One of the ways in which the Tractor is qualitatively better than other methods for measuring the properties of stars and galaxies in imaging is that it can fit multiple images—with different seeing, taken on different nights, and taken through different bandpasses—simultaneously, delivering consistent shapes, colors, and variability information despite heterogenous data and with no requirement of "stacking" before measurement. All that is true in theory but until today most of this functionality was vapor-ware. Today, Lang and I (well, really Lang, with me watching) got the Tractor working on multi-band, heterogenous imaging by permitting the "fluxes" or "magnitudes" of the objects to be arrays of values, one per band. In the future, we hope to work in spaces with well-defined priors, learned hierarchically, but we have a start. When we applied the code to a small snippet of SDSS data, we found some tiny band-to-band astrometric offsets, for which (along with photometric calibration and PSF) the Tractor can also fit.
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