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Dear all , <br>
Here is a summary of discussions on the Obscore data model update,
with several iterations <br>
with Doug Tody, François Bonnarel, Laurent Michel, Daniel Durand and
Pat Dowler.<br>
After the suggestion I made at the last interop in Banff, <br>
(see <a
href="http://wiki.ivoa.net/internal/IVOA/InterOpOct2014DM/obcore-ObsdatasetDM_compatibility.pdf">http://wiki.ivoa.net/internal/IVOA/InterOpOct2014DM/obcore-ObsdatasetDM_compatibility.pdf</a>
) <br>
we iterated with Doug Tody to enhance the axes' description of the
data part exposed in a dataproduct distributed by Obscore.<br>
<br>
These add_ons are meant to enrich the query response of ObsTAP
serving N-D datasets in order to expose the <br>
dimensions and the nature of axes involved in the data part.<br>
<br>
The way the data values are organized in files ( File format, single
or multiple extensions in FITS, etc, ..) is not taken into account
here, <br>
as the goal is discovery and selection of candidate datasets from
the query response.<br>
<br>
These values are not meant to be exact but represent a very good
approximation of the dimension in each physical axis.<br>
So we are proposing the addition of <br>
<ul>
<li>s_dim1, s_dim2 = the coverage in sampling elements ( pixels)
for each spatial axis</li>
<li>em_dim = the coverage in spectral elements along the energy
axis</li>
<li>t_dim = the coverage in the time axis, as number of time bins<br>
</li>
<li>pol_dim = the coverage in the polarization axis, as number of
polarization states <br>
</li>
</ul>
The nature of each axis is given by their prefix (s (spatial), em
(energy), t (time) and pol (polarization) s mentionned in the
Obscore previous version.<br>
<br>
These parameters are not describing the packaging (this is not
providing enough information for cutout yet. <br>
If the value of the <n>_dim is 1, it means that this is a
degenerate axis. <br>
One cannot have a <n>_dim =0 but NULL is allowed (interpreted
as 1). <br>
<br>
Please note that the default polarization for any imaging data is I,
intensity.<br>
<br>
It is important to note that the <n>_dim values are the
overall characteristic of the dataset and NOT the value of its WCS
axis <br>
(it might be true for simple cases, i.e. simple image but certainly
not for mosaic observation where the gap<br>
and/or the presence/absence of a given chip is not specified) .<br>
<br>
Here are some examples of various datasets exposed with this
strategy : <br>
<ul>
<li>MUSE data cube</li>
</ul>
<pre> s_dim1 = 300
s_dim2 = 300
em_dim = 3463
pol_dim = 1
pol_state = I
t_dim = 1
</pre>
<ul>
<li>2MASS: 2D image<br>
</li>
</ul>
<pre> s_dim1 = 300
s_dim2 = 300
em_dim = 1
pol_dim = 1
pol_state = I
t_dim = 1
</pre>
<ul>
<li>MEGACAM: mosaic image<br>
</li>
</ul>
<pre> s_dim1 = 20000
s_dim2 = 20000
em_dim = 1
pol_dim = 1
pol_state = I
t_dim = 1
</pre>
<ul>
<li>STIS spectroscopy (1D):<br>
</li>
</ul>
<pre> s_dim1 = 1
s_dim2 = 1
em_dim = 1024
pol_dim = 1
pol_state = I
t_dim = 1
</pre>
<ul>
<li>STIS spectroscopy (2D long slit):</li>
</ul>
<pre> s_dim1 = 1024
s_dim2 = 1
em_dim = 1024
pol_dim = 1
pol_state = I
t_dim = 1
</pre>
<ul>
<li>ALMA:<br>
</li>
</ul>
<pre> s_dim1 = 1000
s_dim2 = 1000
em_dim = 3000
pol_dim = 4
pol_state = I/U/V/Q
t_dim = 1
</pre>
The more detailed mapping of s, em , etc on NAXIS1, NAXIS2, etc. in
terms of WCS values could be handled in <br>
a future <b><i>getmetadata </i></b>capability of SIAv2, either via
ADQL / Obstap or with a query by parameter, awaiting a <br>
more complete WCS solution for all astronomical observation.<br>
A discussion on WCS implementation for instance at the IVOA interop
meeting in Sesto in June will help to go forward.<br>
<br>
There are suggestions to use an optional UCD tag to specify the
flavour of these axes , like <b><i>em_ucd</i></b>, for instance ,
already in ObsCoreDM <br>
to help to disentangle between frequency , wave and energy in the
query response.<br>
<br>
A draft is currently updated along these lines. <br>
<br>
Your comments , and suggestions welcome . <br>
Mireille <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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