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Persistent weird star shapes in corners even with coma corrector

Astrophotography Collimation Imaging Optics Orion Reflector
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#1 montygill

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Posted 04 May 2022 - 07:03 PM

Relative newbie here. Please see attached images. MainImage.jpg pointing in the general direction of Polaris. I am seeing elongated/triangular/T shaped stars on the top left (close up - TopLeft.jpg) and bottom left (closeup - BottomLeft.jpg) areas of the image. I have experimented for weeks on this persistent issue but to no avail so now, time to ask the experts. Any help is appreciated!

 

The image is a 2 second exposure without tracking.

 

Equipment:

- Orion f/3.9 800mm newtonian astrograph
- Atlas mount 
- Baader MPCC III coma corrector
-16.5mm extender + 20mm filter wheel EFW + 2mm extender + ZWO ASI 2600MM, ASIAir Pro per ZWO camera specs/recommendation.

 

Notes:

- I used 2 second exposures without tracking to catch optical issues vs tracking issues.

- Collimated visually (collimation cap) at times, collimated using Hotech laser at times, Collimated using various star testing methods, same issues.

- Rotated camera 180 degrees to confirm it is not sensor tilt.

- Experimented with changing backfocus distance from 55-58mm (refocusing each time), no luck. 

- Secondary mirror tilt still?

- Astigmatism defect in mirror?

- Rotated coma corrector, defect does not rotate, indicating coma corrector is ok (in terms of defect on one side of the coma corrector).

- Could use CCDInspector but defect is obvious.

 

Just wondering what I should suspect the most or what parameters I can change one at a time to narrow down the issue?

 

Attached files:

MainImage: MainImage.jpeg

TopLeft: TopLeft.jpg

BottomLeft: BottomLeft.jpg

 

 

Thanks guys!

Clear skies

MG


Edited by montygill, 04 May 2022 - 08:48 PM.


#2 SgrB2

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Posted 04 May 2022 - 07:20 PM

Your ZWO camera has a sensor tilt capability.

Have you eliminated that as the cause?

Oh, I see you have....

 

I magnified a portion of the images to better

see the shape of the stars.  The image on

the left is from Top Left above and the

image on the right is from Bottom Left

above.

 

Top Left.jpg Bottom Left.jpg


Edited by SgrB2, 04 May 2022 - 07:53 PM.

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#3 montygill

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Posted 04 May 2022 - 08:08 PM

Yeah @SgrB2. I think it is not sensor tilt based on my experiment of rotating the camera 180 degrees. On rotation of the camera, the elongated stars moved to the other side of the image indicating no sensor tilt issue.


Edited by montygill, 04 May 2022 - 08:08 PM.


#4 Rouzbeh

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Posted 04 May 2022 - 08:09 PM

I doubt the MPCC will fully correct an APS-C chip.

High end coma correctors typically cost a lot more.


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#5 ChrisWhite

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Posted 04 May 2022 - 08:21 PM

The mushroom shaped stars are pretty classic Baader MPCC III.  Unfortunately this coma corrector does not do well with large chips and/or tiny pixels.  I used to have one years ago and experienced the same aberration. 

 

You will find this thread interesting and relevant even though its a few years old:  https://www.cloudyni...iosn/?p=7508775


Edited by ChrisWhite, 04 May 2022 - 08:21 PM.

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#6 montygill

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Posted 04 May 2022 - 08:59 PM

Wow, Thanks @ChrisWhite. Imagine my relief when I saw the exact same mushroom stars in the link you posted. Looks like Paracorr II would do the job? Or are there newer/better coma correctors? Yeah the fast f/3.9 Newtonian I have is cheap but demanding I guess of the coma corrector.


Edited by montygill, 04 May 2022 - 10:26 PM.


#7 ChrisWhite

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Posted 05 May 2022 - 02:31 PM

The paracorr type ii has long been the gold standard for coma correctors. They do have a slight Barlow effect of 1.15x. I've also used the gpu/sky watcher corrector as well, which is very good in my opinion. For aps-c chip the paracorr is probably the best choice. I was using 4/3 chips at the time so I'm not sure how well they correct for the larger sensor, but my gut tells me televue. You can contact them to get an optionion, they make good stuff.
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#8 f300v10

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Posted 06 May 2022 - 08:52 AM

I have been using the Sky Watcher/GPU corrector together with an ASI2600MM for close to a year now and have been pleased with the results.  The paracorr is also a solid choice but costs more and as mentioned has a slight Barlow effect.  I also like that the SW/GPU is designed for 55mm back focus, which makes it easy since ZWO's standard back focus for the OAG/FW/2600MM is also 55mm.

 

The below image is from an older CN thread that compared the performance of the MPCC III, the SW/GPU and the Paracorr.  The SW/GPU and Paracorr are very similar in performance, and clearly superior to the MPCC.  You won't go wrong with either.  If you ever plan to do any visual work with your Newt then I would go with the Paracorr.  If purely imaging then I would choose the SW/GPU for the cost savings and 55mm back focus.

 

AcywTZD.jpg




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