[fitswcs] Paper III and radio/optical velocities

[Richard McMahon at IOA]

Hello,

I have been reading paper III for the first time as a result
of some IVOA work on Data Models and have some concerns about
the above nomenclature. I am a practising mainly optical astronomer
so that is my background.

Comments on section 2:

It would seem to me that introducing the above terms
i.e. "radio/optical" velocities

is arbitrary and I want to propose alternative terms that are more
descriptive.

i.e. why not call them "frequency" velocities and "wavelength" velocities.
The astronomical EM spectrum is not just the optical and radio anymore.

Also, I assume that redshifts are measured in the radio so

z= c(nu_0 etc) = c(Lam etc) not just z=Z/c

best regards richard

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---------- Forwarded message ----------
Date: Mon, 30 May 2005 15:54:34 +0100 (BST)
From: Richard McMahon at IOA <rgm at ast.cam.ac.uk>
To: Jonathan McDowell <jcm at head.cfa.harvard.edu>
Subject: Re: radio vs optical vel


Hi Jonathan,

OK, I'll read the FITS WCS paper III draft. The URL for
the current version is here:

http://listmgr.cv.nrao.edu/pipermail/fitswcs/2005-May/000353.html

http://www.aoc.nrao.edu/~egreisen/

It would seem better to me to call them 'frequency' velocity, 'wavelength'
velocity and 'redshift' velocity if we are trying to describe data. The
optical/radio designations are not terms that real astronomers use.

I would favour a proper descriptive term rather than one
that follows the FITS WCS draft.

I suppose I will have to post some comments on the WCS paper
and see what reaction I get...... However, IVOA could take
the lead on this and agree a more sensible nomenclature?

regards richard

On Mon, 30 May 2005, Jonathan McDowell wrote:

> Date: Mon, 30 May 2005 08:51:22 -0400 (EDT)
> From: Jonathan McDowell <jcm at head.cfa.harvard.edu>
> To: rgm at ast.cam.ac.uk
> Subject: radio vs optical vel
>
>
> See Greisen et al's long dicussion in FITS WCS paper III.
> the 'optical' and 'radio' are shorthand for 'stupid wrong methods
> to calculate a velocity that have been used mostly by optical and
> radio astronomers'. There are plenty of these in the literature and
> as you point out they are not actual velocities, but they define
> a spectral frame. Perhaps you are right that 'DopplerDefinition'
> is not the perfect name. Anyway, the point is that the VO rule
> is to describe, not prescribe, so we want to at least give
> data providers a way to describe which broken convention they
> are using and this is consistent with the approach being taken
> by the FITS community in WCS Paper III (whose exact URL I don't
> have handy right now)
>  - cheers, Jonathan
> (ps further: one can calculate the "radio method velocity" for
> an optical spectrum, and indeed this has been seen in the wild)
>

--------------------------------------------------------------------------
 Dr. Richard G. McMahon    | Phone (office)     44-(0)-1223-337519
 University of Cambridge   |       (switchboard)       1223-337548
 Institute of Astronomy    |       (secretary)         1223-337516
 Madingley Rd              | FAX                       1223-337523
 Cambridge, CB3 OHA, UK.   | mobile                (0)-7885-409019
 Office: Hoyle 18          | home                      1223-359770

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email: rgm at ast.cam.ac.uk         | WWW:  http://www.ast.cam.ac.uk/~rgm
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