NIRSpec Detector Readout Modes and Patterns
The JWST NIRSpec detectors support two readout patterns in each of its two readout modes.
NIRSpec supports four readout patterns, split over two readout modes: traditional and improved reference sampling and subtraction (IRS2). The traditional detector readout mode is the same one that is used by the JWST NIRCam and NIRISS instruments. However, ground test characterization of the NIRSpec detectors showed that this traditional readout did not permit NIRSpec to reach its much more strict 6 e¯ total noise goal (as measured in ~945 s dark exposures).
As a result, the method for improved reference sampling and subtraction (IRS2) was devised by the NASA Goddard detector characterization team (Rauscher et al. 2012). This IRS2 readout mode allows for improved correlated noise performance, and is particularly beneficial for long observations of faint targets. The two readout pattern options, for each of the traditional and IRS2 readouts modes, allow users to save all individual frames or save the averaged groups of frames, as described in the NIRSpec Detector Readout article.
This page includes:
- A comparison of NIRSpec traditional vs. IRS2 readouts
- Traditional vs. IRS2 detector readout performance
For more detailed information, see these associated articles:
A comparison of NIRSpec traditional vs. IRS2 readouts
Figure 1 presents data characteristics of the traditional and IRS2 NIRSpec detector readout modes. Table 1 summarizes the readout patterns available in each of these modes and some key properties. Note that the IRS2 readout cannot be used with subarrays. The best choice of readout pattern should be made using the NIRSpec Detector Recommended Strategies article, and checking the ETC with specific target brightness and noise properties.
Use of the IRS2 detector readout pattern options are recommended for FULL frame NIRSpec integrations that are not in danger of saturating the detector.
Table 1. Available NIRSpec detector readout patterns
Detector readout mode
NIRSpec readout pattern name
Nframes per group
Group time (s)
|Unprocessed data size (one integration)|
|2048 pixels × 2048 pixels x Ngroups|
|1||14.589||3200 pixels × 2048 pixels x Ngroups|
* Bold italics style indicates words that are also parameters or buttons in software tools (like the APT and ETC). Similarly, a bold style represents menu items and panels.
1 NRSRAPIDD1 and NRSRAPIDD2 are used for MSATA target acquisition only.
2 NRSRAPIDD1 reads one frame and drops one.
3 NRSRAPIDD2 reads one frame and drops two.
4 NRSRAPIDD6 reads one frame and drops six.
Switching between traditional and IRS2 readout modes or between full frame and subarrays between science exposures in a visit is not recommended because of additional overheads and a possibility of thermal transients. Target acquisition can only be done using the traditional (NRSRAPID, NRSRAPIDD1, NRSRAPIDD2, or NRSRAPIDD6) readout patterns, but the noise performance benefits of switching to IRS2 for the science exposures following target acquisition outweigh the concern of temporary thermal transients on the detector, especially for long exposures on faint targets.
Performance of NIRSpec traditional vs. IRS2 readouts
The IRS2 detector readout results in raw data that is 3200 × 2048 pixels per frame, compared to the standard 2048 × 2048 pixels in traditional readout data (Figure 1). The IRS2 readout mode includes increased sampling of reference pixels and recorded reference output information, and is described in more detail on the IRS2 readout mode page. The main benefit of IRS2 mode is the reduction in correlated (1/f) noise causing the "striping" which is shown clearly in Figure 2. The use of these readout patterns may allow observers to use distant shutters for background observations, for example. Table 2 presents the noise characteristics for the NIRSpec NRS1 and NRS2 detectors using traditional vs. IRS2 readout. The total noise sees a slight improvement with IRS2 (for the whole focal plane array (FPA)) and it offers significantly reduced 1/f (or correlated) noise, as is evident from the figures below. Figures 2 and 3 show the measured performance improvement between traditional and IRS2 readout modes.
For science observations that might saturate NIRSpec, the best choice of readout pattern should be made by following the guidelines presented in the NIRSpec Detector Recommended Strategies article and by using the ETC for specific targets. Because the IRS2 mode frame time is longer, brighter targets may be at greater risk of saturation.
Table 2. NIRSpec NRS1 and NRS2 detector noise performance in traditional and IRS2 readout
|Parameter||Requirement||Measured NRS1†||Measured NRS2†|
|total noise, traditional readout||<6 e¯||5.55 e¯||6.46 e¯|
|total noise, IRS2 readout||5.17 e¯||6.60 e¯|
† These values were measured from ~945 sec exposures. Values reported are from OTIS testing.
Rausher, B., et al. 2012 SPIE 8543
Reducing the read noise of HAWAII-2RG detector systems with improved reference sampling and subtraction (IRS2)