NIRCam Target Acquisition

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

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

A description of the setup of TA observations within the Astronomer's Proposal Tool (APT) can be found on the NIRCam APT template pages for time-series imaging, grism time series, and coronagraphic imaging. For a description of how to estimate the exposure time required to achieve the necessary centroid accuracy, please visit the JWST ETC NIRCam Target Acquisition article.

 

Dithering for NIRCam Target Aquisition

During Cycles 1 and 2, NIRCam has experienced a number of TA failures due to the appearance of new bad pixels, that, when unflagged, may cause the on-board centroiding to fail. To mitigate these failures, during the execution of the Cycle 3 observing programs NIRCam will switch to a 3-dithers TA approach, similar to NIRISS TA. Users may see a slight increase in their charged time, but this will be accommodated by the observatory. By keeping their TA exposure specification unchanged, users will benefit by a approximately √3 improved SNR in their TA, and thus better centroiding and a reduced risk of TA failure.

NIRCam target location algorithm

The following steps outline the general target acquisition procedure executed by the onboard software:

  1. Two difference images are created from the individual groups in the TA exposure:
    1. Each TA exposure consists of a single integration ramp with an odd number of groups.
    2. Allowed values of the number of groups are 3, 5, 7, 9, 17, 33, or 65, and any NIRCam exposure pattern is allowed.
    3. Resulting integration times depend on the size of the TA subarray (users should refer to the template-specific target acquisition articles listed above).
    4. The difference images are computed from the middle and first group, and from the last and the middle group.
  2. These difference images are then combined via the minimum operator on a pixel-by-pixel basis to reject cosmic rays.
  3. The resulting image is corrected for the flat field.
  4. Known bad pixels are identified using a bad pixel mask and their value is replaced by the median of the nearby good pixels
  5. The image is then smoothed with a 3 × 3 pixel checkbox, as illustrated in Figure 1.
  6. If the subarray is larger than 48 × 48 pixels, a 32 × 32 postage stamp subarray is extracted, centered on the brightest pixel in the smoothed image.
    1. For subarrays 48 × 48 pixels or smaller, the entire subarray is kept.
  7. The background is removed by subtracting the mean of the faintest 40% of pixels 
  8. Final centroiding 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. The target acquisition software then executes a small slew to place the target at the reference position of the target acquisition subarray.
  10. A pre-defined slew then transfers the target to the reference positions of the science aperture (see template-specific target acquisition articles listed above).

Much of this process is generic to all instruments, but the parameters do vary somewhat between them (e.g., checkbox size, centroid box size, fraction of pixels for background subtraction, TA subarray size).

The TA is usually performed on the science target, though 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.

The data from the TA exposure is downlinked and available to users, although the pipeline performs only minimal calibration of the data due to the lack of dark calibration frames for NIRCam's TA subarrays.

Figure 1. Illustration of the general target location algorithm

Note that different instruments and modes use different size TA subarrays, checkboxes, and centroid regions.

References

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



Notable updates
  •  
    Added details about box sizes

  •  
    Added general TA procedure information
Originally published