In the morning, Melissa Ness showed me very strong evidence that not all open clusters are mono-abundance! This is surprising, and violates my expectations based on recent work by Jo Bovy. It doesn't literally conflict with his results, because the clusters in which we find this are not the clusters he looked at, and the elements we find it in are exposed only subtly in the data. Now we have yet another paper to write this summer!
At lunch, Joe Hennawi (MPIA) interviewed me about the argument (often given by the builders of small telescope projects) that the surface-brightness sensitivity of a small telescope is just as good as that of a large telescope, provided that the detectors and f-ratios are similar. I have always been suspicious of this argument, and Hennawi shares my suspicion. We discussed the simplifications in the argument and planned to write out a well-posed data analysis question and answer it. A project for the garden.
Late in the day, Jeroen Bouwman (MPIA) showed me the results of running a CPM-like (modeling data with data) model on some Spitzer spectroscopy of a star through a planet transit, as we had discussed a few days ago. It looks like it does almost as well as a full calibration of the data. In precision, not accuracy though: There are some things that the data-driven model erases. This is the problem in general with data-driven models: When you go data-driven, you lose some aspects of interpretability.