A JWST aperture consists of a unique targetable fiducial point and its associated region, used in support of JWST science operations. Aperture definitions are stored in the Science Instrument Aperture File (SIAF, see Cox & Lallo, 2017).

Each aperture has a reference position which will generally coincide with the observed target at the first dither position, and coordinates defining the field of view for the given instrument configuration.  While in some cases apertures coincide with physical entities in the field of view (e.g. a detector and its center, or a slit and its location), this is not always the case. Apertures can sometimes be defined to represent convenience regions. Detector-specific apertures define the reference position and subarray field of view for each detector. Compound apertures represent, in a simplified way, the overall field of view when multiple detectors are collecting data. This is for example the case of the NIRCam ALL aperture. This aperture encompasses the entire NIRCam FoV, and its reference position is at the nominal center of the FoV, in the intra-modules gap, where no data are collected. In addition to their positions and extents, the SIAF appropriately defines several systems of coordinates for each aperture. These coordinates describe the field of view geometry for the given instrument configuration (see Cox & Lallo, 2017). Some types of apertures may not have all available coordinates systems defined. E.g. compound apertures, not physically bound to a single detector, do not have "det" or "sci" (detector or science) coordinates attached.

For NIRCam — for all observations and for target acquisition — the apertures in use depend on the APT observing template, module selected (e.g. A, B, or ALL), subarray selected, and in some cases, other APT template parameters (e.g. filter). APT uses the relevant apertures to support visualization of observations in the Aladin tool.

Representative entries are given below for the NIRCam apertures corresponding the the FULL subarrays on all 10 detectors and the subarrays. Separate tables are used for the corresponding coronagraphic entries on module A. The coronagraphic field of view is in fact different from e.g. the imaging one, and it is projected onto the detectors when the coronographic pupil elements are in use.

The ALL aperture is used for full frame observations with both modules. It defines the full extent displayed in APT Aladin as well as the default size for each tile in a mosaic. Full frame observations with module B alone use another compound aperture, the "Bs" aperture (aka BSHORT). "Bs" spans the full extent of the short wavelength detectors in module B.

The aperture coordinates may change slightly as further astrometric calibration observations are taken and analyzed. Values here are current as of the beginning of the Cycle 3 observation epoch. 

All NIRCam apertures are nearly aligned (to ~1° of rotation) with both the JWST coordinate system (V2, V3) and all detector rows and columns.

Tables 1–3 show the target reference positions ("_ref") and aperture vertices (_1–4) for the NIRCam detectors (SCAs). Table 1 reports the values in JWST (V2, V3) coordinates and Tables 2, 3 in NIRCam ideal reference systems (XIdl, YIdl) in arcseconds. Note that while the (V2, V3) system is an absolute reference system, tied to the JWST focal plane, the ideal systems are each tied to a specific aperture. An ideal system is a distortion-removed frame, given in the SIAF in units of arcseconds. Ideal frame coordinates can be treated as a location in a tangent plane projection. The tangent point (i.e. XY Idl origin) is defined at the given aperture's fiducial/reference position.

Table 1. NIRCam apertures, center and four corners, for all 10 detectors in the JWST (V2, V3) coordinate system

The ALL, AS (aka ASHORT), and BS (BSHORT) apertures are compound apertures; ALL corresponds to all 10 detectors on both modules, while AS and BS correspond to all 5 detectors on one module. They are all provided as options for Target Placement, defining the telescope pointing, when observing with both modules (MODULE = ALL). When observing with the full module B (MODULE = B), the BS aperture is used. Note that not being tied to a physical detector, compound apertures do not have vertices defined in the (V2,V3) focal plane system. Also note that the AS and BS apertures have reference points located within the footprints of the A3 and B4 SCAs, respectively (see also Figure 1). Values are in arcseconds.

Table 2. NIRCam apertures, center and 4 corners for all 10 NIRCam detectors in the NIRCam ideal coordinate system tied to the NRCALL aperture

This system is used, e.g., by APT to specify the (X,Y) offset special requirement when Module = ALL. Note that while the ALL compound aperture does not have (V2,V3) vertices associated (see Table 1), it does have vertices defined in the Ideal system. Values are in arcseconds.

Table 3. NIRCam apertures, center and 4 corners for the 5 module B detectors in the NIRCam ideal coordinate systems tied to the either the NRCBS or the NRCB5 aperture

These system are used, e.g., by APT to specify the (X,Y) offset special requirement when Module = B, and subarray = FULL (BS) or subarray = 160/320/640 (B5). Note that while the BS compound aperture does not have (V2,V3) vertices associated, it does have vertices defined in the Ideal system. Also note that the BS aperture has its reference point located within the footprint of the B4 SCA. On the contrary, the B5 reference point resides in the intra-SCA gap of the 4 SW module B SCAs (see also Figure 1). Therefore, when using MODULE = B and subarray = 160/320/640, the target is placed within the gap. This approach was chosen to maximize the area of overlap between the SW and LW channels. When using the SUB160/320/640 subarrays, the placement in the gap might not be problematic for extended sources, but for point sources the users will need to specify an offset special requirement in order to place the target within a physical detector on the SW channel as well, or alternatively use the "point source subarrays" provided in APT and located in the upper left corner of the B module (see NIRCam Detector Subarrays).  Values are in arcseconds.

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Table 4. NIRCam subarray apertures for coronagraphy in the JWST V2-V3 coordinate system

Starting in Cycle 2, NIRCam coronagraphic imaging is offered simultaneously on the SW and LW channel. Note that the SW and LW apertures have the same pixels size, but have angular sizes in a 1:2 ratio. When the natural channel for a mask is the SW channel, the apertures correspond to a 640 × 640 pixels subarray, with angular sizes of ~20" and ~40" on the SW and LW channel respectively. When the natural channel for a mask is the LW channel, the apertures correspond to a 320 × 320 pixels subarray, with angular sizes of ~10" and ~20" on the SW and LW channel respectively. The natural channel is the channel corresponding to the wavelength for which the mask is optimized, for example SW for the 210R mask, optimized at 2.1 μm, or LW for the 430R mask, optimized at 4.3 μm.

Scroll right to view the full table. 

Table 4 shows the nominal target reference positions ("Ref") and aperture vertices (1–4) for the coronagraphic occulting masks in JWST (V2, V3) coordinates in arcseconds. Bar occulters (with names ending in "B") have multiple reference positions (not shown here), one for each allowed filter. Note that only module A will be used for coronagraphy.

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References

Cox C. & Lallo M., 2017, JWST-STScI-001550
Description and Use of the JWST Science Instrument Aperture File

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