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:
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