Not totally flat after combining lights and flats

[MKR]

Hi,

I'm imaging with a full frame DSLR plugged on SW 200P. Lights have a significant vignette. I process them with flats but sadly I'm not getting a perfectly flat field. Should I expect it to be flat? What I'm actually seeing is corners are lighter than centre after applying flats. I did an experiment yesterday: used just two lights, two flats (no bias). Converted both, stacked and then processed. All according to https://siril.org/tutorials/tuto-manual/ . I'm fairly sure that I didn't do anything to subtract flats twice. Please find my light + flat CR2s attached - in case there's something wrong with them, instead of my processing. I'm also attaching the jpg result (over-stretched - just to show the problem).

What am I doing wrong?

Many thanks.

flat1.CR2 light1.CR2

Edited July 26, 2021 by MKR

[wimvb]

Your flats are overcorrecting. This can happen if lights and/or flats don't have a proper dark correction. The proper workflow should be

Calibrared Light = (Light - dark) / (Flat - bias)

For a dslr you can experiment with using a bias in place of a dark. (For a cmos camera you need matching darks.)

[alacant]

On 26/07/2021 at 17:04, MKR said:

no bias

You must subtract the bias.

Make a master bias using say 60 frames. 

Use this to calibrate the master flat frame and light frames.

In Siril this is a 5 step process.

Stack bias

Flat - bias

Stack flat

(Lights - bias) / flat

Register and stack light

I'd strongly recommend using bias as bias.

HTH

 

 

 

 

 

 

[MKR]

Thanks, that must be it. I was experimenting with synthetic bias from latest Siril. I had problems with my bias files from the camera due to banding on one side of the photos. I'll try with different shutter speed (so far used 1/4000s), hope I will find one without banding.

[lock042]

Hello. We have a great tutorial that explains everything in appendices on how works the flat correction, and why in some cases, you over-correct it:

https://siril.org/tutorials/synthetic-biases/#appendix-understanding-how-the-flats-correct-the-lights