A procedure to calibrate algorithms for estimating parameters from spectra

I outline a procedure for calibrating the Gaia spectral parameter estimation algorithms, in particular GSP-Phot. It assumes that synthetic spectra accurately reproduce small changes in the APs. In contrast, they may not accurately reflect large changes in the APs or they may have systematic offsets in the absolute fluxes. It is these changes that the outlined calibration procedure can correct for, using Gaia observations of stars with known APs (AP reference stars). The method works by removing from the synthetic data the low frequency variation of flux with AP and replacing it with that determined from the calibration data. It is based on the forward modelling approach discussed in GAIA-C8-TN-MPIA-CBJ-042. To apply in practice would require several hundred stars covering a range of Teff , log g and [Fe/H]. Calibration of the interstellar extinction remains an open issue.