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 four outputs simultaneously (Noutputs = 4, see Figure 1). Each of the four outputs delivers a stripe of data (2048 pixel rows × 512 pixel columns), and the full readout takes 10.73677s. 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.
The NIRSpec traditional readout patterns are NRS 1 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 four 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). Two additional patterns are offered for MSA standard target acquisition (MSATA) called NRSRAPIDD1 and NRSRAPIDD2. 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 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.
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