[fitsbits] Question on angstrom and erg units in FITS standard

[Thierry.Forveille at ujf-grenoble.fr]

Quoting Erik Bray <embray at stsci.edu>:
> I think the idea would be that if somebody wanted to write data to a  
> FITS file that is in some units not supported by FITS then nothing  
> prevents them from doing so--they are free to tell the FITS writer  
> "here are some dimensionless numbers" and then manually tweak the  
> header in the appropriate place to indicate to a (human) reader how  
> they want those numbers interpreted.
>
> But for the sake of automatic serialization of, say, Astropy  
> Quantity objects to a FITS file then there should be no room allowed  
> for ambiguity.
>
> That doesn't mean it should take some existing file and rewrite the  
> data with new units.  Instead if it gets units that it doesn't  
> understand it should just leave them alone.  But when writing *new*  
> data to *new* FITS files I think that it should at least warn when  
> writing an object with units that are not supported by FITS.
>
Since Astropy is (presumably) very seldom used to just copy
the data unchanged (cp does a really good job at that ;-)) and
will normally be used to perform one data transformation/combination
or another, I see no reason for it to fixate on keeping the data
bit for bit identical. For a writing library like cfitsio, the user
can expect that the bits are preserved, but once one starts any
kind of non-trivial arithmetic processing that's just plain
impossible. The numerical noise from dividing by a factor of
10 to convert Angstroms or cgs to SI is then just one more
contribution, and in most cases a minor one.