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.
Words in bold are GUI menus/
panels or data software packages;
bold italics are buttons in GUI
tools or package parameters.
Table 1. NIRSpec traditional readout patterns
|Detector readout type||NIRSpec readout pattern names||Nframes per group||Group time (s)||Measured total noise in ~950 s (e–)|
|Detector NRS1||Detector NRS2|
|NRSRAPID||1||10.737||6.9 †||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.