Challenge 4: Build test data with realistic HR diagram
GSP-Phot draws its training data from a synthetic HR diagram. This synthetic HR diagram currently lacks giants. In particular, it lacks significant numbers of red-clump stars, BHB stars, RR Lyraes and other important stars. Consequently, GSP-Phot cannot perform very well on such stars.
Add red-clump stars, BHB stars, RR Lyraes and other important stars to the synthetic HR diagram used by GSP-Phot.
Adopt definition of red-clump stars from Bovy et al. (2014) ([Fe/H]>-1, 4600K<Teff<4850K, 1.8<logg<2.5 + 0.0018*(Teff - TeffRef) where TeffRef = -382.5*FeH + 4607).
Selecting all available BP/RP spectra in that regime adds a significant number of red-clump stars.
Caveat: The ratio of red-clump stars relative to other types of stars is not realistic (before and after this procedure).
Blue horizontal branch
Adopt definition of BHB stars from Sirko et al. (2004) ([Fe/H]< -1, 7000K<Teff<13000K, 2.9<logg<4).
Synthetic Gaia libraries contain no BHB stars that we could include.