MIRI Coordinate Systems

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It is helpful to discuss the relevant coordinate reference frames pertinent to dithering:

Alpha, Beta: Local coordinate system of the image slicers
V2,V3: JWST coordinate frame, spherical system
RA,DEC: Obvious
XIdl, YIdl: Stands for XIdeal and YIdeal.  This is the coordinate system that APT actually applies offsets in.  For the MRS, XIdeal is defined to lie along -V2, YIdeal to lie along +V3.  Note that XIdl, YIdl are not the same as the (legacy) XAN, YAN frame sometimes used for ground-test data.
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