[fitswcs] Paper III and radio/optical velocities
Richard McMahon at IOA
rgm at ast.cam.ac.uk
Mon May 30 12:40:07 EDT 2005
Dear Arnold,
Just a quick response. I certainly dont want to introduce new terms
unecessarily. I accept you points but if the FITs communities
and IVOA are to use these terms, maybe both the relevant documents
need to explain the historical/scientific origin of the terms. Maybe
they do but I havent gone back and checked.
It depends on your definition but I do not 100% agree that the terms above
are commonly used or understood. I am an observational optical
extragalactic astronomer with over 20 years experience and had never
encountered the terms until I read your comprehensive IVOA documents and
some of the Kyoto IVOA meeting presentations on the IVOA Wiki.
I just did a quick google on the term radio velocity and the second
link was relevent:
http://www.atnf.csiro.au/computing/software/atca_aips/node62.html
It says here:
The radio velocity definition is .......
This expression is often used by Galactic radio astronomers (astronomers
from our Galaxy ?) but is now deprecated by the IAU.
There is also a nice discussion here:
http://www.iram.fr/IRAMFR/ARN/may95/node4.html
Lindegren and Dravins(2003) say in section 2.2
This last expression(i.e. radio velocity) has traditionally been used in
radio astronomy (e.g. Walker 1987), although the practice is discouraged
due to the risk of confusion with the earlier expression (Contopoulos &
Jappel 1974; Muller & Jappel 1977). v(1)r is sometimes called the .optical
velocity. and v(2)r the .radio velocity. (Greisen et al. 2003).
The bottom line, I suppose is that the terms are and have been used, and
we dont want to introduce new ones but I feel the background to the terms
needs exposing possibly with a literature reference like Lindegren
and Dravins(2003).
best regard Richard
On Mon, 30 May 2005, Arnold Rots wrote:
> Date: Mon, 30 May 2005 11:46:43 -0400 (EDT)
> From: Arnold Rots <arots at head.cfa.harvard.edu>
> To: Richard McMahon at IOA <rgm at ast.cam.ac.uk>
> Cc: fitswcs at donar.cv.nrao.edu
> Subject: Re: [fitswcs] Paper III and radio/optical velocities
>
> 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
> > ---------------------------------------------------------------------------
> >
> > _______________________________________________
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> > fitswcs at listmgr.cv.nrao.edu
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> >
> --------------------------------------------------------------------------
> 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/
> --------------------------------------------------------------------------
>
--------------------------------------------------------------------------
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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