Cannot include Quantities in FITS headers

It looks like one cannot put an astropy Quantity into a FITS header (astropy version=3.1.2):

Edit: same on 4.0.dev25972

import astropy.units as u
from astropy.io import fits
temperature = 10*u.deg_C
header = fits.Header()
header['CCDTEMP'] = temperature
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-7-3ce5059bc40e> in <module>
----> 1 header['CCDTEMP'] = temperature

~/lsst/lsstsw/miniconda/envs/lsst-scipipe/lib/python3.7/site-packages/astropy/io/fits/header.py in __setitem__(self, key, value)
    187             # If we get an IndexError that should be raised; we don't allow
    188             # assignment to non-existing indices
--> 189             self._update((key, value, comment))
    190 
    191     def __delitem__(self, key):

~/lsst/lsstsw/miniconda/envs/lsst-scipipe/lib/python3.7/site-packages/astropy/io/fits/header.py in _update(self, card)
   1607         else:
   1608             # A new keyword! self.append() will handle updating _modified
-> 1609             self.append(card)
   1610 
   1611     def _cardindex(self, key):

~/lsst/lsstsw/miniconda/envs/lsst-scipipe/lib/python3.7/site-packages/astropy/io/fits/header.py in append(self, card, useblanks, bottom, end)
   1139             card = Card(card)
   1140         elif isinstance(card, tuple):
-> 1141             card = Card(*card)
   1142         elif card is None:
   1143             card = Card()

~/lsst/lsstsw/miniconda/envs/lsst-scipipe/lib/python3.7/site-packages/astropy/io/fits/card.py in __init__(self, keyword, value, comment, **kwargs)
    189                 self.keyword = keyword
    190             if value is not None:
--> 191                 self.value = value
    192 
    193         if comment is not None:

~/lsst/lsstsw/miniconda/envs/lsst-scipipe/lib/python3.7/site-packages/astropy/io/fits/card.py in value(self, value)
    309                            np.floating, np.integer, np.complexfloating,
    310                            np.bool_)):
--> 311             raise ValueError('Illegal value: {!r}.'.format(value))
    312 
    313         if isinstance(value, float) and (np.isnan(value) or np.isinf(value)):

ValueError: Illegal value: <Quantity 10. deg_C>.

I would have expected to be able to insert a Quantity into the header and have the units round-trip. The FITS standard document suggests putting units in the comment field for that header key (see section 4.3.2 of the FITS Standard document); since astropy Quantities have well defined stringifications, we could just use those?

[timj]

Something like:

CCDTEMP  =     10 / [C]

(modulo me getting the spacing right and remembering how FITS was Celsius to be written). The unit should be inserted at the start of any existing comment (overwriting unit if one is already defined). Parsing the units out on when extracting values from a header would also be wonderful.

[parejkoj]

@pllim : I don’t think so, no. #9197 is about writing to a FITS BinTableHDU (which is already partly supported via mixins). This one is about writing to the FITS headers.