A lot of the second day of #GaiaCal was about "benchmark stars". Indeed, my concerns about "truth" from yesterday got more severe: I don't think there is any possibility of knowing the "true" value of Fe/H or anything else for a star, so we should probably figure out ways of doing our science without making reference to such inaccessible things. One approach is to define the results of a certain set of procedures on a certain set of stars to be "truth" and then warp all models and methods to reproduce those truths. That's worth thinking about. During the day's proceedings, Blomme and Worley showed some incredible spectra and spectral results on the benchmarks, including abundance measurements for some 25 elements.
Along different lines, Cropper gave an astounding talk about the Gaia RVS spectral data reduction pipeline. It was astounding because it involves a complicated, empirical, self-calibrated model of charge-transfer efficiency and radiation damage, which is continuously updated as the mission proceeds. It also includes a spectral calibration (wavelength solution, astrometry) that is fit simultaneously with all spectral extractions in a complete self-calibration. Cropper asked the implicit question in his talk: "How do we extract CTE-undamaged spectra from CTE-damaged data?". I think I know how to answer that; resolved to write something about it.
Liu showed amazing empirical calibrations of the LAMOST spectra using SEGUE parameters and support vector machines. He has done a great job of tranferring the one system onto the other data, which gives hope for the benchmark plans that are emerging. Bovy gave a nice talk about ages (birth dates), chemical abundances, and orbital actions serving as stellar "tags" which can then be understood statistically and generatively.