NIRCam Grism Time-Series Target Acquisition
JWST NIRCam target acquisition (TA) positions the source with subpixel accuracy on a specific part of the detector. A TA is required for NIRCam grism time-series observations.
Observations in NIRCam’s grism time-series mode require a target acquisition (TA) to precisely place the target at specific points on the detector. Precise positioning is required in order to achieve the highest possible calibration stability and enable enhanced flat field determination. The grisms are in the NIRCam pupil wheels and are always paired with filters in the filter wheel, either F277W, F322W2, F356W, or F444W for grism time-series observations. Depending on the selected filter, some portion of the spectrum from 2.4–5 μm is dispersed onto the detector, with an undeviating wavelength at 3.95 μm. To ensure that the spectrum falls completely on the detector, the target is placed at either of two field points, one for F444W or another for the other 3 filters.
GRISMR is required for time-series observations, so spectra will disperse in the row direction. When F277W, F322W2, or F356W are used, TA places the target towards the right of the long wavelength detector (yellow stars on the right in Figure 1), and the spectrum disperses towards the left. When F444W is used, TA places the target at one of at the yellow stars on the left of Figure 1, and the spectrum disperses towards the right. The positions of these field points are discussed in more detail on the main grism time-series article.
The TA subarray is 32 × 32 pixels in size on the long wavelength detector and is offset from all field positions to avoid saturating the pixels at the field points prior to science observations. All TAs are performed with the F335M filter for operational simplicity, and because it offers a good combination of sensitivity and saturation limit (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.