NIRSpec Detector Recommended Strategies
NIRSpec offers two readout modes for science observations and each mode offers two readout patterns. This article informs the observer about the available strategies to select the detector configuration for the MOS, IFU, Fixed Slits, and BOTS observing modes.
To summarize, NIRSpec offers two different readout modes for observations: the traditional readout mode and the improved reference sampling and subtraction (IRS2) mode. Each mode offers two readout patterns for science, without and with frame-averaging:
- NRSRAPID (traditional with 1 frame per group)
- NRS (traditional with 4 frames per group)
- NRSIRS2RAPID (IRS2 mode with 1 frame per group)
- NRSIRS2 (IRS2 with 5 frames per group)
IRS2 readout mode can only be used for full frame science data (not target acquisition exposures), whereas the NRS and NRSRAPID patterns can be used for subarrays as well as full frame data.
As described in the NIRSpec Detector Readout article, an integration or ramp consists of one or more groups plus the initial reset. For NIRSpec's readout patterns, each group consists of one or more (1, 4 or 5) frames, depending on the readout pattern. The length of an integration depends on the number of groups and the number of frames per group (which determines the group time). Finally, an exposure consists of 1 or more integrations. This is illustrated in Figure 1.
Which readout pattern should I choose?
The general recommendation is to use NRSIRS2RAPID whenever full frame data is acquired, i.e., in the MOS, IFU, and Fixed Slits observing modes. If early saturation in NRSIRS2RAPID is an issue due to bright sources, NRSRAPID with full frame should be used for MOS and IFU, and subarrays (e.g., ALLSLITS) with NRSRAPID in the fixed slits observing mode. Better performance for such very bright sources may be achieved due to the shorter frame time in NRSRAPID compared to NRSIRS2RAPID. Frame averaging (NRSIRS2 and NRS readout patterns) can be used if data volume becomes an issue. This could be the case for e.g., very long integrations with many groups or when taking data with a second instrument in parallel.
The BOTS observing mode was designed for bright sources and no full frame readout is available. Thus, one should select the subarray best suited for the science (usually SUB2048 for the gratings and SUB512 of the prism) with NRSRAPID. For very bright sources, even smaller subarrays are available, but usually at the cost of reduced spectral coverage.
Flowcharts designed to guide the decision process on which readout pattern and how many groups per integration to use are shown in Figure 2 (MOS and IFU), Figure 3 (fixed slits) and Figure 4 (BOTS).
How many groups and integrations should I use?
While the optimal combination of groups and integrations depends on the science case and the observing mode used, some general recommendations can be given:
- The “ideal” number of groups: starting with the best matching readout pattern and subarray, do as many groups as possible without saturation. Up to a certain maximum (see next point), longer integrations with more groups are always better than multiple shorter integrations. For some science cases and instrument setups, it might even be beneficial to saturate some part of the spectrum in order to gain more in other parts. The JWST Exposure Time Calculator (ETC) should be used to assess saturation and achieved SNR.
- The maximum recommended integration length is 1500 seconds (corresponding to ~100 groups in NRSIRS2RAPID). Longer integrations can be taken, but the benefits overtaking two shorter integrations will be limited due to cosmic ray effects.
- The minimum recommended number of groups is 2, although 1 can be used if really necessary (e.g., very bright targets in BOTS observations). The higher the number of groups the better (better total noise, better duty cycle/efficiency), see first point above.
If one integration with a number of groups determined as outlined above is not enough to reach the desired signal to noise ratio, multiple integrations can be taken. This can be as either one exposure with multiple integrations or as several exposures with one integration each or as a combination thereof. While one exposure with multiple integrations would be most efficient, in many cases there will be other factors, like e.g. the desired dithering strategy, that will drive this decision.