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JWST NIRCam Target Acquisition (TA) positions the source with sub-pixel accuracy on a specific part of the detector. A TA is required for all NIRCam Time-Series observations.


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Introduction

Parent pages: NIRCam Operations → NIRCam Target Acquisition Overview 
Main article: NIRCam Target Acquisition Overview
See also: NIRCam Time-Series Imaging

Observations in NIRCam’s time-series imaging mode require a target acquisition (TA) to precisely place the target at specific pixels. Precise positioning is required in order to achieve the highest possible calibration stability and enable enhanced flat field determination.

This is particularly critical for the data from the long wavelength (LW) channel, because the in-focus images there will be concentrated on just a few pixels. Time-series observations will often use the WLP8 weak lens element in the short wavelength (SW) channel to defocus the light of a bright source, improving the saturation limit and photometric precision. TA precisely positions the target so that most/all of the defocused light is on the detector and/or within a selected subarray. The diameter of the defocused SW image for WLP8 is approximately 132 pixels.

Time-series imaging uses one SW detector and the LW detector in module B. In addition to the FULL 1 array, users can select the point source subarrays located at the upper right of the module: SUB64P, SUB160P, or SUB400P (Figure 1). These subarray locations were selected to minimize the number of bad pixels on the detectors and to include reference pixels along two edges of the subarrays. The TA subarray is a 32 × 32 pixel subarray on the LW detector, located near the point source subarrays. The center of the TA subarray is offset from the other subarrays to avoid saturating the pixels used for science observations. All TAs are performed using the F335M filter for operational simplicity, and because it offers a good combination of sensitivity and saturation limits (see below). The TA procedure is as follows: 

  • The telescope slews to place the target in the TA subarray.
  • One TA exposure is taken with the F335M filter.
  • The on-board TA software processes the image as needed (to realign the image, flag bad pixels, remove cosmic rays, and subtract the background level), and applies a centroiding algorithm to determine the target coordinates.
  • A small slew then moves the target to the nominal center of the TA subarray.
  • A larger slew is then executed to precisely position the target at the science position on the detector.

Data from the TA exposure will be delivered to observers along with that from subsequent science exposures.

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Figure 1. Target acquisition for time-series imaging

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Target acquisition is performed with a 32 × 32 pixel subarray (yellow square) on the long wavelength detector B5, near the point source subarrays. The target acquisition pointing is centered on the TA subarray, followed by a telescope slew to one of the yellow stars at the center of the SW subarray for the science exposures. For simplicity, only the SW subarrays are shown here. For information on the location of the LW subarrays, see the NIRCam Detector Subarrays article.


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Target acquisition saturation and sensitivity limits

See also: NIRCam Bright Source Limits

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References

JWST technical documents

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Last updated

Published October 4, 2017


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Published March 02, 2017