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Flux differences that originate from post-image calibration should be small, however.  Gross differences in photometry are much more likely to result from not applying the non-linear flux transformation.

PS1 Images

How can I convert pixel values on the stack images back to the original counts?

The short answer is you can't really get from the stack pixel values back to measured counts.

The long answer is it depends on why you want to these values The stacks are a weighted combination of the single-epoch warp images, with some scaling to get the zero-point to 25, so although you can get back to the numbers on the stack frames, it is not clear how these will relate to the warp counts, although they probably agree within a factor of 2 or better.

It is not clear that you could ever reverse engineer the stacking to recover the actual sum of the input warp counts (although there is some information in the image headers which might help).

It is actually worse than this in some cases, because the exposure time recorded is not the true exposure time (warps can be dropped from the stacking process after the total exposure time has been calculated). In this case the numbers in the stack could be quite different from warp counts.

Also, sky has been removed, so if you wanted the number of total counts (object + background) to do photon counting S/N stats that would be tricky.

With these caveats, to get back to a version of the numbers on the stack frames, you can use this equation:

10**((zeropt+2.5*log10(exposure_time) - catalogue_mag)/2.5)

where the zeropt is 25 for the stacks.