[fitsbits] Stokes parameters coordinate system

[Peter Teuben]

Bill

       If an instrument doesn't measure one of the standard I,Q,U,V 
(stokes 1,2,3,4) or XX,YY,.... (stokes -5,-6,...) or RR,LL,... (stokes 
-1,-2,...) for which we have an integer convention (such that even 
CDELT3 etc. work), i'm inclined to say to assign it a non-standard 
integer (but negative), and indeed put in the comment fields that these 
are non-standard and not calibrated yet. For example -12,-13 might be 
nice if they somehow reflect Q and U ?

Hopefully the calibration routines will then output proper stokes Q and 
U. presumably the FITS header will have other parameters that the 
calibration routines know how to deal with....

just a wild guess

- peter


On 3/2/20 4:09 PM, Thompson, William T. (GSFC-671.0)[ADNET SYSTEMS INC] 
via fitsbits wrote:
>
> Folks:
>
> WCS Paper I describes a convention for Stokes parameters.  I’ve been 
> asked how to best represent the coordinate system that these Stokes 
> parameters are expressed in, specifically the Q and U parameters.  Is 
> there a standard practice of how this should be done?  It appears that 
> some space-based instruments define Q and U relative to instrument 
> (pixel) coordinates. However, the comment was made that this is not 
> really practical for a ground-based alt-az telescope due to field 
> rotation.  Another possible way to encode the Stokes parameters would 
> be relative to the real-world coordinates of the data.
>
> Suppose that one had a three dimensional cube with the following axis 
> definitions:
>
> CTYPE1 = ‘RA---TAN’
>
> CTYPE2 = ‘DEC--TAN’
>
> CTYPE3 = ‘Stokes’
>
> And that the PC matrix had cross terms between axes 1 and 2, i.e. 
> image rotation.  Is there a convention for how the Q and U values 
> should be interpreted?  For example, if Q/I=1, would you interpret 
> that as polarization aligned along the first pixel direction, or along 
> the RA direction as defined by the PC rotation matrix?
>
> Is there a better way to distinguish between these cases other than 
> putting a comment into the header?
>
> Thank you,
>
> Bill Thompson
>
>
> _______________________________________________
> fitsbits mailing list
> fitsbits at listmgr.nrao.edu
> https://listmgr.nrao.edu/mailman/listinfo/fitsbits
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://listmgr.nrao.edu/pipermail/fitsbits/attachments/20200302/ed7bb867/attachment.html>