[fitswcs] Polarization codes

[Doug Tody]

Hi Paddy -

This sounds like a visualisation or analysis issue.  Probably the
physical dataset should stick with (I,Q,U) if that is how data is
observed.  If a rendered value is computed, that could result in a
packed value represented as a 2-D image or image plane, with a UCD
or some layered convention used to describe what the pixel quantity
represents (or alternatively just use JPEG or some such).  As you say,
there is no standard way to describe things in this level of detail,
but it would be best to not over complicate the core standards.

	- Doug


On Thu, 13 Mar 2008, Paddy Leahy wrote:

> 
> On Thu, 13 Mar 2008, Doug Tody wrote:
> 
> > Hi All -
> >
> > I am sure there are plenty of other lurkers following this discussion,
> > including myself.
> >
> > We are currently having a similar discussion within VO, in connection
> > with the new version of the VO image access interface, which will
> > support cube or N-D data including spectral and time cubes, and
> > polarization.  We want this to be as compatible as possible with FITS.
> > FITS pretty much addresses this already, at least for spectral data
> > and polarization, but not I think for time cubes.
>                                          ^^^^^^^^^^
> 
> A Google search indicates active work on this...    ;)
> 
> The point of my original post is that the current FITS WCS definition does 
> not address the polarization case in enough generality.
> 
> Simple example: A common move in polarimetry is to convert (I,Q,U) 
> triplets into (Angle, Fractional polarization, I) triplets which are 
> easier to interpret (e.g. they map nicely to an HSV display). This 
> conversion may not be trivial if you correct the polarized amplitude for 
> noise bias.  But there is no standardised way of saving the latter triplet 
> (even one layer at a time) in a FITS file...I mean: a way that allows 
> programs to correctly interpret the file without the user having to look 
> at the header. Hopefully the VO does better...if so, FITS should go with 
> the VO convention.
> 
> regards,
>  		Paddy
> 
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