The JWST field of view (FOV) is reviewed in this article. An example script to retrieve precise aperture vertices locations is also provided.

See also: JWST Target Viewing Constraints, JWST Micrometeorite Avoidance Zone (MAZ), JWST Instrument Ideal Coordinate Systems, JWST Position Angles, Ranges, and Offsets

Each JWST instrument observes an area on the sky bounded by the coordinates given in the telescope's (V2, V3) coordinate system.

Figure 1 shows NIRSpec, NIRCam, MIRI, NIRISS, and FGS fields of view in the JWST focal plane. Figure 2 shows illustrations of data obtained with various JWST observing modes.

Figure 1. The JWST field of view

 

Those wishing to obtain precise locations of aperture references and vertices in the focal plane can do so with pysiaf (Sahlmann, 2019). It is a python package that allows users to work with JWST apertures. 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).

The pysiaf code, with details on its installation and use, can be found in the GitHub repository: https://github.com/spacetelescope/pysiaf

A basic example for retrieving the reference location, orientation angle, and and vertex locations for (in this case) the NIRCam module A LW channel, is provided below:

import pysiaf  # Make sure that pysiaf is installed before importing
siaf = pysiaf.Siaf(‘NIRCam’)
aper = siaf[‘NRCA5_FULL’]  # NRCA5_FULL is the SIAF aperture name for NIRCam Module A LW channel

# The aperture reference positions in telescope coordinates (arcsec) and the rotation of the aperture’s Y-axis relative to V3 (degrees, counterclockwise):
v2ref, v3ref, v3angle = aper.V2Ref, aper.V3Ref, aper.V3IdlYAngle
print(v2ref, v3ref, v3angle)

# Vertices of the quadrilateral in telescope coordinates (arcsec)
v2_1, v3_1 = aper.idl_to_tel(ap.XIdlVert1, ap.YIdlVert1)
v2_2, v3_2 = aper.idl_to_tel(ap.XIdlVert2, ap.YIdlVert2)
v2_3, v3_3 = aper.idl_to_tel(ap.XIdlVert3, ap.YIdlVert3)
v2_4, v3_4 = aper.idl_to_tel(ap.XIdlVert4, ap.YIdlVert4)

print("%.2f  %.2f  %.2f  %.2f  %.2f  %.2f  %.2f  %.2f" % \
               v2_1, v2_2, v2_3, v2_4,
               v3_1, v3_2, v3_3, v3_4))

Note that there may be periods of time where new SI team updates to SIAF values may be in the process of being delivered to the Science Operations Database. Those with demanding use cases that require specific SIAF awareness should contact the relevant SI team for recommendations and support via the JWST helpdesk.

References

Sahlmann, J.S., 2019, Technical Report, JWST-STScI-007030
pysiaf, a Python package for the interpretation, generation, and maintenance of space telescope Science Instrument Aperture Files (SIAF)

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