Today was the first day of the Astrophysical calibration of Gaia and other surveys meeting at Ringberg Castle in Tegernsee, Germany. The meeting is about methods for combining data to produce good astrophyiscal parameters from current and future surveys. There were many great talks today, but highlights were the following:
Andrae showed work on simulated data to demonstrate that the final Gaia Catalog will have good parameters (effective temperature, surface gravities, and so on) for stars. He showed that the differences between models is much larger than any precision of the data, and that therefore no models are expected to be "good fits" in any sense. He showed that they can recalibrate or adjust the input spectral models to force them to agree with the data, and that this seems to work extremely well.
At the end of a talk by Korn, Fouesneau commented that every model incorporated into the Gaia pipelines ought to include (if it has it) predictions for wavelengths outside the Gaia bandpasses. This is because then the results of fitting to Gaia data can be used to make predictions in other surveys. This comment resonated with me, because in the talks today there was a lot of talk about the "true" values of astrophysical parameters, but since these latent values are inaccessible to direct observation, we need other ways to assess credibility.
In between talks, Bovy made a nice point, which is that if Gaia wants end users to be able to compute likelihoods (and not just get point estimates or best-fit values) for astrophysical parameters, they should project all models onto their various systems (the G magnitude system, the BP/RP low-resolution spectral system, and the RVS high-resolution spectral system). This would permit end users to re-compute likelihoods for all data for all models.