How often do I have to acquire new flatfields ?
It is difficult to specify a time. Some use the same flatfields for month, some acquire new flatfields daily. Due to many possible reasons (mechanically, temperature, electronical lifetime, etc.) the sensitivity of the CCD chip changes, also there might be a change in the scintillator coupling, etc. All those reasons might change the uncorrected image in such a way, that the formerly acquired flatfields will no longer correct the image as desired. This is the latest moment where you will have to acquire new flatfields.
