Optimal Fields for WFSS with NIRCam

Coordinated observations can mitigate the source confusion often encountered in wide-field slitless spectroscopy (WFSS)—but only over a limited field of view (FOV) in the detector: the "optimal fields."

In a single WFSS observation, a grism produces a first-order spectrum of all sources in the FOV of a long-wavelength (LW) detector. The range of this spectrum is the bandpass of the filter selected to be series with a grism—either of 2 grisms with orthogonal directions of dispersion. However, for any source in the FOV, only a fraction—none, some or all—of the spectrum actually falls on the detector to be recorded. The truncated portion is permanently lost.

The scientific potential of a single WFSS observation is further reduced by the possible confusion of overlapping spectra from multiple sources. Unlike truncation, source confusion can be mitigated by a program of multiple observations of the same astronomical scene.

For any LW filter (λ, λ+), for either grism (GRISM-R, GRISM-C), and for either module (A, B), the optimal field is defined as the union of all possible source positions within the FOV of the detector for which the entire spectrum from both grisms falls on the detector. The rationale for this requirement is that at least 1 of the 2 orthogonal spectra may avoid corruption by confusion.

As a further precaution, obtaining a multi-filter set of direct-imaging observations may help disambiguate confused sources by their colors.

The accompanying figure and table report the optimal fields in the A and B modules for the 12 NIRCam filters supported for WFSS. Each field is a square with center and side given in pixels. The area of a field is expressed both as a fraction of the full detector area and in arcmin2.

Optimal fields: MODULE-A to the left, MODULE-B to the right.

X center
Module A
X center
Module B
Y center

Length on
each side

Fraction of
detector area


Greene, T. et al. 2017, JATIS, 035001
λ = 2.4 to 5 μm spectroscopy with the James Webb Space Telescope Near-Infrared Camera

Robberto, M. 2017, JWST-STScI-00NNNN, 
An Analysis of the Sky Areas Mapped by NIRCam LW Grisms 

Last updated

Updated September 25, 2017

Updated March 21, 2017

  • New sections on out-of-field sources and direct imaging

Published December 24, 2016