My only research today was writing a paragraph or two in the Sloan Atlas photometric method paper. Several people asked me to explain my cryptic comments from yesterday, so here is a (draft, needs work) version of the first paragraph from the paper:
There is a deep sense in which obtaining precise and accurate galaxy photometry is fundamentally impossible. The reasons are multiple, but the dominant reasons are, first, that the angular outskirts of galaxies can contain significant luminosity but at incredibly low surface brightness and unknown morphology, and secondly, that the imaging point-spread function (PSF) can also have large-angle contributions that are unknown. The latter problem affects stellar photometry also, but so long as the PSF is constant, precise and accurate stellar photometry is possible. The difference between galaxies and stars is that all point-like stars will illuminate the PSF (correlate with it) identically. Stellar photometry does not rely on getting all this right so long as it deals with it consistently across stars. Not so for galaxies, each of which might have very different correlations with the PSF at large angles. There is no way to produce consistent photometry without knowing things about every galaxy and every PSF that are—almost in principle—unknowable.
Sorry, but I don't follow this. A discussion of this sort is in need of understanding what the goal is, what the algorithm is, and so on. There are strategies for relatively local measurement of ultra-low surface brightness, and knowledge of the PSF at large angles is possible to obtain. One can then quantify how any measurement strategy will work with a given galaxy model. A good part of this is understanding how to declare what a galaxy is apart from its surrounding environment. One approach might be understanding how to separate an arbitrary extragalactic SB distribution from the MW foreground. One can then correct that SB for the PSF. From that point on it becomes an issue of what exactly you want to measure from that distribution and why. (BTW, de Vaucouleurs was one of the first to think deeply about this in late 70s early 80s...)
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