We need a science (end) user-friendly description of astrometric calibration,
including how we will treat DCR, what assumptions we will be making in
JointCal, what external inputs, what quantities will be persisted, the impact
of SEDs, etc. We also need to describe what we will do about proper motion
and trigonometric parallax computations.
As use cases, think of Richards et al. and quasar classification, and the use
of proper motion and trigonometric parallax for Galactic structure studies.