The locations of the JWST instruments in the focal plane and a description of instrument ideal coordinate reference frames are provided.
Figure 1 depicts the locations 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. Each of the pairs of red arrows represent the right-handed ideal coordinate system (XIdeal, YIdeal) for one of the science apertures on that detector.
The ideal coordinate system is a distortion-removed frame used for dithers and other pointing offsets. As such, ideal frame coordinates represent a location in a tangent plane projection when applied over scales of the dimension of a detector. The ideal coordinate frame is defined locally for each aperture in the SIAF (Science Instrument Aperture File) and it is the output of a polynomial of up to 5th order that is a function of pixel coordinates in the science frame. The latter is used in the uncalibrated images produced by the JWST calibration pipeline, while the ideal frame is used in geometrically rectified images (e.g., the _i2d image products of the pipeline).
The orientation and parity of the ideal coordinate system are equal to those of the science coordinate system.
For the MIRI Medium Resolution Spectrometer (not shown), the Ideal coordinate system is not aligned with the detector arrays (since these map discontinuously 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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