JWST ETC NIRSpec Target Acquisition

The JWST Exposure Time Calculator has a NIRSpec target acquisition (TA) mode that will allow the user to choose instrument and detector parameters that will achieve the necessary signal-to-noise ratio for the TA source. The TA procedures are important to be able to place the targets into one of the FSs, the IFU, or within the MSA shutters.

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After a JWST slew, the NIRSpec target acquisition (TA) will fine-tune the telescope’s pointing to align the target within the specific aperture of the observation.

As presented in the Astronomer's Proposal Tool (APT), NIRSpec has 4 options for TA: Micro-Shutter Array TA (MSATA), Wide Aperture TA (WATA), VERIFY_ONLY TA, and NONE. The VERIFY_ONLY option is an imaging mode that is executed at the end of the observation and does not run the TA algorithm, it is meant to enable the assessment of fine field pointing in post-analysis. The NONE option forgoes the TA algorithm as well, and additionally, it skips the TA verification imaging. Fixed slit (FS) NIRSpec spectroscopy can use the WATA, MSATA, or NONE options. Bright object time-series (BOTS) observations can use either the WATA or NONE options. NIRSpec integral field unit (IFU) and multi-object spectroscopy (MOS) can use any of the 4 TA options.

In APT, MSATA, WATA and VERIFY_ONLY modes can use brightness estimates from the ETC calculations. MSATA observations are always acquired in FULL subarray detector readout, while WATA observations can be obtained with the FULLSUB32, or SUB2048 subarrays. For NIRSpec MSATA and WATA modes, the detector setup has a fixed number of groups (3), integrations (1), and exposures (1), which cannot be altered. Also, for these 2 modes the readout pattern and the filter are the only parameters that can be adjusted (NIRSpec readout patterns) to tune the signal-to-noise (SNR) for the TA observation. Information regarding brightness ranges for NIRSpec target acquisition can be found in Table 2 for MSATA and Table 2 for WATA.

Creating a TA calculation

Users will first create a source and a scene in the Scenes and Sources tab (or modify the default source if this is the first calculation of the workbook), and then specify parameters for the instrument and detector setups in the Calculations tab after selecting Target Acquisition mode from the NIRSpec drop-down menu. 

Defining the TA scene and source

The scene definition and source definition for TA calculations are defined along the same lines as for the other observing modes.

Creating a calculation

Target Acquisition is one of the modes available for all JWST instruments within the ETC. To initialize a TA calculation, select Target Acquisition from the NIRSpec instrument drop-down menu. Once selected, the calculation will load a default scene and source. This can be changed at the user's discretion by accessing the Scenes and Sources tab where the Select a Scene pane can be selected to change the pre-defined TA source.  

Figure 1. Creating a NIRSpec Target Acquisition calculation

Create a target acquisition calculation by selecting the Target Acquisition mode from the NIRSpec instrument drop-down menu.

What's supported

The ETC currently supports 2 NIRSpec TA modes: WATA and MSATA (Single Object). The WATA mode always uses the S1600A1 aperture, and therefore the aperture is not displayed in Instrument Setup as a selectable parameter. The MSATA (Single Object) mode, which is appropriate for MSATA standard TA calculations, does a full detector readout (no subarray) calculation using the MSA model. Both WATA and MSATA have a fixed number of groups (3), integrations (1), and exposures (1) and cannot be changed by the user in the ETC. Therefore, to increase the S/N for either ETC TA mode, it is recommended that users choose another star for TA, or select different filter and/or readout options in the instrument and detector configuration areas in the Calculations tab.

Figure 2. NIRSpec TA supported modes and filters

Top: The ETC supports the Wide Aperture Target Acquisition (WATA) and Micro-Shutter Array Target Acquisition (MSATA)
Bottom: Filters F110WF140X, and CLEAR are available for both acquisition modes. 
The readout patterns available in the ETC for MSATA mode are NRSRAPID, NRSRAPIDD1, NRSRAPIDD2 and NRSRAPIDD6. Note that selection of NRSRAPIDD1 or NRSRAPIDD2 is only valid when used in conjunction with the CLEAR filter. For WATA mode, only NRSRAPID and NRSRAPIDD6 are available. Selection of the readout pattern defines the TA exposure duration (more information on readout patterns can be found at  NIRSpec Readout Patterns). 
Figure 3. NIRSpec TA subarrays and readout patterns

The Detector Setup tab, showing the Subarrays for WATA (top) and Readout patterns for MSATA (bottom).

WATA mode

WATA is the only TA mode available for high SNR spectrophotometric observations using the BOTS, FS, MOS and IFU templates.

The ETC WATA mode can be used with the following filters: CLEARF110W, or F140X, just as in APT. Both the mode and the filter are selected in the Instrument Setup sub-tab within the Calculations pane. With the choice of filter, a throughput versus wavelength plot is presented.

The subarray choices for the ETC WATA mode are FULL, SUB32, and SUB2048. The SUB32 subarray reads a window of 32 × 32 pixels, while the SUB2048 reads a window of 32 × 2048 pixels. The FULL subarray reads 2048 × 2048 pixels.

The readout patterns available for the ETC WATA mode are NRSRAPID and NRSRAPIDD6.

MSATA (Single Object) mode

MSATA mode can be used with FS, IFU and MOS templates.

The ETC MSATA (Single Object) mode can be used with the CLEAR, F110W, or F140X filter. As previously mentioned, the mode and the filter are selected in the Instrument Setup sub-tab within the Calculations pane.

The single subarray choice for the MSATA (Single Object) mode uses the FULL subarray, which reads the entire 2048 × 2048 pixel array.

The readout patterns available for MSATA (Single Object) mode are NRSRAPID, NRSRAPIDD1, NRSRAPIDD2 and NRSRAPIDD6. The NRSRAPIDD1 and NRSRAPIDD2 readout patterns can only be used in conjunction with the CLEAR filter.

The ETC allows for the calculation of optional imaging with the MOS spectroscopy mode to acquire confirmation images of science sources using the NIRSpec imaging mirror. This imaging mode can be used to analyze the position of science sources through the open shutter slitlets and are acquired after the science observations. The ETC implements confirmation imaging using the NIRSpec MOS Confirmation Imaging mode in the NIRSpec drop-down menu on the Calculations page. This mode uses the Imaging Aperture Photometry strategy and the same 5 science filters and 4 subarrays as does the ETC NIRSpec Multi-Object Spectroscopy mode. The number of groups can be specified to produce a SNR that is comparable to the science observations. 


For VERIFY_ONLYNRSRAPIDD1 and NRSRAPIDD2 are not supported in APT.

For VERIFY_ONLY, the pointing relies only on the guide star acquisition performed by the FGS. This method will be used for placing extended objects in the field for IFU observations, or when using the MSA as a long slit, or when using the FS S1600A1 aperture. VERIFY_ONLY is an imaging mode in the APT and can be found in the ETC as modes IFU Verification Imaging and MOS Verification Imaging.


For ETC TA calculations, the optimal range for the SNR is from 20 at the faint end all the way to saturation at the bright end. If the SNR of the calculation does not reach 20, the ETC will issue a warning that "TA MAY FAIL." However, it is possible that TA could be successful at lower S/N ratios, and these thresholds may be revised in the future.

For the on-board centroiding algorithm to work properly, at most only one pixel can be saturated. Otherwise, the coarse location routine in the centroid algorithm may fail and the TA process will derive an inaccurate slew to place science targets in their observing apertures. The ETC will issue a warning when one or more pixels is saturated.

Latest updates

    Updated for ETC 3.0.

  • Updated for ETC 1.7.

  •  Updated for ETC 1.6.

  • Updated for ETC 1.5.

  • Fixed incorrect links.

    Updated for ETC 1.3.
Originally published