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This step of the astrometric calibration is done in two steps. In the first step the subarray coordinate
system, that is the the integer coordinates (p,q in the figure below) of the pixel centers as displayed in IA,
have to be transformed into the the cartesian coordinate system of the PACS focal plane (u, v, respectively,
in mm), which reproduces the real misalignment and rotation of the submatrices, as shown in the bottom
figure below. The transformation coefficient between p,q to u,v coordinates are contained in the spatial
calibration file PCalPhotometer_SubArrayArray_version.fits.
As a second step the coordinates u,v on the PACS focal plane have to be transformed into orthogonal local
coordinates on the tangential plane on the sky, y,z. The coordinates y,z, as shown in the figure below,
correspond to the offset in arcsecond of the individual pixel coordinates with respect to the the virtual
aperture. They are approximated by two polynomials in the three-dimensional space of u,v and chopper
and alpha (CHOPFPUANGLE entry in the status table output of convertchopper2angle task):
/Users/mromanie/tmp/Pxml/dp_workshop.xml
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