def determine_refframe(phdr):
    """Determine the reference frame and equinox in standard FITS WCS terms.

    Parameters
    ----------
    phdr : `astropy.io.fits.Header`
        Primary Header of an observation

    Returns
    -------
    out : str, float
        Reference frame ('ICRS', 'FK5', 'FK4') and equinox

    """
    # MUSE files should have RADECSYS='FK5' and EQUINOX=2000.0
    equinox = phdr.get('EQUINOX')
    radesys = phdr.get('RADESYS') or phdr.get('RADECSYS')

    if radesys == 'FK5' and equinox == 2000.0:
        return 'FK5', equinox
    elif radesys:
        return radesys, None
    elif equinox is not None:
        return 'FK4' if equinox < 1984. else 'FK5', equinox
    else:
        return None, None