Problems with Flat Field Correction for Color Camera

[TKnightKY]

The flat field correction doesn't seem to be correcting the image for my ASI294MC Pro camera in preprocessing. So, I ran preprocessing without the flat field and manually used divide with my flat field and the results were much better. Not sure why it is not working in preprocessing. Images are attached. I had to reduce image size to send.

[einari]

You are aware how should do flats with ASI294MC?
They should have longer exposure, like 2 seconds. And accompanied with flat darks, ie bias with equal exposure as flats.
Have you done that?

[TKnightKY]

Yes, I am aware. 25 flats @ 10 seconds & 25 flat darks @ 10 seconds as well as the 25 darks for the exposure time of the images.

[fabdev]

Hi, the image "without flat processing" shows a strong left-to-right gradient, so that flat field cannot correct the image in any case. Maybe there was a strong light pollution or dew.
Here is a quick processing on that JPEG with Remove Gradient Adaptive 25, after having masked the nebulas:

(the mask does not need to be precise, it's only a suggestion for the algorithm, all the image will be processed anyway).

[TKnightKY]

I have also removed the gradient with points in the image after I divide the image manually with the flat. My question is why doesn't preprocessing apply the flat so I don't have to do it manually.

[fabdev]

To solve it would be needed to see the original images. I think that Preprocessing is actually applying the flat, but the correction is partial because the images also have an extra gradient caused by light pollution.

[TKnightKY]

The area I image from does have a lot of light pollution but it has never been a problem with preprocessing the flat until I started using a color camera. Why does manually applying the flat work while preprocessing the flat does not? Not sure how to send the raw images because of their size. Thanks for your input.

[fabdev]

There is no solution, a flat field can only correct vignetting and dust. To correct also light pollution it should be taken on the same zone at the same hour, but that would mean a different flat field for every object. Also light pollution "adds" light to the image, instead of "multiplying" like vignetting, so a perfect correction is not possible. Unfortunately on color images the problem is worse because the gradient is colored, and it's even worse on modern cameras because the FOV is larger.
By the way, using the normal flat correction is good, because later a gradient removal via image processing is easier.

[TKnightKY]

I'm not trying to correct light pollution with the flat. How is the flat correction applied during preprocessing? When I manually apply the flat correction it does correct the image. I just don't know why it doesn't work in preprocessing.

[fabdev]

Hi, in preprocessing, a GRGB RAW flat field (so grayscale) is used as luminance (unless it's already a color master flat field), so it cannot correct chroma variations. The menu command "Image / Divide" instead divides each R/G/B planes separately.

To solve, prepare a color master flat field (averaging several flats, with darks) using the "Images" box, with the usual option "Demosaic RGB". This master flat can then be used as a color flat field and will correct chroma variations.

An option for automatic debayering of GRGB RAW flat fields is interesting by the way, it will be added to the next SP in february. If you like to have the pre-release, just write me by PM or email.
Fabio.

[TKnightKY]

Thanks for information. I am still using version 8. I am assuming the SP will be for version 9. Is that correct?

[fabdev]

Yes, but I can add that option also to an experimental version of Astroart 8, it's easy to add.
Fabio.