NIRCam Detector Overview
JWST NIRCam has ten 2K × 2K HgCdTe detectors: eight for 0.6–2.3 µm observations (0.031"/pixel) and two for 2.4–5.0 µm (0.063"/pixel).
Each detector contains 2048 × 2048 pixels; the interior 2040 × 2040 pixels are light-sensitive photodiodes, and the outer 4-pixel wide border consists of reference pixels used to measure temperature and bias voltage drifts during exposures. These drifts are removed by the image reduction pipeline.
Eight of the detectors, manufactured with a 2.5 µm cutoff, are used in the short wavelength channel (0.6–2.3 µm). Two detectors with ~5 µm cutoffs are used in the long wavelength channel (2.4–5.0 µm). The short wavelength detectors have lower dark current (see NIRCam Detector Performance).
Each detector is referred to as a sensor chip assembly (SCA). In each NIRCam module, the 4 short wavelength detectors are arranged in a 2 × 2 focal plane array (FPA). When imaging the sky, the short wavelength pixels have twice the spatial resolution (0.031″) as the long wavelength pixels (0.063"). The field of view of each short wavelength FPA overlaps with that of the single long wavelength SCA in each module. Both wavelength channels cover approximately the same 2.2' × 2.2' area on the sky with each module, for a total 9.7 arcmin² field of view when observing with both modules.
During an integration, the detectors accumulate charge while being read out multiple times non-destructively according to readout patterns. Users may opt to either read out the full detector or smaller subarrays for faster readout times. When the integration is complete, the detectors are reset, releasing the charge.
Accumulated analog charges are converted to 16-bit digital signals with up to 65,535 data counts (analog-to-digital units, or ADU) in each pixel. The gain values are roughly two electrons per data count (2 e–/ADU) on average, varying among the detectors as well as among pixels within each detector (see the Detector Performance article).
Table 1. Detector properties
|Detector properties||Short wavelength channel|
|Long wavelength channel|
|Wavelength response †||0.4–2.5 µm||0.4–5.1 µm|
|Number of detectors||8||2|
|Pixels (including reference)||2048 × 2048 pixels||2048 × 2048 pixels|
|Pixels (light sensitive)||2040 × 2040 pixels||2040 × 2040 pixels|
|Pixel pitch (physical size) ‡||18 µm||18 µm|
|Pixel scale (on sky)||0.031″/pixel||0.063″/pixel|
|Nyquist wavelength *||2.0 μm||4.0 μm|
|Field of view (each detector)||64" × 64"||129" × 129"|
|Field of view (all detectors)||2 × 2.2′ × 2.2′ (with 4–5” gaps)||2 × 2.2′ × 2.2′|
† Filters and dichroics limit these wavelength ranges.
‡ Distance between centers of adjacent pixels.
* PSF FWHM ~2 pixels at this wavelength; undersampled at lower wavelengths. For coronagraphy, the Nyquist wavelength is shorter.
1 System for Image Digitization, Enhancement, Control, and Retrieval Application-Specific Integrated Circuit
Rieke, G. H., 2007, ARA&R, 45, 77
Infrared Detector Arrays for Astronomy