Today was hack day with Ruth Angus (Columbia), Megan Bedell (Chicago), Dan Foreman-Mackey (UW), and I all working on various things in parallel at NYU. Bedell and Foreman-Mackey got the Gaussian-Process stellar spectroscopy model working for Bedell's HARPS data, and it is blazingly fast. It is much faster than the code Bedell and I wrote that does a dense spline. The fastness comes from magic that Eric Agol (UW) and Foreman-Mackey are making happen for GP kernels of a particular (very flexible) form. We made various pragmatic decisions in this project today, like to work in log flux (rather than flux) and to optimize an error model along with the other hyper-parameters. These all look like good decisions in the short run.