As fallout from
RFC-549, we have to transition our existing reference catalogs from having fluxes in Jansky (most only implicitly: most don't have units on their flux fields at all!) to nanoJansky. We do not have versioning for reference catalogs (the afw.table version has a different purpose and is not propagated to the butler), so we have no way to identify "old" refcats other than attempting to guess from the various fields they do or don't contain.
I propose to do this by writing a script that will convert refcat files to have nJy units and having LoadReferenceObjectsTask.loadSkyCircle raise an exception pointing to that script if it encounters a refcat that has anything other than "nJy" for flux units. This will greatly simplify this transition and will allow us to also fix the fluxSigma->fluxErr values in those refcats, while maintaining the old refcats in-place for older versions of the stack.
For example, on lsst-dev, we would copy each of the existing directories in /datasets/refcats/htm, appending "-nJy"; run the script on those new directories; and update the obs package config defaults to point to the new refcats. Those old refcats could then be kept in place until at least v18.
All newly created refcats will have fluxes in units of nJy.
This would be made much easier if we could drop support for astrometry.net reference catalogs at the same time (RFC-562): a.net refcats are persisted in a way that makes them much harder to modify in-place. I believe the only important a.net refcat that we are still using is in testdata_jointcal, and I have a plan for dealing with it soon.