Crazy flat field correction, what am I doing wrong?

Software RawTherapee
1

I’m trying to do a flat field correction for my camera but something is going horrifically wrong.

I have a Tokina 11-20 f2.8 AT-X pro lens for use doing astrophotography. It has some pretty bad vignetting, which I have not tried to correct before, but it make milky way photography practically impossible. So I am now trying to sort this.

I took several flat field photos, but the lighting was artificial fluorescent, so they are very yellow. I checked the quality of them by using them to correct each other and I get a very flat uniform field.

However, when I try to correct a night sky image I get a large, well defined blue ring in the image centre and yellow corners. What is going on here? I’ve attached a screenshot of RawTherapee showing the flats with default processing (you can see the yellow jpegs at the top) and the night sky image after applying the flat field.

I taped the lens to ensure the focal length stayed the same for the flats and the night shots. Both show 18mm f2.8 in the image metadata.

Thanks for any suggestions

2022-03-18 (1)
2022-03-18 (1)1366×728 831 KB

2022-03-18 (2)
2022-03-18 (2)1366×728 412 KB

2

It looks like the red channel is clipped in your histogram?

I can’t easily tell if that’s the raw or output histogram, if it’s the raw histogram that would probably explain the strange behaviors.

3

Indeed. Clipped channels is cause for trouble.

Take one shot for every major aperture value (f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, etc.). Remember to press the auto-exposure button each time to keep the histogram centered without clipping.

From: Flat-Field - RawPedia

4

Thank-you
That indeed seemed to be the problem. It didn’t look overexposed so I just thought it was fine, obviously not.