Dark Frames, flats

I am sure this was asked before but searching the form did not give me what I was looking for.

  1. Is it necessary to take dark frames at every session? There seems to be split on this. Some say once year and some say every session.
  2. In a session if I take multiple exposures at different exposure times, what would be the ideal dark frame exposure be?
  3. What about flats and darkflats? Do I take every session.

What is the recommended method for Siril to process the image best?


  1. You need darks that were taken at the same temperature as your lights. How you get that depends on your camera. If you take photos at a controlled sensor temperature, you can keep darks for a while (months, anyway). If you use an uncooled camera (like a dslr) then you need some way to match up the sensor temps. Many Canons include an indirect measure of sensor temp in the RAW file header, Nikons do not.
  2. You should have darks that match the lights, so multiple sets of darks. It is possible in some circumstances to “scale” darks, but I don’t know much about that.
  3. If you keep your imaging train exactly in place you can reuse flats. If you move filters or remove or rotate the camera you need new flats. Dust has a way of moving around, so you can’t keep using one set of flats forever in any case.

Thanks dx_ron for the answer, I’ll just add:
2. in siril it’s called dark optimisation, and it requires having a master bias to subtract to the dark frames to adapt the dark signal to each frame. It’s quite possible and in general works well
3. The flat will evolve with dust and dew, so it’s better to do it often, the dark-flat won’t change much, like an usual dark or bias.

Thanks a lot. I have ASI533 and control it -10c. So I guess I can keep the dark. Regarding exposure for dark, what should I take 30 secs or 300secs? The lights exposures are anywhere from 30sec to 300 secs I have taken. Also how many darks I should take?

For flats,I don’t generally move the camera etc but I do take it down every night and put it back next day due to weather is crazy now a days. I guess I need to start taking flats and dark flats for every sitting then.

A factor of 10 is a bit large maybe, if you have many short ones and many long ones you could process them as two sequences, otherwise, I guess 165s could be ok for your darks.
Regarding the number it depends on the camera, maybe that can give you an idea, every body has his own rule… https://siril.org/tutorials/tuto-scripts/#are-you-ready