The day started with planning between Bedell (Flatiron), Foreman-Mackey (Flatiron), and I about a possible tri-linear model for stellar spectra. The model is that the star has a spectrum, which is drawn from a subspace in spectral space, and doppler shifted, and the star is subject to telluric absorption, which is drawn from a subspace in spectral space, and doppler shifted. The idea is to learn the telluric subspace using all the data ever taken from a spectrograph (HARPS, in this case). But of course the idea behind that is to account for the tellurics by simultaneously fitting them and thereby getting better radial velocities. This was all planning for the arrival of Ben Montet (Chicago), who arrived later in the day for a two-week visit.
At lunch time, Mike Blanton (NYU) gave the CCPP brown-bag talk about SDSS-V. He did a nice job of explaining how you measure the composition of ionized gas by looking at thermal state. And etc!