SED Data Model: Questions and Comments
amsr at jb.man.ac.uk
Wed Feb 16 02:17:19 PST 2005
> The issue is ensuring that the model includes all the necessary meta-data
> needed to convert *from* MKSA units to other more esoteric systems. For
> instance, if all flux density-like data is required to be stored in
> W/m^2/Hz (or do I mean W/m^2/Angstrom?) within the DM but the end user
> wants to see antenna temperature, then the DM needs to include all the
> necessary efficiency factors, etc, needed to do this conversion. Likewise,
> converting to/from magnitudes.
Interesting point - I think that we should be able to convert between
standard units but not necessarily instrument-specific ones in all cases.
That is, we should convert _from_ Tant to Jy _if_ the data provider
(possibly with VO guidance) provides the necessary metadata, and the user
should be able to get data from that particular telescope either in its
'natural' units or in standard units. However I don't think that the VO
has to convert some other data to the equivalent antenna temperature for
the Lovell. Similarly, we should handle magnitudes on standard systems
only. Of course, some telescopes are so productive that their systems
become definative but I think the rule of thumb is when other people start
adopting Sloane i etc.
So, don;t lose metadata, but don't promise too much - the problem is that
conversions are often date- or object-brightness-dependent etc. and can't
be applied blindly.
We also need to be able to quantify the accuracy of conversions. e.g. if
I want Lovell data and I just have a spectrum in Tant (K) and a blanket
conversion factor 1.1 Jy/K, then either I can get it in K (warning not
suitable for quantitative comparison with other instruments) or in Jy
(accurate to e.g. 10%). Or the data provider can give a more
sophisticated algorithm taking into account gain-el, the receiver being
used, the target contribution to Tsys etc.... accurate to e.g. 1%.
I would like to see some input from x-ray and other high energy people
here, I suspect that they ahve some good solutions already unerway.
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