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Problem with "Extract-Ha"  

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(@mseningmail-com)
Molecular Cloud Customer
Joined: 10 months ago
Posts: 5
April 26, 2020 07:52  

Hello. My camera is ZWO ASI294MC Pro. Having an issue with "Extract Ha" algorithm. The image was taken with HaOIII filter. After integration, if I zoom in I'm seeing weird patterns. This only happens with Bayer Drizzle, in this case 3.0 but can happen with any scale. If drizzle is off there's no pattern. Also no pattern in colour images, regardless of drizzle setting.
How to fix this pattern?

Attached is the example
- Top left is "HaOIII: Extract H-alpha" with drizzle disabled.
- Top right is "HaOIII Mono" with 3.0 bayer drizzle
- Bottom left is "HaOIII: Extract H-alpha" with 3.0 bayer drizzle
- Bottom right: "HaOIII color" with 3.0 bayer drizzle

And finally the image of the pattern I'm having issue with. Why isn't it "solid" colour?

pattern
4 panel

 


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(@neverfox)
White Dwarf Customer
Joined: 4 months ago
Posts: 11
September 15, 2020 00:16  

I see the same thing. Extracting without drizzle or doing Ha-OIII color with drizzle works, but extracting with drizzle gives that pattern and/or doesn't result in different Ha and OIII outcomes.


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(@vincent-mod)
Quasar Admin
Joined: 3 years ago
Posts: 2453
September 15, 2020 10:02  

How did you capture the data? It is important for drizzling that your data is undersampled (or at least it will have a good effect on that) and very well dithered.


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(@neverfox)
White Dwarf Customer
Joined: 4 months ago
Posts: 11
September 15, 2020 19:36  

What counts as sufficiently under-sampled or "very well dithered"? I can say that I'm what astronomy.tools would call slightly under-sampled at 2.24"/pixel. For dithering, I used 1 pixel dithering (and I have the option to use 2, 3, 5, or 10) after each frame and required 1" stability to start again with a 5s settle time.


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(@vincent-mod)
Quasar Admin
Joined: 3 years ago
Posts: 2453
September 17, 2020 14:19  

I'd say that somewhat undersampled can already benefit from it, regarding dithering I would go higher, probably at least 3 as that will then cover more pixels for it to do the drizzling with. The more data the better for it as well.


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(@mseningmail-com)
Molecular Cloud Customer
Joined: 10 months ago
Posts: 5
September 18, 2020 03:50  

Dark and brighter grey-scale pixels are alternating in a specific pattern in each line:
D B D B D B D B...
B D B D B D B D...
D B ...
I thought it has something to do with de-bayering in combination with drizzle, or maybe there's some other reason. While dithering is important it's not relevant to producing such a pattern in the image.

I confirm what Roman had said: extracting OIII or Ha from HaOIII-filtered colour image plus drizzle with result in this alternating pattern of pixels instead of a smooth image.

Also the Ha-extract and OIII-extract are the same image:

https://www.astropixelprocessor.com/community/main-forum/duo-band-l-enhance-and-drizzle/#post-12492

This post was modified 1 month ago by Maxim Senin

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(@vincent-mod)
Quasar Admin
Joined: 3 years ago
Posts: 2453
September 18, 2020 10:32  

Yes, I'll have a look at the data today, it might point to why this is the case.


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