[fitswcs] Paper III and radio/optical velocities

[Arnold Rots]

Richard,

I grant you that the terms "optical" and "radio" are somewhat
arbitrary, but they are commonly used and understood - something that
cannot be said of "frequency" and "wavelength", aside from the fact
that one might quibble about what a "wavelength velocity" actually
would mean; in my mind it's no more descriptive than "optical
velocity".

And, no, the definition of "redshift" is uniform: delta(lambda)/lambda0

The use of the radio definition is mainly limited to Galactic radio
work and usually tied to LSR as reference frame, though there are
occasional cross-overs which can cause great confusion.

This issue is messy enough as it is; don't make it worse by
introducing new terms.

  - Arnold


Richard McMahon at IOA wrote:
> 
> 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
> 
> --------------------------------------------------------------------------
>  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
> 
> ---------------------------------------------------------------------------
> email: rgm at ast.cam.ac.uk         | WWW:  http://www.ast.cam.ac.uk/~rgm
> ---------------------------------------------------------------------------
> 
> ---------- 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
> 
> ---------------------------------------------------------------------------
> email: rgm at ast.cam.ac.uk         | WWW:  http://www.ast.cam.ac.uk/~rgm
> ---------------------------------------------------------------------------
> 
> _______________________________________________
> fitswcs mailing list
> fitswcs at listmgr.cv.nrao.edu
> http://listmgr.cv.nrao.edu/mailman/listinfo/fitswcs
> 
--------------------------------------------------------------------------
Arnold H. Rots                                Chandra X-ray Science Center
Smithsonian Astrophysical Observatory                tel:  +1 617 496 7701
60 Garden Street, MS 67                              fax:  +1 617 495 7356
Cambridge, MA 02138                             arots at head.cfa.harvard.edu
USA                                     http://hea-www.harvard.edu/~arots/
--------------------------------------------------------------------------