How to do background subtraction of photometric reference field

Currently the illumination correction is derived without doing any background subtraction.

E.g. see the association matrix (as generated by c2e16e07, this is the first version that actually contains mcd_img_illum):

Should background subtraction be performed on the input to the routine that derives the illumination correction?

Currently the background subtraction is done in the mcd_img_detrend_final recipe (name still tentative), which also performs the illumination correction. It is probably possible to reuse the mcd_img_detrend_final recipe by creating an OCA Action where the illumination correction is not an input.

How should the background subtraction be done for the photometric reference field, if we decide to do it? Can we use the same procedure as for the science images? This should be possible if the photometric reference field observation is very similar to the science observations and only differ in the number of exposures; is that true?

@verdoes can you share your thoughts?

Edit: improved association diagram