VOunits draft

[Francois Ochsenbein]

Rob,

I tried to understand your analogy:

>By analogy, consider celestial transients.  A single data set will
>include many classes of event.  Planetary scientists may be looking
>for KBOs, but cosmologists for distant SNe.  VOEvent has to be able to
>describe both, however, because both appear in the data and each class
>of event contaminates the other.  Similarly, cgs is a contaminant to
>SI purists.

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 ? 

And about:

>Perhaps VOunits needs three sections?  1) a list of "thou shalts", 2)
>a list of "never under any circumstances" - or simply a statement that
>"units not mentioned (eg, from firkins to fortnights) will not be
>referenced in IVOA documentation and will not be used in IVOA systems
>or applications", and 3) a (hopefully brief) list of units that are
>deprecated within IVOA, but that may be encountered in external data
>sets and that IVOA applications and external interfaces *may* choose
>to support conversions to/from.

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.

Anyway, following Anita, provided a unit is defined in terms of SI
equivalent, it should not be technically difficult to understand it;
but the 'Crab' for instance can't be expressed in SI units (as would
be a 'Vega' unit in the optical/near-IR domain :-). 

--Francois
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Francois Ochsenbein    ------   Observatoire Astronomique de Strasbourg
   11, rue de l'Universite 67000 STRASBOURG  Phone: +33-(0)390 24 24 29
Email: francois at astro.u-strasbg.fr (France)    Fax: +33-(0)390 24 24 17
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