Coordinate Systems and Transformations

The coordinates of JWST data are transformed from the raw coordinate system to the science coordinate system.

DMS produces data that has been rotated from the raw (“native” readout coordinate frame) orientations to the science orientations, putting all the detectors in the same orientation with respect to the V2, V3 axes in the focal plane. This requires flipping and rotating the reference pixels and science coordinates for NIRSpec, NIRCam, NIRISS, and FGS. MIRI by design has the same detector and science (X, Y) coordinates so no transformations are needed. The figure below shows with the red box the origin of the raw coordinates of the detector. The green box shows the origin of the science orientation, with the X and Y axis indicated with the black arrows. The lines across the the NIR detectors indicate the regions for each of the four output sublocks (512 × 2048) pixels in size, with the slow scan direction going opposite to the red box parallel to the black lines. The default fast scan directions are indicated with the arrows at the bottom of the NIRISS detector.

The orientation between the spacecraft V2/V3 coordinate system and the FITS science image x/y coordinate system is shown below. The telescope V1/V2/V3 coordinate system is right-handed with +V1 pointing toward the sky and out the page.

This figure compares the orientation of the raw images with those produced by DMS, also known as SCI orientation. The red box indicate the origin of the raw coordinates while the green box is the origin for the science coordinates. The black arrows are the axis for the science orientation.  Although not shown here, the raw orientation is the same as the detector orientation, except for NIRISS.  Note that NIRCam LW is offset.  Science image frame orientations with respect to V2V3-frame V2 to the left and V3 up.


References

JWST-STScI-001256

JWST-STScI-002111




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