NIRSpec Detector Readout Modes and Patterns

The JWST NIRSpec detectors support two readout patterns in each of its two readout modes.

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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:

For more detailed information, see these associated articles:



A comparison of NIRSpec traditional vs. IRSreadouts

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.

Figure 1. Traditional vs. IRS2 data structure

Traditional vs. IRS2 data structure

The data structure of the traditional readouts (left) versus IRS2 detector readouts (right). The data acquired in traditional readout mode has a frame of four reference pixels surrounding the 2040 × 2040 regular pixels used for science. The IRS2 readout, in addition to the frame of four reference pixels, intersperses reads of reference pixels within the regular pixels and also records a fifth reference output, resulting in a frame size of 3200 × 2048. Note that this size corresponds to raw data which the pipeline will process to the final 2048 × 2048 size. Users will not see this data format unless they repeat the processing on the raw 3-D data cubes.

Table 1. Available NIRSpec detector readout patterns

Detector readout mode

NIRSpec readout pattern name1

Nframes per group

Group time (s)

Unprocessed data size (one integration)

TraditionalNRSRAPID1

10.737

2048 pixels × 2048 pixels x Ngroups
NRSRAPIDD1 2,3121.474
NRSRAPIDD2 2,4132.210
NRS442.947
IRS2NRSIRS2RAPID114.5893200 pixels × 2048 pixels x Ngroups
NRSIRS2572.944

1  Bold italics font style is used to indicate parameters, parameter values, and/or special requirements that are set in the APT GUI.

2 NRSRAPIDD1 and NRSRAPIDD2 are used for MSATA target acquisition only.

3 NRSRAPIDD1 reads two frames and drops one.

4 NRSRAPIDD2 reads three frames and drops two.

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 (NRSNRSRAPID, NRSRAPIDD1, or NRSRAPIDD2) 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

For full frame NIRSpec readouts in MOSIFU, and FS observing modes that are not in danger of saturating the detector, the IRS2 readout mode is recommended because of its improved noise performance.

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

ParameterRequirementMeasured NRS11Measured NRS21
total noise, traditional readout<6 e¯ 5.55 6.46
total noise, IRS2 readout5.17 6.60

1 These values were measured from ~945 sec exposures. Values reported are from OTIS testing.  

Figure 2. Performance of tradition vs. IRS2 readouts

Performance of tradition vs. IRS2 readouts

A visual view of the traditional readout performance (left) vs. IRS2 readout (right). IRS2 readout is efficient at removing vertical striping caused by 1/frequency noise inherent in the JWST near-infrared detector system.
 Figure 3. A pixel cut showing the noise improvement of the IRS2 readout

A cross-cut of pixel noise in ADU vs. column for one row of the NIRSpec NRS1 detector. Traditional readout noise is plotted in red, and IRS2 noise is shown in black.


References

Rausher, B., et al. 2012 SPIE 8543
Reducing the read noise of HAWAII-2RG detector systems with improved reference sampling and subtraction (IRS2)




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