FAQ

Q: How does fcmaker deal with proper motions?

A: fcmaker relies on astropy.coordinates.SkyCoord.appl_space_motion() routine to propagate proper motion between epochs. This routine assumes that the target is moving in a straight line (in space). It also requires the distance along the-line-of sight to the target. In comparison, the VLT propagates proper motions by assuming that the target moves along a Great Circle on the sky. The error associated with this mismatch will be negligible in most cases, particularly if user-provided coordinates are at recent epochs.