NIRCam Time-Series Imaging Target Acquisition
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.
Target acquisition saturation and sensitivity limits
See also: NIRCam Bright Source Limits
The TA subarray frame time is 0.015 s. It is recommended that users choose a TA exposure time that achieves a total integrated signal-to-noise ratio (SNR) of >30, which enables a centroid accuracy of <0.15 pixel. Any readout pattern is available for TA, with Ngroups = 3, 5, 9, 17, 33, or 65. The saturation and sensitivity limits for the TA subarray are summarized in Table 1. Sensitivity assumes SNR ~ 30 with Ngroups = 65. Saturation limits are derived for Ngroups = 3. All calculations use the F335M filter. Users should use the Exposure Time Calculator (ETC) to estimate saturation and sensitivity for their targets.
Table 1. Bright source saturation limits and sensitivity for the TSO target acquisition sub-array (SUB32)
Saturation (Vega Mags)
Sensitivity (Vega Mags)
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TA centroid accuracy is a function of the source brightness as well as the location of the source within a pixel. Pixels that saturate prior to the second group of the three that are used to create the target location algorithm input image will appear with little or no signal, as seen in Figure 2. This will negatively impact the centroiding results of the algorithm.