Jointcal's current astrometry model fixes one visit to be the identity, to break the sensor/visit model degeneracy. This leaves one visit unable to correct for focal plane/sky distortions. We should instead fix one sensor, fitting the rest of the sensors to that one.
This may mean making one sensor's transform be the identity, or it may mean taking canonical values for one sensor but not letting them vary during the fit.
Because I'm digging into the astrometry code now, I'm going to try to do this as part of