are there galaxies behind M31?

The answer to this question is yes of course, but Lang and I spent a bit of time trying to find them, in the places where multi-epoch HST data overlap, for all the obvious astrometric reasons you might want to do that. The day also included a talk by Martin Elvis on mining asteroids (of all things!), and discussions with Tsalmantza and Adam Bolton about finding double-redshift objects in spectra.