The locations of the JWST instruments in the focal plane and a description of instrument ideal coordinate reference frames are provided.
The ideal coordinate system is a distortion-removed frame used for dithers and other pointing offsets. These coordinates correspond to a functional transform from the pixel coordinates in the science frame. The orientation and parity of the ideal coordinate system are equal to the pixel coordinates, but pixel coordinates are used in the uncalibrated images produced by the JWST calibration pipeline while the ideal coordinates are used in geometrically rectified images (e.g., the _i2d image products of the pipeline). The ideal coordinate frame is defined locally for each aperture in the SIAF (Science Instrument Aperture File).
Figure 1 depicts the right-handed ideal coordinate system (XIdeal, YIdeal) for one of the science apertures on each of the JWST instrument detector arrays in the JWST focal plane and its V2/V3 coordinate system. (The +V1 axis is the telescope boresight, which points into the screen in this figure.) For NIRCam, only the 2 long-wavelength detectors are shown for simplicity. For the MIRI, the Medium Resolution Spectrometer (MRS) ideal coordinate system (not shown here) is not aligned with the detector arrays (since these have a degenerate and discontinuous mapping to the sky) but is instead defined to be aligned with the V2/V3 coordinate system in the sense that +XIdeal lies along -V2, and +YIdeal lies along +V3.
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