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dear DMers, <br>
<br>
I am trying to recap on the discussion held since 2013 about the
addition of a redshift or doppler axis in Obscore. <br>
there has been various opinions expressed and typically two
different ways to think about it : <br>
<br>
1. Some dataset contain velocity measurements and are sampled inside
a velocity range , have velocity resolution , etc .<br>
this is the case for Marco's Usecase for instance Then quite
naturally we could think a new axis is needed .<br>
this was the first suggestion we had before and at the Banff interop
meeting.<br>
<br>
2. The doppler shift is always derived from an initial set of
spectral measurements . Therefore it can be described as a spectral
axis , with added reference position , rest frequency , etc. ,
specific unit in km/s , specific ucd.<br>
if I consider the Spectral FITS WCS specification,( <a
href="http://www.aanda.org/articles/aa/pdf/2006/05/aa3818-05.pdf">http://www.aanda.org/articles/aa/pdf/2006/05/aa3818-05.pdf</a>)
the velocity/doppler axes can have several flavors that are
characterized precisely. Table 1 and Table 2 of the spec , provide
all tags to differentiate the various cases.<br>
<br>
I also gave a second read to the most recent version of ImageDM
published in March , and from the description of the Spectral
coordinate class ,Fig 20. p 55 compared to the RedshiftCoordinate
Fig 21, it is clear the two subtrees have a lot in common . This
means the Redshift coordinate could just be a derived class of the
spectral coordinate construct, with the addition of all the
reference frequency, positions , etc. , needed for that. <br>
<br>
For the needs of the Obscore v1.1 update , I suggest that velocity
cubes be discovered by using <b>em_ucd</b>, with the possible
values equal to the following , as extracted from the UCD list:<br>
<br>
<i>E | spect.dopplerVeloc | Radial velocity, derived from the shift
of some spectral feature</i><i><br>
</i><i>E | spect.dopplerVeloc.opt | Radial velocity derived from a
wavelength shift using the optical convention</i><i><br>
</i><i>E | spect.dopplerVeloc.radio</i><i> | Radial velocity derived
from a frequency shift using the radio convention</i><i><br>
<br>
</i>em_dim, em_min, em_max, etc will describe the dimension and
range along the velocity axis for the returned datasets.<br>
<i><br>
</i>In a more detailed description , as required for instance for
the <i>getmetadata</i> capability in SIAv2 , then the full WCS axis
description could be exposed.<br>
<br>
Many thanks for comments , Mireille.<br>
<br>
<br>
<pre class="moz-signature" cols="72">--
Mireille Louys , Maître de conférences
Centre de Données ( CDS) Icube & Télécom Physique Strasbourg, Pôle API
Observatoire de Strasbourg 300, boulevard Sébastien Brant
11, Rue de l'Université CS 10413
67000 Strasbourg F - 67412 ILLKIRCH Cedex
<a class="moz-txt-link-freetext" href="http://astro.unistra.fr">http://astro.unistra.fr</a> <a class="moz-txt-link-freetext" href="http://www.telecom-physique.fr">http://www.telecom-physique.fr</a>
tel : 03 68 85 24 34</pre>
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