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 (FoV) 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. For example, the NIRCam ALL 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 FoV 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 (e.g. A, B, or ALL), and/or Subarray selected (e.g. SUB320, SUB400P), and in some cases, other APT template parameters (e.g. filter). Apertures for the Time Series and Grism Time Series templates are described in the linked articles. APT uses the relevant apertures to support visualization of observations in the Aladin tool.

SIAF uses two coordinate systems. V1, V2, V3 system is is common to all instruments and is referenced to the telescope boresight (V1) and anti-sunshield (V3) vectors. Apertures have a reference position defined in V2, V3 that is used to point at targets. Each detector and each subarray have reference positions in this system. The compound apertures also have V2, V3 reference positions. The ideal coordinate system is defined with axes parallel to detector rows (XIdl) and columns (YIdl) at the reference position on the detectors. For NIRCam, the V2 and V3 axes are nearly aligned (to ~1° of rotation) with both the ideal coordinate system at all locations in the focal plane. The ideal system is used to calculate offsets of the pointing within an observation (e.g., dither positions, mosaic tile positions, Offset special requirements). Ideal frame coordinates can be treated as a location in a tangent plane projection, with the origin defined as the given aperture's fiducial/reference position.

Representative entries are given below for the NIRCam apertures corresponding the the FULL subarrays on all 10 detectors and some subarrays. Separate tables are used for the imaging and coronagraphic imaging entries (on modules B and A, respectively). The coronagraphic field of view is in fact different from, e.g., the imaging one, and it is projected onto the detectors only when the coronagraphic 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 for each detector and tile in a mosaic. Full frame observations with module B alone use another compound aperture, the "Bs" aperture (a.k.a BSHORT). "Bs" spans the full extent of the short wavelength detectors in module B.

The aperture coordinates have changed slightly as astrometric calibration observations have been analyzed, but are extremely accurate and expected to be quite stable. Values here are low precision and informational only. Users interested in the most up-to-date and precise data should use the PySiaf package, as outlined in the article JWST Field of View.

Tables 1–3 provide the target reference positions ("_ref") and aperture vertices (_1–4) for the NIRCam detectors (SCAs), the subarrays used for imaging, and those used for coronagraphy in JWST (V2, V3) coordinates. Tables 4 and 5 give the FULL detector aperture information in NIRCam ideal reference systems (XIdl, YIdl). 

Table 1. NIRCam apertures (reference position and four corners) for all 10 detectors, and the All, A- and B-module compound apertures, in the JWST (V2, V3) coordinate system

For Table 1, ALL, AS (a.k.a 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. These three compound apertures are provided as options for Target Placement, defining the telescope pointing, when observing with all detectors on both modules (MODULE = ALL), or module B (MODULE = B in which case the BS aperture is used). Note that the 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 imaging subarray apertures (reference position and four corners) for Module B detectors in the JWST (V2, V3) coordinate system

In reference to Table 2, these systems are used, e.g., by APT to specify the (X,Y) offset special requirement when Module = B, and Subarray = 160/320/640. or 64P/160P/400P. When using Module = B and Subarray = 160/320/640, the target is placed within the short wavelength detector gap, so dithers, a mosaic or an Offset special requirement will be required to provide coverage at the target position. These subarrays are primarily intended to provide imaging of bright, extended sources. For imaging of bright, confined or point sources, use the "point source subarrays" (Subarray = SUB64P/SUB160P/SUB400P) which are located in the upper right corner of the B module as pictured in Figure 1 (see NIRCam Detector Subarrays).  Values are in arcseconds.

Table 3. NIRCam subarray apertures for coronagraphy (reference position and four corners) 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.

Table 3 gives the nominal target reference positions for the coronagraphic occulting masks ("Ref") and subarray aperture vertices (1–4)  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 is used for coronagraphy.

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

Table 5. NIRCam apertures, center and 4 corners for the 5 module B detectors in the NIRCam ideal coordinate systems referenced to the NRCBS and the NRCB5 apertures

Values are in arcseconds.

---

References

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

---

---