JWST Field of View

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 to be obtained with various JWST observing modes.

Table 1 provides the vertices of each region in the observatory's coordinate system (V2, V3). This is a small excerpt of data from the JWST Science Instrument Aperture File (SIAF). SIAF is a reference file used in operations that contains the official information on all apertures (e.g., NIRCam Apertures) and internal instrument coordinates.

Figure 1. The JWST field of view

Each JWST instrument observes an area on the sky shown here in (V2, V3) coordinates. Colors indicate observing modes: imaging, coronagraphy, grism spectroscopy, slit spectroscopy, NIRISS AMI, NIRSpec MSA, NIRSpec IFU, MIRI MRS (IFUs), and guidance with FGS.

Figure 2. Illustration of observations in the JWST field of view

Example illustrations of the types of data that can be obtained with many (not all) JWST observing modes.

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.


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 

Latest updates
    Added example on how to use pysiaf to pull aperture coordinates
    Reviewed and updated for Cycle 1 Call for Proposals release.
Originally published