[obs-tap] draft summary topic 6: *Polarised data*

[Mireille Louys]

Topic 6: *Polarised data*
This is the case when the observed flux was recorded for various  
states of a polarizer.
Then the data set can be : a set of images, a set of spectra , a set  
of spectral cubes with various polarisation flux at each data point,  
etc..
It seems there is no need for a specific data product_type value here.

ObsTAP can be used to expose these data set by :

1. use o_ucd (mandatory) with appropriate ucd value containing the  
substring phys.polarisation like in  
"phot.flux.density;phys.polarisation" . (cf list of observables)

2. add an optional data model field: pol_states
It contains a  list of polarisation states.
Its format is an enumeration based on the FITS polarization axes  
labels (by code, not by number, i.e. I Q U V RR LL RL LR XX Y Y XY Y X  
POLI POLA UNDEF
(see http://www.ivoa.net/Documents/Notes/Polarization/ by Anita  
Richards et al.)

Examples of such data sets should be worked out, and included as  
examples in the document.

--ML