VOunits draft

[Rob Seaman]

Hi Francois,

> but I don't catch the relation with units -- solar system objects
> and supernovae are both physical events, and energies or distances
> can share a common system of units, even if these are not studied
> by researchers of the same sub-discipline ?

The VO wants to clarify its preferred subset of units (and of various  
other concepts and quantities).  In order to define that preferred  
subset, the VO must characterize the subset it is explicitly NOT  
supporting internally.  These exo-VO units must be handled at the  
interfaces between VO and the larger community.  It is unrealistic to  
expect the larger community to develop those interfaces, so it falls  
to the VO to provide a certain minimal level of support converting  
from (and to) "unsupported" units.

> why not keeping the same sections as IAU, i.e.
>    1) SI units and derivates, which have to be understood by the VO
>    2) Astronomical extensions, which should be understood by the VO
>    3) Deprecated units, which may be encountered and used by IVOA
>       applications
> ... and no other ones.

I don't have a problem with that.  Just don't leave any holes - that  
is, "... and no other ones" is a fourth category either explicitly or  
implicitly.

Rob