u.Magnitude() does not produce a FITS conform output

Description

When creating a table column with u.mag() as unit, the FITS writer refuses to take it as a FITS mag standard column but serializes it in a astropy specific manner.

Expected behavior

I’d expect that both u.mag and u.mag() result in a FITS unit mag, as their difference is quite subtle.

Steps to Reproduce

>>> from astropy.table import QTable
>>> import astropy.units as u
>>> from astropy.io import fits
>>> QTable({'fmag': [1.0] * u.mag}).write('mag-units.fits')
>>> fits.open('mag-units.fits')[1].header['T*1']
TTYPE1  = 'fmag    '                                                            
TFORM1  = 'D       '                                                            
TUNIT1  = 'mag     '                                                            
>>> QTable({'fmag': [1.0] * u.mag()}).write('mag-magnitude.fits')
WARNING: The unit 'mag(1)' could not be saved in native FITS format and hence will be lost to non-astropy fits readers. Within astropy, the unit can roundtrip using QTable, though one has to enable the unit before reading. [astropy.io.fits.convenience]
>>> fits.open('mag-magnitude.fits')[1].header['T*1']
TTYPE1  = 'fmag    '                                                            
TFORM1  = 'D       '                                                            

System Details

• Linux-5.18.0-2-amd64-x86_64-with-glibc2.33
• Python 3.10.5 (main, Jun 8 2022, 09:26:22) [GCC 11.3.0]
• Numpy 1.21.5
• pyerfa 2.0.0.1
• astropy 5.1
• Scipy 1.7.3
• Matplotlib 3.5.2

[olebole]

@plim Not really, but the other way: ST is a flux unit; mag(ST) converts it to a magnitude. If one wants to convert a surface flux, the natural way is mag(ST/arcsec²). STmag/arcsec² is just lazy writing and not to be taken literally.

[pllim]

Ah, okay. Thanks for clarification and sorry for the noise!