Plotting with .get_transform?

Astropy
4

The two FITS keywords to look for are RADESYS and EQUINOX. It sounds like RADESYS is not defined in your FITS header, so I’m guessing that EQUINOX is explicitly defined in your header to be 2000. The FITS standard says:

RADESYSa – [string; default: ’FK4’, ’FK5’, or ’ICRS’: see
    below]. Name of the reference frame of equatorial or eclip-
    tic coordinates, whose value must be one of those speci-
    fied in Table 24. The default value is ’FK4’ if the value of
    EQUINOXa < 1984.0, ’FK5’ if ’EQUINOX’a ≥ 1984.0, or
    ’ICRS’ if ’EQUINOX’a is not given.

That is, you’re getting FK5 rather than ICRS because EQUINOX is defined.