Red images after Pre-Processing

Software Siril
1

Hi there,
I am using my Canon EOS R for astrophptography. Because of unstable temperatures of the sensor I dont use DARKs but rather only a BIAS/OFFSET.

I debayer the image during conversion. The result has “natural” colors, but of course is not flat.
After preprocessing with BIAS and FLAT (I subtracted a BIAS from the FLAT as well), the resulting color image has a brightly red background. Only after a photometric color correction, colors get back to normal. The result of the flat field correction is perfect, i.e. the image is flat with no indications for any vignetting anymore. So I would think that the preprocessing is working, except for the red background, which requires the extra step of color calibration.
I dont understand where the red background is coming from and how I could avoid it. Any ideas ?
Regards,
Götz

2

This is because your flat has a natural (non white) color. When you divise your image by the flat you change colorimetry.
TO avoid this behavior there is the “Equalize CFA” option, provided in the preprocess step. Using that, you will have the “normal” green color.

3

Thanks, this is very helpful. However I am using the CFA option in preprocessing of the LIGHTs.
I am not sure if I used the option during pre-processing of the FLAT, though. I will check asap

4

Equalize CFA is not valid when I only subtract the BIAS from the FLAT.
What I can see is that the flat is green, and the red channel is very weak compared to the other two channels of the FLAT.

5

Please, take a look at our tutorial. It explains when you need to apply equalize cfa.

6

Hello,
maybe a misunderstanding. I always use “equalize CFA” when preprocessing the Lights, and still they have the red background. So it seems that “equalize CFA” is not working as expected in this case.
Greetings
Götz

7

Please send a flat file.

8

The Flat has 78MB, can I upload it here ? I could also post it on my website.

9

There is wetransfer too

10

Here is the URL to download the flat:

11

Let me know if you need any other images.

12

Why your flat is demosaiced?
We should never do that with flat, biases or darks.

13

:slightly_smiling_face: I was always wondering whether I should first debayer and then preprocess, or the other way round. Since I got good results with first debayering (except for the redshift … ) I thought thats the right way.

I will redo may FLATs and BIASES right away and report about the result. Thank you !

14

You should really read the tutorial I gave you. It explains everything.

15

The tutorial is great, but when I started working with siril 2 years ago there was no tutorial, so I learn to work without it.

16

I finished the complete processing with the recommended way of debayering only after preprocessing of the lights. These are the differences:

  • Of course, as you said, there is no problem with a red background in the processed lights anymore.
  • The final image looks the same, BUT:
  • There is a higher dynamic range in the new picture. The old picture had (min,med,max) of (1900, 3300,10800) whereas the new picture has (10100,18650,60600) in the red channel.

This is a great improvement, much more than I was hoping for. So thank you once again, Siril is a great software.

Götz

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