NIRSpec Detector Readout

The JWST NIRSpec detectors sample pixels in MULTIACCUM up-the-ramp exposures.

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The NIRSpec detectors (sensor chip assemblies; SCAs) operate in MULTIACCUM mode, meaning that the charge in each pixel may be sampled (read out) multiple times as it accumulates during an integration. The readout process is non-destructive, leaving charge unaffected and in place. Hence, charge is not transferred between pixels as in charged coupled devices (CCDs). These non-destructive up-the-ramp readouts offer several advantages:

  • They allow increase in the measurable dynamic range by recording the signal in pixels before they reach saturation
  • They allow for detection of cosmic rays and the possibility of recovering the incoming flux (in data processing)
  • They permit fitting to multiple up-the-ramp reads to provide a means for reducing the net effective read noise

At the end of an integration, the pixels are read out a final time and then reset, releasing their charge. In order to maintain thermal stability, pixels are constantly read or reset at a fixed cadence (10 µsec per pixel). To increase the readout speed and maintain low power consumption, each SCA is read out using four output channels. Each channel of normal pixels in the SCA comprises a region of 512 × 2048 pixels, and all channels are read simultaneously. The four outputs in each SCA appear in the data as thick bands aligned with the dispersion direction for NIRSpec. This alignment minimizes calibration difficulties in spectra that would otherwise span multiple outputs. The SCAs can also be read out in a subarray mode in which only a contiguous rectangular subset of pixels is recorded from each SCA using a single output channel. NIRSpec subarray readout mode can be used in fixed slit (FS) or bright object time-series (BOTS) observing modes for bright sources that would cause pixel saturation with the minimum full frame readout time.



NIRSpec detector readout definitions

Frames

The unit of data that results from sequentially clocking through and reading out a rectangular area of pixels is called a frame. The unit of time needed to clock a frame is called frame time and it depends on the size of the area read out and on the readout mode. Each detector readout delivers a frame of data which takes about 10.7 s for full frame traditional readout and about 14.6 s for IRS2. Shorter frame time can be achieved in traditional readout by selecting a dedicated subarray.

Modes  

NIRSpec offers two different readout modes. The traditional detector readout mode is the same as the NIRCam and NIRISS readout modes. This readout mode can be used for full frame or subarray readout. In addition, in order to meet strict noise performance requirements, NIRSpec also offers a noise reduction mode called improved reference sampling and subtraction, or IRS2. The IRS2 mode is only available for full frame readouts (no subarrays) and is recommended for all observations because of better read noise performance.

Groups

In order to reduce data volume and enable longer integrations, multiple frames can be averaged in the onboard focal plane processor. The averaged frames are referred to as one group. Table 1 lists the available NIRSpec readout patterns. For each of the two main readout modes (traditional or IRS2) there are two different readout patterns. By selecting one of the NIRSpec readout patterns, users may either save all individual readout frames (each Groups = N, Frames = 1; Figure 1) or averaged groups of frames (Figure 2).

Integrations

Multiple groups of non-destructive reads are generally taken consecutively, resulting in an integration ramp for each pixel. Integrations are terminated by a reset, which clears accumulated charge from the pixels. The result of the NIRSpec detector readout is a 3-D data cube that has x and y detector pixels and flux ramps in the z dimension (Figures 1 and 2).

Exposures

Multiple integrations can be executed without interruption in a single exposure (Figure 3).  An exposure with a single long integration can yield better noise performance, particularly on extremely faint sources.

The longest single integration multiaccum ramp time is recommended to be 1,500 s to balance noise effects and optimize cosmic ray rejection.  Using multiple integrations for exposures longer than 1,500 s can allow for longer on-source exposure times, to the maximum exposure duration limit of 10,000 s. Multiple integrations can also limit hard saturation or can provide time-resolved spectroscopy (e.g., as for bright object time-series (BOTS) mode).


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)
TraditionalNRSRAPID*1

10.737

2048 pixels × 2048 pixels x Ngroups
NRSRAPIDD1 1,2121.474
NRSRAPIDD2 1,3132.210
NRSRAPIDD6 4175.159
NRS442.947

IRS2

NRSIRS2RAPID

114.5893200 pixels × 2048 pixels x Ngroups
NRSIRS2572.944

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.

Figure 1. Example detector readout with single frame groups

Figure 1. Example detector readout with single frame groups

An example of up-the-ramp readout (left) shown with one frame per group. The resulting data cube structure is shown to the right.
Figure 2. Example detector readout with four frame-averaged groups

Example detector readout with 4 frame averaged groups

An example of up-the-ramp readout (left) shown with four frames averaged into each group. The resulting data cube structure is shown to the right. Groups with more averaged frames result in lower data volume, allowing longer integration times, or additional exposures, in a single visit.
Figure 3. An example multi-integration exposure

NIRSpec science exposures can consist of a single integration (one ramp) or multiple integrations as shown here to increase dynamic range or provide a time series dataset.



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