what are these strange donuts in my newtonian reflector stacked image

ZWO ASI294MM Reflectors Acquisition Deep Sky Artifacts
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Oscar Shu avatar
Oscar Shu avatar
I am a new to newtonian reflector, just bought a GSO 8" F5 with GSO 2" coma corrector. still not very good at precise collimation but I tried my best with collimation cap and laser. the issue is I always got these strange donuts like unfocused star in the background in the stacked image, they only shows up when image is stacked and stretched, I don't see them in the single sub, and I can not notice them during shooting. not sure if this is light leakage, I have a shower cap put on the end of the newtonian reflector to block light leakage of the primary mirror but does not have anything else modified yet. the camera is ASI294MM, I don't see any of the donuts in the flats.


Does anyone know what is the cause of this? Appreciate any suggestion from the forum, thank you!
Oscar Shu avatar
Oscar Shu avatar
sorry for the rush to ask. I find the problem. the problem is the flat has those "donuts", I need to retake the flats, the stacked are impacted by the bad flats
Carastro avatar
Carastro avatar
Was just going to say the same thing.
Brian Puhl avatar
Brian Puhl avatar
Not gonna lie, if you have that many donuts, you might wanna consider cleaning things up 😁😁
Eddie Bagwell avatar
Eddie Bagwell avatar
Check your filters and camera sensor for dust. I use a rubber dust air bulb blower to remove the dust off the glass. They are very cheap and a better alternative than using glass cleaner.
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Rob avatar
Rob avatar
You have significant dust on at least two elements, based on the two sizes of dust rings.   Here's a calculator to show you how far from your sensor they are:

http://astronomy.tools/calculators/dust_reflection_calculator

I use a filter drawer, leaving me with three different locations these dust rings might source from; on the camera itself, filter, and coma corrector.  If I have one that persists even after flats are calibrated, I will use that calculator to figure out where the culprit is.
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Ashraf AbuSara avatar
Ashraf AbuSara avatar
Rob:
You have significant dust on at least two elements, based on the two sizes of dust rings.   Here's a calculator to show you how far from your sensor they are:

http://astronomy.tools/calculators/dust_reflection_calculator

I use a filter drawer, leaving me with three different locations these dust rings might source from; on the camera itself, filter, and coma corrector.  If I have one that persists even after flats are calibrated, I will use that calculator to figure out where the culprit is.

That's a very cool tool actually and makes sense. Thanks for sharing it.
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Peter Hannah avatar
Peter Hannah avatar
1. Looks to me like your flats are over-correcting during calibration. The light corners of the image are another indication of this. What software were you using to calibrate?

2. Were your flats taken around the same time as your lights? If so the dust donuts must also be present in your light frames. Have you tried over-stretching the image to check? (The donuts will look dark on the light frames).
It's best to take sets of flats regularly so the dust donuts in your flats match those in your lights. Doesn't have to be the same day, but as time passes more dust will settle on your optics.

3. If the dust donuts *are* present in the lights, cleaning your optics now will produce flats that don't correct for the donuts.

Following on from @Rob 's post above, the smallest donuts will be from the camera sensor window, the larger ones from your filter(s). I can only see 2 sizes in your image.
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