? im using 1.25" filters which always have some sort of vignetting. The pic with the Perseus cluster of galaxies was a quick DBE in pixinsight, only for the purpose of showing the star shapes (I figured since it has more stars than the leo triplet image). Ill recheck collimation my next imaging session, it would be a bit embarrassing if this whole problem was just collimation, but I figure theres some tilt issue as well

Haakon Rasmussen

Feb 2, 2022

Hallo Aastro123,

i thought the hole time that i had tilt in my image train. ASTAP image inspector told me so. After getting the collimation in order, i have no more tilt according ASTAP. So in my case bad collimation was seen as tilt in the image train.

CS
Haakon

Concise

Ryan Jones

Feb 2, 2022

=16pxI think you are right, the imaging train is probably tilted. I may try to add a 3rd screw to the focuser?... Might be worth a shot, its not an expensive focuser (which also may be part of the problem)

I found on mine ( GSO ) it was the draw tube in the focuser that was tilting in the focuser body. I added some grub screws through the body of the focuser onto the bearing guides so I could adjust it to keep the draw tube central. The addition of an auto focuser also helped as I no longer need to do up the focuser lock which also introduces tilt. I also intend to add a third screw to keep my coma corrector centralized. Keep us posted on your efforts to correct it.

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Aastro123

Topic starter

Feb 6, 2022

Hello everyone!

After some testing and fiddling, I think i fixed the problem (mostly). My collimation was definitely a bit off, so that was fixed using my cheshire eyepiece. I just had to adjust my seconday mirror a bit. I also added a 1mm spacer between my camera and the efw. I also took mastermerlin's advice and added the ring back onto the baader mpcc (pic attached). I originally took this off because it did not fit onto the OAG when i originally had it asi1600mm-efw-11mm ring-oag-mpcc (I switched it to asi1600mm-efw-oag-11mm-baader mpcc. It now fits when i use the other attachment ring that came with my oag. Now the mpcc sits flush in the focuser.

In conclusion, I think Ive got the coma under control where I dont feel like im wasting time imaging. If this happens again, ill check collimation. The coma is perfect, but given the price compared to other coma correctors, I think its about as good as its going to get. Maybe Ill tweak around more with the spacing and see if adding another spacer helps...

Thanks everyone!!!!

(ignore the poor background calibration! My neighbors turned on their porch light after I went to bed. The m106 image had background calibration selected in DSS, giving it that checkered look, especially in the bottom left (you can turn that off to "No background calibration" under "light" in "stacking parameters" in case anyone else if having that issue))

All photos using luminance filter 120 sub

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Haakon Rasmussen

Feb 6, 2022

Hallo Aastro123,

the stars on the M106 photo are really good. Glad you fixed your problems.

CS
Haakon

Dave B

Feb 8, 2022

I shimmed my back focus on my mpcc to 57.1mm, lose the m42 fitting on the mpcc and use the m48 fitting, fitted the baader steeltrack focuser (yet to test) but oozes quality honestly, but also very important check if your focuser is square to the OTA more often than not they aren’t, line a long metal rule along spider vane and extend focuser out maximum then eye the edge of ruler to focuser body, also invest in a concentre 2”eyepiece , hope this helps .

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