NIRISS Apertures

JWST NIRISS apertures define target reference positions and nominal extent in the field of view for all observations.

Main article: JWST Instrument Ideal Coordinated Systems
See also: JWST Observatory Coordinate System and Field of Regard, NIRISS Detector Subarrays

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.

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. 

Figure 1. NIRISS apertures used for telescope pointing

The large cyan rhombus represents the footprint of the NIRISS detector in JWST focal plane V2-V3 coordinates. The (X, Y) coordinate system of this plot corresponds to the (X, Y) coordinate system of the science images delivered by the JWST calibration pipeline. The NIS_CEN aperture is situated at the geometric center of the NIRISS detector and represents the pointing location for WFSS observations. The NIS_SOSSTA aperture corresponds to the target acquisition subarray for SOSS observations; the center of this subarray is the location of the object while SOSS grism exposures are executed (see SOSS article for more information). The NIS_AMITA aperture corresponds to the target acquisition subarray for AMI observations. After the AMI target acquisition procedure has finished, the object is moved to the NIS_AMI1 location on a separate, nearby subarray (of 80 × 80 pixels) for the AMI science observations.
Figure 2. NIRISS apertures associated with the subarrays used in the SOSS observing mode.

The large cyan near-square represents the footprint of the full NIRISS detector in JWST focal plane V2-V3 coordinates (similar to in Figure 1). The two rectangular apertures named NIS_SUBSTRIP256 (which encompasses both rectangles) and NIS_SUBSTRIP96 (the bottom rectangle) show the footprints on the sky of the SUBSTRIP256 1 and SUBSTRIP96 subarrays used in SOSS observations, respectively. The plus sign corresponds to the reference location of both apertures shown, which is the same location as that shown for NIS_SOSSTA in Figure 1 above.
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