# New UCD list

Sebastien Derriere derriere at newb6.u-strasbg.fr
Thu May 12 12:08:01 PDT 2005

```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
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```