Understanding flat correction

Software Siril
1

Hi,

I would like to dig a bit deeper in flat correction maths. I’ve read the article Siril - Synthetic biases which is quite interesting. However I have still some doubts regarding that particular explanation:

It starts from the formula L = a - b (x - W/2)^2 + drate x tlights + o

The formula represents the intensity of background pixels. But we are not only calibrating the background, but the whole image. This is the first point I don’t get. We would need a formula representing the image itself, in order to calibrate, no?
Also, what is the term “b”? It is measured in ADU/px^2 but I don’t know that represents.
The formula for flats is the same but multiplied by a factor. But that leads me to the first point. The L formula does not represent the light, but the background.

I have some more nuances to clarify but let’s start with these.

Any clarification and/or explanation is welcome.

Regards

2

Hi,

indeed, we did not want to complexify the (already-complex) formula. If you want to add an image term I(x), the L term would be:

L = I(x) * (a - b * (x - W / 2)^2) + d_rate * t_lights + o

And then if you simplify the (L-D)/(F-O) formula again, you end up with:
Lc = I(x) / K

Cheers,

C.