Donut ring around the bright star

[SamuraiOZ]

Hi all,

As shown in the attached picture, I'm annoyed by the donuts ring around the bright star.
It was stacked and enhanced from 20 shots of 300sec exposure with h-alpha filter on 200/800 F4 reflector telescope.
Before stacking, each shot is subtracted by dark frame and divided by (flat - dark) frame.
Flat frame is taken at the neighboring sky with translucent cover using exactly same telescope settings.

Has anyone had the similar experience using reflector telescope?
I appreciate if someone can share the technique to avoid this donuts ring or remove it in the post-processing.

[Merlin66]

Hi!, from the Bellarine.....

What Ha filter were you using? Looks like a reflection...

 

[SamuraiOZ]

Hi, thanks for the quick comment.

I'm using ZWO's 7nm Ha filter:

https://astronomy-imaging-camera.com/product/zwo-new-narrowband-1-25-filter

This donuts ring appears not only on this filter but also any filter such as L, R, G, B, and Oiii.

So I'm doubting this caused by reflector telescope's optics.

[Merlin66]

I was thinking of reflections from the filters....

[Astrobug]

Reflection from your coma corrector. It happens on verry bright stars.

[SamuraiOZ]

Coma corrector! Yes, I'm using MPCC-MARK III with the filters.

Is there any idea to reduce reflection from coma corrector such as anti-reflection coating or something?

Edited January 27, 2020 by SamuraiOZ

[tooth_dr]

@Peje I think Pete has some experience with this scope and CCs?

Nice image though @SamuraiOZ hope you get the issue sorted.   A quick look at the star shapes suggests that the spacing isnt quite right.

Edited January 27, 2020 by tooth_dr

[SamuraiOZ]

Hi, thanks for the suggestion. Does spacing mean the optical axis alignment?

Yes, it is another issue on my telescope. Since my F4 Newtonian has offset, I can not use laser collimating tool. I'm wondering how I can align it.

[tooth_dr]

10 minutes ago, SamuraiOZ said:

Hi, thanks for the suggestion. Does spacing mean the optical axis alignment?

Yes, it is another issue on my telescope. Since my F4 Newtonian has offset, I can not use laser collimating tool. I'm wondering how I can align it.

What I was suggesting is that the distance between the CC and CCD plane isn’t exactly what it should be.  That might be 55 or 57mm for example. 

[Peje]

I've never attributed rings to be down to the CC but that's perhaps because I haven't thought of it. I'd like to see some comparitive images. 

This is a well known, challenging star for all scopes. 

The problem I had with the Baader CC was that it was causing my stars to bloat. This could have been because my scope is f/4 but it does seem to be a widespread problem when you start search the internet for it. 

EDIT: Just noticed you're also running f/4. Mine is the SW Quattro. If it's the same I would suggest getting the CC made for the Quattro. 

Edited January 27, 2020 by Peje

[SamuraiOZ]

Yes, I'm using exactly same newtonian SkyWatcher Quattro 200/800 F4.

OK, I see. The issue caused by the incompatibility of Baader CC and my telescope.

I will consider purchasing SkyWatcher CC for F4.

Anyway, thank you guys for a lot of reply and suggestions!