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.
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, IRS2, will deliver enhanced noise performance and is recommended above the traditional readout patterns for all science cases that are not in danger of saturating.
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Table 1. NIRSpec traditional readout patterns
|Detector readout type||NIRSpec readout pattern names||Nframes per group||Group time (s)||Measured total noise in 1,000 s (e–)|
|Detector NRS1||Detector NRS2|
|NRSRAPID||1||10.737||5.55 †||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.