Apertures define targetable areas and their extents in the JWST field of view. Each aperture has a reference position to coincide with the observed target as well as outer bounding coordinates defining the nominal sky coverage of each observation. These definitions are stored in the Science Instrument Aperture File (SIAF).
The coordinates of NIRISS apertures used for science observations are shown in Figure 1. They may change slightly depending on the outcome of calibration measurements obtained in flight.
All NIRISS apertures are nearly aligned (to within ~0.5° of rotation) with both the JWST focal plane coordinate system (V2, V3) and all detector rows and columns.
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Note that there is an important difference between apertures and subarrays: apertures delineate the sky coverage of an observation, while subarrays refer to parts of the detector that are being read out. While the region of the sky covered by an aperture and by the corresponding subarray are often the same, this is not always true. A case in point is the apertures and subarrays used in the SOSS mode, which are distinct from one another and do not overlap (due to the cross-dispersion by the GR700XD grism, which projects the spectral orders away from the incoming beam). The apertures associated with the SOSS mode are shown in Figure 2.