At MPIA Galaxy Coffee, Rix talked about mono-abundance populations in the Milky Way, and Dan Weisz talked about combining photometric and spectroscopic observations to constrain the properties of distant, compact star clusters. In the first talk, Rix showed that at low abundances, the stellar populations want negative scale lengths: This is because they are like "donuts" or "rings" around the Milky Way: Low-abundance stars in the disk appear to form far out. In the discussion, we argued about how we could use his results to test models for the thickness of the disk and its dependence on stellar age and orbital radius.
In the second talk, Weisz showed that inclusion (and marginalization out) of a non-trivial calibration model makes the inferences from badly-calibrated spectroscopy much more accurate. They also permit learning about calibration (he showed that you learn correct things about calibration) and they permit principled combination of different data sets of different calibration veracity or trustability. All calibration data are wrong, so you really want to leave it free in everything you do; that's a nice challenge for the next generations of The Cannon.