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SamuraiOZ

Donut ring around the bright star

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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.

DonutRingOnIC434.jpg

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Hi!, from the Bellarine.....

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

 

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

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

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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 by SamuraiOZ

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@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 by tooth_dr
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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.

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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. 

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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 by Peje

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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!

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