The JWST Near-Infrared Camera (NIRCam) provides target acquisition (TA) capabilities for the time-series imaging, grism time series, and coronagraphic imaging modes.

Target acquisition (TA) is a procedure that computes the centroid of an imaged source and places it precisely within an aperture or subarray. TA for relevant NIRCam observing modes is discussed in the following articles:

• NIRCam Time-Series Imaging Target Acquisition

• NIRCam Grism Time-Series Target Acquisition

• NIRCam Coronagraphic Target Acquisition

Instructions for configuring TA observations within the Astronomer's Proposal Tool (APT) are found in the NIRCam APT template pages for time-series imaging, grism time series, and coronagraphic imaging. Guidance for estimating the exposure time needed for computing accurate centroids is provided in the JWST ETC NIRCam Target Acquisition article.

NIRCam target location algorithm

The following steps describe the NIRCam target acquisition procedure executed by the onboard software:

1. Record three TA subarray exposures, each offset from the others (dithered) by a small integer number of pixels in the X and Y directions. 
2. Divide each TA exposure by a normalized flat field image.
3. Register the three TA exposures so that the target images are aligned.
4. Combine the three TA exposures using the minimum value of each pixel in the overlapping regions. This step removes bad pixels and cosmic rays from the combined image.
5. Smooth the image with a 3 × 3 pixel checkbox, as illustrated in Figure 1.
6. For subarrays larger than 48 × 48 pixels, extract a 32 × 32 subarray centered on the brightest pixel in the smoothed image.
7. Subtract the background using the mean of the faintest 40% of pixels in the 32 × 32 subarray .
8. Locate the target image using at 1st-moment calculation with a N × N pixel centroid box (iterative).
   1. For NIRCam coronagraphy, a 5 × 5 pixel centroid box is used, as shown in Figure 1.
   2. For other NIRCam modes, a 9 × 9 pixel centroid box is used.
9. Execute a small slew to place the target at the reference position of the TA subarray.
10. Using a predefined slew, move the target to the reference position of the science aperture.

The dimensions of the three dithered TA subarray exposures depend on the utilized NIRCam observing mode. (See the template-specific TA articles listed above.)

TA is usually performed on the science target, but it can also be performed on an object within the visit-splitting distance of the science target's position. An offset <20″ is recommended to maximize the final pointing precision.

NIRCam TA exposures are down-linked and available to users, but the data are only minimally calibrated due to the lack of dark reference frames.

References

Perrin, M. et al. 2013, JWST-STScI-003472
Sample Target Acquisition Scenarios for JWST