NIRSpec Traditional Detector Readout Mode

The JWST NIRSpec traditional readout mode is also used for the near-infrared detectors in NIRCam and NIRISS. Two readout patterns are available for NIRSpec science observations in traditional readout mode, and 3 additional patterns were added for target acquisition.

Readout of a full NIRSpec detector (2048 × 2048 pixels) in the traditional mode is performed with 4 outputs simultaneously (Noutputs = 4, see Figure 1). Each of these 4 outputs reads a rectangular area of the detector (2048 pixel rows × 512 pixel columns), and the total time needed to read the entire SCA (a.k.a. the frame time) is 10.73677 s. The orange border in Figure 1 indicates the 4 rows and columns of reference pixels (not to scale), which are insensitive to light. During the readout, each row is read one pixel at a time, beginning with the pixels along the bottom marked with small open black squares in Figure 1. Arrows show the default scan directions; pixels are read in fast scan and then slow scan directions. Readout of a subarray is done using just a single output, and the frame time for a subarray depends on its size.

The other NIRSpec readout mode, IRS2, has a superior noise performance and is recommended for all science cases that are not in danger of saturating.

Figure 1. NIRSpec 2048 × 2048 traditional detector readout

Visualization of the traditional readout sequence for JWST near-infrared detectors. The yellow and green regions each designate one of the 4 detector channel outputs, each of which reads out a (2048 x 512) pixel area. The orange "frame" outlines the 4 rows/columns of light-insensitive reference pixels. The readout progresses along the "fast" scan direction for each output before moving on along the "slow" direction, with a 10 µsec "dwell time" per pixel. The readout of the full SCA takes 10.73677 s.

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The 2 readout patterns available in NIRSpec traditional mode are NRS and NRSRAPID, which are summarized in Table 1 and illustrated in Figure 2. The NRSRAPID readout pattern saves each frame as a group, so the integration ramps are sampled every 10.737 s. The NRS readout pattern averages 4 frames into a group onboard the spacecraft, thus only saving one group every 42.947 s. In practice, using the NRSRAPID readout pattern with frequent recording of data groups is preferable, and encouraged for all science programs without data volume limitations (the JWST Astronomer's Proposal Tool (APT) will issue a warning if data volume is a concern for an observing program). Three additional patterns are offered for MSA standard target acquisition (MSATA) called NRSRAPIDD1, NRSRAPIDD2, and NRSRAPIDD6.  These patterns are used to optimize the duration of the TA exposures, e.g. to reach fainter reference stars.

Table 1.  NIRSpec traditional readout patterns

Detector readout typeNIRSpec readout pattern namesNframes per groupGroup time (s)Measured total noise in ~950 s (e)
Detector NRS1Detector NRS2

NRSRAPID110.7376.9 7.3




Detector noise measured during commissioning in NRSRAPID dark exposures, includes effects of cosmic rays after mitigation.

NRSRAPIDD1 and NRSRAPIDD2 are used exclusively for MSATA target acquisition. NRSRAPIDD6 is used only for WATA and MSATA, not for science exposures.

Figure 2. Up-the-ramp presentation of the NRSRAPID and NRS readout patterns

The NRSRAPID (left) and NRS (right) traditional readout patterns. Each blue diamond denotes a (2048 × 2048) pixel MULTIACCUM frame in the 3-D data cube. For the NRSRAPID readout pattern, each individual frame is saved as a group, i.e., frames = groups = 1. In order to save data volume, the NRS readout pattern can be used, in which every 4 subsequent frames are averaged onboard, and saved into a group as indicated by the red diamonds.

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