José Garmilla (Princeton) showed up for group meeting, and talked to us about star–galaxy separation, which is of great interest to us at CampHogg. He has been trying many things—including many of the methods we at CampHogg been advocating—all with HyperSuprimeCam data, and he has been hitting many of the things Fadely and I have been hitting over the years: Things break down at faint magnitudes, even the existence of a truth table or training set is hard to establish once you get below about 25th magnitude.
In that same group meeting, Blanton introduced the idea of SDSS fiber collisions and the methods that are used to mitigate them. This is a seemingly trivial question: What to do with the galaxies that don't get redshifts because of proximity to another galaxy that does get a redshift? But when it comes to two-point statistics, there is no trivial answer.
After lunch, Kopytova and I pair-coded some of her spectral-fitting code, implementing the ideas we had about fitting simultaneously for a bunch of calibration or normalization parameters along with the stellar parameters. The code we (very hastily) wrote seems to work well, but of course it has model-complexity parameters (or, really, hyperparameters). That's to think about tomorrow!