The JWST NIRISS instrument is equipped with a Hawaii-2RG detector which is an array of 2048 × 2048 pixels. It has a pixel scale of 0.065"/pixel and provides a 2.2′ × 2.2′ field of view.
The NIRISS detector is a HAWAII 2RG detector manufactured by Teledyne Imaging Sensors. It has a single sensor chip array (SCA) of 2048 × 2048 pixels, covering a field of view (FOV) of 2.2' × 2.2'. The detector is made of HgCdTe with specific composition tuned to provide a wavelength cutoff at approximately 5 μm. Detector readout is controlled by a SIDECAR (System for Image Digitization, Enhancement, Control, and Retrieval) ASIC (Application Specific Integrated Circuit) that sets the bias voltages and performs the analog-to-digital conversion of the pixel voltages.
The detector has a 4-pixel wide border of reference pixels which do not respond to light, but are sampled and digitized in the same way as the photosensitive pixels. These reference pixels can be used to monitor and remove low-frequency bias drifts on a frame-to-frame basis. The full array of sensitive pixels is 2040 × 2040.
The pixel size of the NIRISS detector is 18 × 18 μm which offers a plate scale of 0.0654 × 0.0658"/pixel. The full-well capacity of the detector is approximately 105 e–. Properties of the NIRISS detector are summarized in Table 1. A photo of the detector is shown in Figure 1.
Further information about the NIRISS detector properties that are important to consider when planning science observations can be found at the following articles:
- NIRISS Detector Subarrays - smaller subarrays mitigate the risk of saturation and enable faster read times than full-frame readout mode.
- NIRISS Detector Readout - the NIRISS detector is read non-destructively ("sampling up the ramp")
- NIRISS Detector Readout Patterns - NIRISS allows two readout patterns that determine the sampling cadence for non-destructive readout
Table 1. NIRISS detector properties
2048 × 2048 pixels
18 × 18 μm (physical)
Plate scale in X
Plate scale in Y
95000 to 105000 e–
>0.8 between 1.5 and 5.0 μm
Read noise (correlated double sampling)
Dark current @ 37K
0.0257 ± 0.0021 ADU/s
NIRISS observing modes
Doyon, R. et al., 2012, Proceedings of the SPIE, Vol 8442
The JWST Fine Guidance Sensor (FGS) and Near-Infrared Imager and Slitless Spectrograph (NIRISS)