<div dir="ltr"><div><div><div><div><div>I strongly urge to keep the two separate.<br></div>Yes, they have much in common, but they serve very different purposes.<br></div>And it is possible to create a 4-D cube where the 4th axis is actually a spectral axis and the pixels are different lines (rest frequencies).<br></div>STC2 will treat the spectral and redshift/Doppler as distinct axes.<br><br></div>I also note that you are missing redshift and that spect.dopplerVeloc makes no sense: it's either radio or optical (or, in rare cases, so-called relativistic), and one should not be allowed to gloss that over.<br><br></div> - Arnold<br></div><div class="gmail_extra"><br clear="all"><div><div class="gmail_signature"><div dir="ltr">-------------------------------------------------------------------------------------------------------------<br>Arnold H. Rots Chandra X-ray Science Center<br>Smithsonian Astrophysical Observatory tel: +1 617 496 7701<br>60 Garden Street, MS 67 fax: +1 617 495 7356<br>Cambridge, MA 02138 <a href="mailto:arots@cfa.harvard.edu" target="_blank">arots@cfa.harvard.edu</a><br>USA <a href="http://hea-www.harvard.edu/~arots/" target="_blank">http://hea-www.harvard.edu/~arots/</a><br>--------------------------------------------------------------------------------------------------------------<br><br></div></div></div>
<br><div class="gmail_quote">On Mon, Apr 27, 2015 at 2:26 PM, Louys Mireille <span dir="ltr"><<a href="mailto:mireille.louys@unistra.fr" target="_blank">mireille.louys@unistra.fr</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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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" target="_blank">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.<span class="HOEnZb"><font color="#888888"><br>
<br>
<br>
<pre 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 href="http://astro.unistra.fr" target="_blank">http://astro.unistra.fr</a> <a href="http://www.telecom-physique.fr" target="_blank">http://www.telecom-physique.fr</a>
tel : 03 68 85 24 34</pre>
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