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Problem with "Extract-Ha"
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?
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.
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.
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.
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.
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:
Yes, I'll have a look at the data today, it might point to why this is the case.