In looking at the Source table outputs (SRC*.fits files) in the validation_data_hsc repository, we noticed that the headers are not consistently applied in the FITS files generated by afw.table output. In particular, the TUNIT keywords are missing for the coord_ra and coord_dec columns, making it difficult for a client to determine that the columns are persisted in units of radians, especially since the FITS convention is for the default units for angles to be degrees.
REFERENCE: FITS Standard
COMMENT: column units
DEFINITION: The value field shall contain a character string describing
the physical units in which the quantity in field n, after any
application of TSCALn and TZEROn, is expressed. The units of all FITS
header keyword values, with the exception of measurements of angles,
should conform with the recommendations in the IAU Style Manual. For
angular measurements given as floating point values and specified with
reserved keywords, degrees are the recommended units (with the units,
if specified, given as 'deg').
Accepting that we are not conforming to that convention, the proper value for TUNITn is "rad" for data in units of radians.
While much access to afw.table-produced files will be through the afw.table interfaces, where this issue is irrelevant as long as round-tripping is successful, for exploratory and debugging purposes it is very useful to be able to access the file contents with standard tools.
In addition, if the FITS tables produced from afw.table are to be used as the input to the database ingest, providing units for all columns will assist the database system in providing and validating correct column metadata to its clients, whether through VO standards or other means.