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<p>Bill <br>
</p>
<p> 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 ?<br>
</p>
<p>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....</p>
<p>just a wild guess</p>
<p>- peter</p>
<p><br>
</p>
<div class="moz-cite-prefix">On 3/2/20 4:09 PM, Thompson, William T.
(GSFC-671.0)[ADNET SYSTEMS INC] via fitsbits wrote:<br>
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<p class="MsoNormal"><span style="font-size:11.0pt">Folks:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">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.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Suppose that
one had a three dimensional cube with the following axis
definitions:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">CTYPE1 =
‘RA---TAN’<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">CTYPE2 =
‘DEC--TAN’<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">CTYPE3 =
‘Stokes’<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">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?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Is there a
better way to distinguish between these cases other than
putting a comment into the header?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Thank you,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Bill
Thompson<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
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