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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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