A new NIRCam capability that allows grism time-series observations in the short wavelength (SW) channel will be available to the scientific community in Cycle 4. This new capability (available in APT 2024.5 or higher, to be released on August 22, 2024) further enhances JWST’s ability to perform cutting edge studies of bright, time-variable sources by extending the brightness limit of JWST at wavelengths shorter than 2µm.

Previously, the grism time-series mode performed spectroscopy only in the long wavelength (LW) channel. Now, observers will also be able to select spectroscopy in the SW channel to obtain spectra from 0.6 to 5 µm via the usage of several blocking filters. The LW spectra (2.4 to 5 µm) have R~1600 at 4 µm, while the new SW spectra (0.6 to 2.3 µm) have R~300.

The new capability takes advantage of the Dispersed Hartmann Sensor (DHS), which produces 10 separate spectra of the target dispersed along detector rows (see the schematic in Figure 1). New subarrays have been added to accommodate observations of very bright objects. For more details, see the NIRCam Short Wavelength Grism Time Series JDox article. Observers who wish to use this new capability should also review the related Observing Strategies JDox page for special instructions on how to assess the DHS bright limits, and use the newly defined readout patterns to manage the data excess limits.

The accuracy of the JWST ETC for the SW grism time-series calculations may not meet the 10% accuracy requirement for Cycle 4. As a result, this capability is being offered on a shared-risk basis in Cycle 4. Calibration observations will be obtained and used to improve the accuracy of the ETC and the pipeline during and after Cycle 3. Please contact the JWST Help Desk, using the NIRCam card, with any questions.