# New UCD list

Thu May 12 12:43:14 PDT 2005

```You may want to look the gory detail up in the STC document.
I'd much rather tie "radial velocities" to redshift and, yes, they are
basically just formalisms, not physical velocities; but your
description is not entirely accurate.

- Arnold

Sebastien Derriere wrote:
> Arnold Rots wrote:
> >
> > Andrea Preite Martinez wrote:
> > > ...
> > >   You first suggested em.veloc.radio to account for the 2 conventions to
> > > derive velocities from spectra (optical and radio conventions). If
> > > nobody
> > > ever applies the optical convention to a radio spectrum (or vice versa),
> > > then we can live with spect.veloc;em.opt and spect.veloc;em.radio
> > >   What do you think?
> > >
> >
> > O yes!  The optical definition needs to be (and is) used for plenty of
>
>   Hi,
>
>   In order to find the relevant UCD, we need an accurate description/
> explanation of the quantity we want to describe.
>   My understanding is the following: there are 3 kinds of velocities
> 1. physical velocities (ratio of a distance and a time)
> 2. radio "velocity" representing a frequency shift
> 3. optical "velocity" representing a wavelength shift
>
>   2 and 3 have no physical meaning, they only correspond to a physical
> velocity for motions << c (speed of light). The difference is that (2)
> depends
> on the rest frequency, while (3) depends on the observing frequency. But
> (2)
> seems to be deprecated by the IAU.
>
>   In the UCD, you have the word "src.veloc" for case (1). - Again, the
> velocity"
> sorry for the confusion.
>
>   There is also a word "spect.veloc" for so-called velocities derived
> from a spectrum. We had in mind that it could be complemented by a word
> indicating what kind of spectra was used, e.g :
> spect.veloc;em.opt     -> radial "velocity" derived from optical
> spectrum
> spect.veloc;em.line.HI -> radial "velocity" derived from a shift of the
> HI line
>
>   With your input, I understand that we should have two words, for cases
> (2)
> and (3), for example: spect.opticalVeloc and spect.radioVeloc , and we
> could
> measurement-of-a-frequency-shift-in
>   My concern with this is that if someone asks the proper UCD for a
> because
> the description given by the user is ambiguous.
>
>
> > Do you distinguish between LSR, geocenter, barycenter, topocenter,
> > Galactic center (to name the most popular ones)?
>
>   There are a few UCD words which describe some reference frames:
> pos.geocentric
> pos.heliocentric
> pos.galactocentric
>
>   You are allowed to append them to indicate in which frame the velocity
> was measured:
> src.veloc;pos.geocentric
> src.veloc;pos.cartesian.y;pos.galactocentric   (to indicate one
> component of the velocity)
>
> In 3 cases where the frame was only used for velocities, some specific
> words exist, as noted by Jonathan:
> src.veloc.cmb
> src.veloc.lsr
> src.veloc.lg
>
>   Sebastien.
> --
>     _______
>    /  ~   /, Sebastien Derriere   mailto:derriere at astro.u-strasbg.fr
>   / ~~~~ //  Observatoire de Strasbourg    Phone +33 (0) 390 242 444
>  /______//   11, rue de l'universite     Telefax +33 (0) 390 242 417
> (______(/    F-67000 Strasbourg  France
>
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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/
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