ObsCore update discussion : adding Axes information in Obscore table

[Louys Mireille]

Dear all ,
Here is a summary of discussions on the Obscore data model update, with 
several iterations
with Doug Tody, François Bonnarel, Laurent Michel, Daniel Durand and Pat 
Dowler.
After the suggestion I made at the last interop in Banff,
(see 
http://wiki.ivoa.net/internal/IVOA/InterOpOct2014DM/obcore-ObsdatasetDM_compatibility.pdf 
)
we iterated with Doug Tody to enhance the axes' description of  the data 
part exposed in a dataproduct distributed by Obscore.

These add_ons are meant to enrich the query response of ObsTAP serving  
N-D datasets in order to expose the
dimensions and the nature of axes involved in the data part.

The way the data values are organized in files ( File format, single or 
multiple extensions in FITS, etc, ..) is not taken into account here,
as the goal is discovery  and selection of  candidate datasets from the 
query response.

These values are not meant to be exact but represent a very good 
approximation of the dimension in each physical axis.
So we are proposing the addition of

  * s_dim1, s_dim2 = the coverage in sampling elements ( pixels) for
    each spatial axis
  * em_dim = the coverage in spectral elements along the energy axis
  * t_dim = the coverage in the time axis, as number of time bins
  * pol_dim = the coverage in the polarization axis, as number of
    polarization states

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.

These parameters are not describing the packaging (this is not providing 
enough information for cutout yet.
If the value of the <n>_dim is 1, it means that this is a degenerate axis.
One cannot have a <n>_dim =0 but NULL is allowed (interpreted as 1).

Please note that the default polarization for any imaging data is I, 
intensity.

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
(it might be true for simple cases, i.e. simple image but certainly not 
for mosaic observation where the gap
  and/or the presence/absence of a given chip is not specified) .

Here are some examples of various datasets exposed with this strategy :

  * MUSE data cube

     s_dim1   = 300
     s_dim2   = 300
     em_dim   = 3463
     pol_dim      = 1
     pol_state = I
     t_dim      = 1

  * 2MASS: 2D image

     s_dim1   = 300
     s_dim2   = 300
     em_dim   = 1
     pol_dim  = 1
     pol_state = I
     t_dim = 1

  * MEGACAM: mosaic image

     s_dim1   = 20000
     s_dim2   = 20000
     em_dim   = 1
     pol_dim  = 1
     pol_state = I
     t_dim = 1

  * STIS spectroscopy (1D):

     s_dim1   = 1
     s_dim2   = 1
     em_dim   = 1024
     pol_dim  = 1
     pol_state = I
     t_dim = 1

  * STIS spectroscopy (2D long slit):

     s_dim1    = 1024
     s_dim2    = 1
     em_dim    = 1024
     pol_dim   = 1
     pol_state = I
     t_dim = 1

  * ALMA:

     s_dim1    = 1000
     s_dim2    = 1000
     em_dim   = 3000
     pol_dim  = 4
     pol_state = I/U/V/Q
     t_dim = 1

The more detailed mapping of s, em , etc on NAXIS1, NAXIS2, etc. in 
terms of WCS values could be handled in
a future */getmetadata /*capability of SIAv2, either via ADQL / Obstap 
or with a query by parameter, awaiting a
more complete WCS solution for all astronomical observation.
A discussion on WCS implementation for instance  at the IVOA interop 
meeting in Sesto in June will help to go forward.

There are suggestions to use an optional UCD tag to specify the flavour 
of these axes , like */em_ucd/*, for instance , already in ObsCoreDM
to help to disentangle between frequency , wave and energy in the query 
response.

A draft is currently updated along these lines.

Your comments , and suggestions welcome .
Mireille

-- 
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
http://astro.unistra.fr			http://www.telecom-physique.fr
tel : 03 68 85 24 34

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