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

[William Thompson]

I know that the solar physics community would be *very* unhappy if the FITS 
software suddenly started changing Angstroms into nanometers.

On 11/04/14 16:02, Thierry.Forveille at ujf-grenoble.fr wrote:
>
> 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.
>
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-- 
William Thompson
NASA Goddard Space Flight Center
Code 671
Greenbelt, MD  20771
USA

301-286-2040
William.T.Thompson at nasa.gov