NIRSpec Traditional Detector Readout Mode

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

The NIRSpec "traditional" detector readout mode is the same as that used by the JWST NIRCam and NIRISS instruments. 

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 the 4 outputs delivers a stripe of data (2048 pixel rows × 512 pixel columns), and the full readout takes 10.73677 s. The orange border shown in Figure 1 represents the reference pixels, which are 4 pixels across (not to scale) and insensitive to light. During these readouts, 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. The readout time of a subarray is dependent on its size.

The other NIRSpec readout mode, IRS2will deliver enhanced noise performance and is recommended above the traditional readout patterns for all science cases that are not in danger of saturating.

Figure 1. NIRSpec 2048 × 2048 traditional detector readout

A summary of the traditional readout of JWST near-infrared detectors. The yellow and green regions each designate one of the 4 detector channel outputs. The detectors are rimmed by 4 light-insensitive reference pixels. The 10 µsec pixel readouts progress from fast through slow scan directions for each output. The readout of a full NIRSpec detector takes 10.73677 s.

Words in bold are GUI menus/
panels or data software packages; 
bold italics are buttons in GUI
tools or package parameters.

The NIRSpec traditional readout patterns are NRS and NRSRAPID, which are summarized in Table 1 and presented 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, saving one group every 42.947 s in a ramp. In practice, using the NRSRAPID readout pattern with frequent recording of data groups is preferable and encouraged for all science use cases with no data volume limitations (the JWST Astronomer's Proposal Tool (APT) will warn users if data volume is an issue 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 extend the duration of the TA exposures to reach fainter magnitude reference stars.

Table 1.  NIRSpec traditional readout patterns

Detector readout typeNIRSpec readout pattern namesNframes per groupGroup time (s)Measured total noise in 1,000 s (e)
Detector NRS1Detector NRS2

NRSRAPID110.7375.55 6.46




Detector noise was measured during ground tests by investigating ~945 s (88 group) NRSRAPID dark exposures. Values reported are documented in ESA-JWST-SCI-NRS-RP-2017-003.

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. As in all JWST detector readouts, each blue diamond presented in these graphs represents one 2048 × 2048 pixel MULTIACCUM frame in the 3-D data cube. For the NRSRAPID readout pattern, the blue diamond is also the group that is saved, where frames = groups = 1. In the NRS readout pattern, 4 frames are averaged into a single saved group, shown as the red diamond.

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