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So how do you combine multiple imaging sessions with different calibration frames? Well I had the same question and now I know how to do it with Siril.
Let’s say you have an imaging session on one night and then you shoot new flats the next night. If you were to combine the raw image files and run the pre processing script now, you only have the choice to use one set of flats.
Fortunately there is a way to overcome this. On any given night, you process the data using scripts and then you need to keep the pp_light files found in the processing folder. All other data there can be deleted each night.
These pp_light files are already calibrated, so you can add more files on subsequent nights and then simply stack the stacked images. Just add all the pp_lights to a sequence, register them and then do a sum stack and it’s done.
This is a great way to manage varying flat files (or darks for that matter) over multiple nights of imaging.
You can read this discussion here on Cloudy Nights