Tutorial:
Siril for pre processing astrophotos

Content:

1. My personal parameters when using Siril
2. Desaturate stars
3. Star Color calibration
4. Calibration tests
5. Dark calibration analyze
6. Adjust the flat calibration with a fake bias
7. Meridian flip and stacking
8. Deconvolution
9. Post Processing
10. Star Synthesis
11. To be continued

Note:
I take no responsibility or liability for what are written here, you use the information at your own risk!

6, Adjust the flat calibration with a fake bias

Sometimes when doing flat calibrating it can over compensate or under compensate the vignetting. If it's only a minor difference there is a chance that it can be corrected by a faked bias constant. The bias of my Canon 6D camera is = 2048 ADU. I used the bias to replace the MasterDark and can then fine tuning this bias constant. If MasterDark is used a small Bias constant, pos or neg, can be added to do the same fine tuning.

With an extra bias the level of the image will be different and the flat calibration will do a wrong calibration. But with only a small extra bias added / subtracted it can be used to fine tune the flat calibration. When changing the bias when calibrating it change the shape of the vignetting curvature. If it's good or bad depends of how the vignetting looks like from the optics.

Here are examples of how my optics behave in vignetting: Analyzes of vignetting from my lenses and telescopes . If the curvature of the vignetting is smooth there is better chance to correct it, if it has sharp bents it's not good.

Bias set to = 2048:

With the standard value of bias set to = 2048 it looks like this. It's a little too bright at the edges.

Bias set to = 2058:

With increased Bias the center get brighter and the edges darker. Her the Bias is increased by 10 units. The center got a little bit brighter, a very small correction. If a normal MasterDark hade been use I had set the Bias = 10 to give the same effect. Every camera will be different so trail and error method must be used.

Bias set to = 2100:

I increased the Bias even more, now Bias set to = 2100. Now the center get more push to the brighter. It works, but in my case the flat correction problem is something else. In Siril's tutorial this is called a synthetic biases and can be found here: Synthetic biases.

Today I don't worry much about this, with GraXpert (Siril ver 1.4.0) I remove the background and it handle this problem very well.