Hi @whwang,
No problem,
” Let’s say the images were taken under different sky brightness (strong light pollution and less light pollution). So obviously there is a need for an additive term. ”
The additive term is for differences in sky background between your lights indeed due to additive terms like differences in light pollution indeed. But also for the ever changing sky brightness during the night.
In most cases, the add + scale is the preferred option.
“Also, for some reason, the images were taken with different ISO. So a multiplicative term is also needed. In that case, which is the proper option to use?”
This is a very good question. We still want to correct the light pollution and changing sky brigthness additively, but to account for different iso (or gain) me need multiplicative correction as well to normalize the lokation value of the data.
I think the best way to approach this is to use the method which is applicable for the factor which influence is the biggest.
For instance:
we have ISO400 and ISO800 light frames with equal exposure.
The multiplier between these frames would normally be 2 for the ISO800 data relative to the IS400 data.
If the additive terms in the data, like light pollution diffferences, are less than this factor, use the multiplicative normalization for the lokation. So use multiply + scale.
Would you agree, possibly there are other arguments to choose between additive or multiplicative normalization for the lokation?
Possibly you know of a better way to handle this?
Mabula