Log units (was Re: SED Data Model: Questions and Comments)

[Igor Chilingarian]

Hello,

On Sat, 19 Feb 2005, David Berry wrote:

> Ed,
>    So you are suggesting that the IVOA specifies that no data should have
> logarithmic units? Anyone else got any views on this?

Does it mean, that stellar magnitudes will not be allowed, or I'm missing
something? It is impossible, astronomers probably will not agree on this!

For example, quite a usual way of representing photometric data,
by the way not suffering from zero-point problem, is AB magnitudes.

m(AB)=-2.5*log(f_nu)-48.60  (Oke, J.B. 1974, ApJS, 27, 21)

where f_nu is expressed in "erg*cm^-2*s^-1*Hz^-1".

If this is not allowed, we may forget about having the photometric data
in the VO: data providers will not like to convert all the magnitudes to
fluxes.

>
> Obviously my example of log(Jy)/sr was a bit contrived, but what about
> spectra in which the spectral position is given in units of log(Hz)?
>
> David
>

With best regards,
						Igor