The JWST Exposure Time Calculator (ETC) has a target acquisition (TA) mode for the Near Infrared Imager and Slitless Spectrograph (NIRISS) which allows the user to estimate the exposure time required to obtain sufficient signal to noise for the TA source to achieve the desired centroiding accuracy.

The JWST Near Infrared Imaging and Slitless Spectroscopy (NIRISS) instrument uses target acquisition (TA) for 2 of its observing modes, single object slitless spectroscopy (SOSS) and aperture masking interferometry (AMI). The NIRISS SOSS mode enables slitless spectroscopy of a bright target and is a key mode for exoplanet transit spectroscopy, while AMI enables high-contrast imaging to identify faint companions close to bright targets. The NIRISS ETC TA mode can be used to estimate the exposure times to obtain the required signal-to-noise ratio (SNR) for the NIRISS TA by using options available under instrument and detector setups. The recommended SNR for the NIRISS target acquisition is an integrated SNR = 30 or higher to obtain a centroid accuracy of 0.15 pixels for the TA source. Centroiding accuracy improves to about ≤0.10 pixel at SNR = 50 and to about ≤0.05 pixel at SNR = 100.

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How to create a TA calculation

The steps involved in creating a TA calculation are: (1) define the TA scene with a source having appropriate spectral type and magnitude on the Scenes and Sources pane, and (2) specify the instrument and detector setup on the Calculations pane.  

Defining the TA scene and source

The scene definition and source definition for the TA calculations are defined along the same lines as for the other observing modes. The default ETC TA scene has a single TA source located in the center of the scene. The spectral type for the source can be selected from the various options available for the continuum, and be normalized as required.

Creating a TA calculation

Target Acquisition is one of the mode options available for each instrument. To initialize a NIRISS TA calculation select Target Acquisition from the NIRISS instrument drop-down menu. This default calculation uses the default scene with a single point source with flat continuum. If the user wishes to change the default to a pre-defined TA source, use the Scene tab on the Configuration pane to select the scene that contains the pre-defined TA source.

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What's supported

The ETC supports TA for the following NIRISS observing modes: single object slitless spectroscopy (SOSS) and aperture masking interferometry (AMI). 

Instrument Setup

The Instrument Setup has options which are common to SOSS and AMI because the TA for both these modes are operationally similar except that the 64 × 64 pixels subarrays are located in different regions on the detector. The SOSS or AMI Faint option performs a normal imaging calculation using the imager or CLEARP aperture, while the SOSS or AMI Bright will use the NRM aperture to reduce the flux from very bright targets. See the NIRISS Target Acquisition article for information on how the "bright" and "faint" are defined for NIRISS TA. 

The only filter choice available is F480M which is the filter used for SOSS and AMI TA.

Detector Setup

The detector subarray setup for NIRISS TA uses SOSS or AMI TA subarray which is 64 × 64 pixels. Both SOSS and AMI TA observations use the same size for the subarray. While the 2 TA subarrays are on different locations on the NIRISS detector, the ETC does not take detector location into account. The TA region for SOSS is located at 1923 ≤ X ≤ 1986 and 1167 ≤ Y ≤ 1230 on the detector (in the science coordinate frame), while it is at 1054 ≤ X ≤ 1117 and 81 ≤ Y ≤ 144 for AMI.

Readout pattern

The Readout pattern used for NIRISS TA in the ETC is NISRAPID when using the SOSS or AMI Bright mode. Both the NISRAPID and NIS readout patterns are available when using the SOSS or AMI Faint mode. The subarray (SOSS or AMI TA) is a fixed value and no choices are available to the user. The number of groups available are from 3 to 19 and allow only odd numbers to account for the weighting scheme used by the TA observing program scripts. The minimum number of groups is 3. The TA mode allows only one exposure with one integration and cannot be changed by the user.

Strategy

The NIRISS TA mode only offers Target Acquisition as the option for strategy. The signal to noise is computed within a region of size 5 × 5 pixels. There is no background subtraction that is currently implemented for the TA strategy. It is assumed that the SNR is dominated by the photon noise from the bright target and the contribution from the sky is negligible. If the scene has multiple sources, the user should select the TA source from the Aperture centered on source drop-down menu in the Strategy tab.

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Outputs

The exposure specification for the TA should be chosen to obtain at least the minimum required SNR = 30 to achieve a centroiding accuracy of ≤0.15 pixel for the TA source. The ETC will issue a "TA may fail" warning if the SNR is below the required value. However, increasing the exposure time for TA (by increasing the value of Groups) has a negligible effect on the total time needed for the TA procedure, since it is dominated by overheads; this should be considered when planning observations for which accurate centroiding is deemed crucial. For example, the centroiding accuracy improves to about ≤0.10 pixel at SNR = 50 and to about ≤0.05 pixel at SNR = 100. 

The TA procedure uses bright targets and it is important to note whether there is saturation and how it affects the TA. For NIRISS, it is possible to accommodate up to 5 partially saturated pixels in the 5 × 5 pixel aperture used for the TA strategy. However, the TA will likely fail if there are fully saturated pixels, or if the number of partially saturated pixels exceeds the maximum number of pixels (5) allowed to ensure a successful target acquisition. The recommendation is to adjust the detector setup (e.g., decreasing the number of groups) or instrument setup (e.g., by using SOSS or AMI Bright) to avoid saturation.  

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References

Goudfrooij, P. 2017, JWST-STScI-005934
NIRISS Target Acquisition: the sensitivity of centroid accuracy to the presence of saturated pixels

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