NIRISS Detector Overview
The JWST NIRISS instrument is equipped with a Hawaii-2RG detector, a single 2048 × 2048 pixel sensor chip array (SCA), providing a 2.2′ × 2.2′ field of view. It has an average pixel scale of 0.066"/pixel.
Manufactured by Teledyne Imaging Sensors, the NIRISS detector is made of HgCdTe with a 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 an average plate scale of 0.0653"/pixel × 0.0658"/pixel which varies slightly across the detector and from filter to filter. 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"); and
- NIRISS Detector Readout Patterns—NIRISS allows 2 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 X1
Plate scale in Y1
Full well capacity
95,000 to 105,000 e–
0.88 from 0.5 μm - 5 μm
Read noise (correlated double sampling)
Dark signal @ 39K
1 Reported plate scales are the average value. There is a slight dispersion in plate scale across the detector and from filter-to-filter.
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)