Today was my first day at MPIA. I worked with Hans-Walter Rix (MPIA) and Christina Eilers (MPIA) on her new version of The Cannon, which simultaneously optimizes the model and the labels, with label uncertainties. It is a risky business for a number of reasons, one of which is that maximum likelihood has all the problems we know, and another of which is that optimization is hard. She has taken all the relevant derivatives (analytically), but is stuck on initialization. We came up with some tricks for improving her initialization; this problem has enormous numbers of local optima!
We also spoke with Kareem El-Badry (Berkeley) about a project he is doing with Rix to find binary stars among the LAMOST spectra. Here the problem is that the binaries will not be resolved spectrally or spatially, so it is up to seeing that the one-d spectrum is better explained by two stars (at the same distance and metallicity) than one. He is finding (not surprisingly) that because the spectral models are not quite accurate enough, a mixture of two stars is almost always better than a single star fit. So he decided today to try implementing (his own, bespoke, version of) The Cannon. Then the model will (at least) be accurate in the spectral domain, which is what he needs.
I got started on a new project with Jessica Birky (UCSD) who is here at MPIA to work with me on M-dwarf spectra in the APOGEE project. Our first job is to find a training set of M dwarfs that have APOGEE spectra but also known temperatures and metallicities. That isn't trivial.