RADECSYS and co are deprecated in FITS standard 4.0

[maxnoe]

RADECSYS (4)
(4) These eight-character keywords are deprecated; the seven-character forms, which can include an alternate version code letter at the end, should be used instead.

The replacement should be RADESYSa

We use this in the events table, however we use a global header for this. But the FITS standard also mentions keywords to specify this for columns of bintables:

BINTABLE vector
RADEna

Where n is the field number