JWST Instrument Ideal Coordinate Systems

The locations of the JWST instruments in the focal plane and a description of instrument ideal coordinate reference frames are provided. 

See also: JWST Position Angles, Ranges, and OffsetsJWST Observatory Coordinate System and Field of RegardJWST Field of View

The ideal coordinate system is a distortion-removed frame used for dithers and other pointing offsets. These coordinates correspond to a functional transform of the pixel coordinates in the science frame. The orientation and parity of the ideal coordinate system are equal to the pixel coordinates.

Pixel coordinates are used in uncalibrated images produced by the JWST calibration pipeline. The ideal coordinate system is used in geometrically rectified images (e.g., the _i2d image products from 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. (In this figure, the +V1 axis—the telescope boresight—points into the screen.) 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.

Figure 1. JWST instrument detector locations in the JWST focal plane and ideal coordinate system axes
JWST Focal Plane with Instrument Detectors
JWST focal plane V2/V3 coordinate system with instrument detectors' fields of view shown as cyan rectangles. (For NIRCam, only the 2 long-wavelength detectors are shown.) For one SIAF aperture on each detector, the right-handed ideal coordinate system is shown in red. For reference, the +V1 axis (boresight) points into the screen. Click on figure for a larger version.

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