[obs-tap] Spectral axis - meters

[Petr Skoda]

>
> The problem is that Hz (or energy) and m have an inverse relationship. Hence, 
> if you have a bandwidth which is a large fraction of the observing 
> wavelength/frequency, the spectral resolution is linear on one unit but 
> changes appreciably across the bandwidth in the other unit.


> characterization of the coarse-level quantities in any unit, i.e. upper and 
> lower bounds, representative wavelength, spectral resolution..
>
> It is not possible to give a single precise value for e.g. spectral 
> resolution in wavelength units, for a radio spectrum.

I agree - it is the case of echelle optical spectroscopy as well but as I 
understand for discovery purpose (the ObsTAP is meant for) its enough to 
have Spectral Resolving Power (em_res_power (sec.18 pf March draft) which 
is dimensionless number) -
and I consider more natural to ask for optical spectrum with SRP about 
50 000 to get idea about the detailed line profiles or 1900 (the Sloan 
case) which gives me all spectrum with barely distinguishible line 
profiles.

It is as well the reason why SRP is a optional query parameter for SSAP
(called SPECRP)   while the Char.Spectral.Axis.Resolution is only in
characterization metadata returned by the server.


> So, at a coarse level we can adopt one unit but for tools which require 
> precision we need Hz and m
Sure - one unit for discovery, and more detailed understanding for tools - 
but it is the problem of returned format, data product subtype  etc ....

The question of the tools to consume obsTAP was already written by me 
several days ago....

Petr

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*  Petr Skoda                         Phone : +420-323-649201, ext. 361 *
*  Stellar Department                         +420-323-620361           *
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