NIRCam Performance on JWST In Flight

TL;DR

The paper evaluates NIRCam’s in-flight performance on JWST, detailing its dual-module, dichroic-split design that enables wide-field imaging and wavefront sensing with high redundancy. It demonstrates that NIRCam delivers higher-than-expected throughput, robust wavefront sensing via dispersed Hartmann sensors, and versatile observing modes (imaging, WFSS, grism time series, photometric time series, and coronagraphy) that enable groundbreaking exoplanet and high-redshift galaxy science. Commissioning reveals practical issues such as cosmic-ray snowballs, persistence in a subset of detectors, and scattering/glints, along with established mitigation strategies. The work also outlines planned enhancements, including parallel DHS operation with grisms and simultaneous multi-wavelength coronagraphy, to further improve efficiency and coverage in future JWST observing cycles.

Abstract

The Near Infrared Camera for the James Webb Space Telescope is delivering the imagery that astronomers have hoped for ever since JWST was proposed back in the 1990s. In the Commissioning Period that extended from right after launch to early July 2022 NIRCam has been subjected to a number of performance tests and operational checks. The camera is exceeding pre-launch expectations in virtually all areas with very few surprises discovered in flight. NIRCam also delivered the imagery needed by the Wavefront Sensing Team for use in aligning the telescope mirror segments (\citealt{Acton_etal2022}, \citealt{McElwain_etal2022}).

Figures (18)

• Figure 1: NIRCam's location in the JWST field of view. Designations for the short wavelength arrays are shown with the long wavelength arrays called NRCALONG and NRCBLONG with the fields of view outlined in blue. The long wavelength arrays cover the same area as the four short wavelength arrays in a module. The small green squares show the locations of the coronagraphic fields. V2 and V3 refer to the telescope coordinate system where V3 points away from the sunshield and V2 is perpendicular to V3 to form a right-handed coordinate system.
• Figure 2: One half of NIRCam showing its optical train and mechanisms. Starlight from the telescope enters the instrument from the upper left, enters the module after reflection from the pick-off mirror (1) and then reflects into the main body of the camera where the dichroic beamsplitter (5) separates the long and short wavelength light. The other module is a mirror image of the one shown here, mounted back-to-back behind what is shown.
• Figure 3: Throughputs of NIRCam's filters. All terms affecting throughpout including detector quantum efficiency and telescope reflectivity are included. The gray bar denotes the dichroic deadband. The letter P indicates filters that are mounted in the pupil wheels.
• Figure 4: On the left, a ramp for a pixel illuminated by a star showing the linear behavior of the charge collection. On the right, the low signal portion of the ramp illustrates the non-linearity present at low signal levels.
• Figure 5: Ramps from adjacent pixels illustrating how the slope changes when the adjacent pixel saturates and charge migration begins.