digitized plates

I spent some time learning about the horrendous photometric and astrometric issues with photographic plates that have been digitized. The photometric and astrometric solutions are non-trivial functions of stellar brightness, because as stars saturate, only the halo (outer part of the PSF) can be measured. The detectable size of the halo of each star grows slowly with magnitude (this is good, but it needs to be calibrated), and it shifts radially relative to the star (the outer part is not concentric with the inner parts). The former effect means that magnitudes are a function of "flux" in the scan and apparent angular size of the star. The latter effect means that the plate scale is a function of stellar magnitude! If we are going to get all this right, we need a generative model of the optics and the photographic process. Uh-oh.