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

[Erik Bray]

> Quoting Phil Hodge <hodge at stsci.edu>:
>
>> On 11/04/2014 02:26 PM, Thierry.Forveille at ujf-grenoble.fr wrote:
>>> Deprecated means no new uses, so I would recommend (resources
>>> allowing, of course) that Thomas accept those units when reading
>>> external files but convert to standard SI on output.
>>
>> I wouldn't use a FITS writer that changed my data.

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.