STC-S Velocities

[Arnold Rots]

Actually, the argument has been laid out in the STC document,
sections 2 and 4.4.1.4 and a mention in sections 1 and 4.4.2.2.

The issue is that Doppler velocities are formal, not physical,
velocities - the values are calculated using one of at least three
different formalisms.
There can be a fine line: One can analyze Doppler velocities in
a galaxy and derive a rotation curve with physical velocities.
The the conversion from delta_nu, using the radio, optical, or
"relativistic" formula, only yields a formal velocity that is not
necessarily physical.
I have seen the most awful examples resulting from confusion
over which definition was used.
It's a lot cleaner to keep these things properly separated.

Cheers,

  - Arnold


-------------------------------------------------------------------------------------------------------------
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 cfa.harvard.edu
USA
http://hea-www.harvard.edu/~arots/
--------------------------------------------------------------------------------------------------------------



On Fri, Jun 21, 2013 at 3:07 AM, Markus Demleitner <
msdemlei at ari.uni-heidelberg.de> wrote:

> Dear DM group,
>
> Over on the DAL list I've posted in the context of datalink an
> example for noting an STC description in STC-S, viz,
>
>     Time TT "Date"
>     Position ICRS SPHER3 Epoch J2010 "alpha" "delta" "distance"
>     Velocity "mualpha" "mudelta" "radialvelocity"
>     Redshift OPTICAL "z"
>
> [Fixed the errorneous CART3 in the original to SPHER3]
>
> Arnold commented:
>
> On Thu, Jun 20, 2013 at 05:40:36PM -0400, Arnold Rots wrote:
> > One more aside: I noticed that Markus had an example which listed
> > three velocity components (mu_ra, mu_dec, and radial_velocity) and
> > a redshift. However, if that radial velocity is actually a Doppler
> > velocity (and I may be wrong on this), then it is in the wrong
> > place and should be on the Redshift coordinate axis (which then
> > should be called Doppler velocity, to be more helpful).
>
> Since this is unrelated to the original discussion but sounds like
> something that might be relevant to others, too, I've taken this
> here, although the question probably is mainly for Arnold -- why
> do you want to move radialvelocity?  Given that this thing has
> distances, it seems fairly natural to me to have the derivative of
> that distance in Velocity.  Would clients be expected to do things
> differently if the radial velocity came in the redshift phrase rather
> than as the third component of Velocity?
>
> Also, since Position and Velocity share a common system, I have a
> third Velocity component anyway -- so, what would I write there?
> If I understand correctly, you'd like the above to roughly look like
> this:
>
>     Position ICRS SPHER3 Epoch J2010 "alpha" "delta" "distance"
>     Velocity "mualpha" "mudelta" ???
>     Redshift VELOCITY OPTICAL "radialvelocity"
>
> -- to me,  it would seem odd to me to put some artificial NULL value
> to where there's ??? now.  Or am I missing something fundamental
> here?
>
> Remark 1: The situation for this dataset is even worse -- it is a
> simulation of the Gaia result set and thus we are dealing with a
> radial velocity that
> would-be-a-doppler-velocity-if-it-were-a-measurement-in-the-first-place.
> *And* they have redshifts, too, mainly for the convenience of the
> quasars that Gaia will observe, too, and for which the radial
> velocities look a bit daunting.   But that's probably beside the
> point, and I might even be convinced that that Redshift belongs to
> another STC instance.
>
> Remark 2: If you're worried  about all the quotes in these
> expressions because you've never seen those mentioned in STC-S:
> Don't.  These are DaCHS-local extensions to STC-S that (essentially)
> allow column references whereever STC-S has actual literals and that
> I don't propose for standardization since I don't think anything of
> this kind should ever leave a data center.  It's how we give assign
> STC roles and metadata to our tables -- for the question here, you
> can just as well write 0.1 whereever there's something quoted.
>
> Cheers,
>
>       Markus
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.ivoa.net/pipermail/dm/attachments/20130621/127e6abc/attachment.html>