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 highly recommended for NIRCam grism time-series observations.
See also: NIRCam Target Acquisition, NIRCam Grism Time-Series
Observations in NIRCam’s grism time-series mode should normally include 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. For applications requiring precision at the 1% level, and/or short observations where the TA would significantly increase the time to acquire the data, TA may not be necessary.
The grisms are in the NIRCam long wavelength (LW) pupil wheels and are always paired with filters in the filter wheel, either F277W, F322W2, F356W, or F444W for grism time-series observations. Module A is used for grism time-series observations because of the higher throughput relative to module B. GRISMR is required for grism time-series observations, so spectra will disperse in the row direction with longer wavelengths to the left (decreasing pixel index in science data, Figure 1).
Depending on the selected filter, some portion of the spectrum from 2.4–5 μm is dispersed onto the detector. To ensure that the spectrum falls completely on the detector, the target is placed at either of 2 locations in the along-dispersion direction. When F277W, F322W2, or F356W are used, TA places the target towards the right of the long wavelength detector (yellow star on the right in Figure 1). When F444W is used, TA places the target at the yellow star on the left of Figure 1. The positions of these field points are discussed in more detail on the main grism time-series article. They are used for both subarray and full-frame observations. The single TA subarray (yellow box in Figure 1) is used to place targets at either of the science field points.
The generic aspects of NIRCam TA are described in the NIRCam Target Acquisition article.
Words in bold are GUI menus/
panels or data software packages;
bold italics are buttons in GUI
tools or package parameters.
- The TA subarray is 32 × 32 pixels in size and is on the long wavelength detector on module A.
- Users may select either the F335M or F405N filter for TA.
- Using F405N allows acquisition of targets approximately 3.0–3.5 magnitudes brighter than F335M, depending on the spectrum of the target.
- Using F405N allows acquisition of targets approximately 3.0–3.5 magnitudes brighter than F335M, depending on the spectrum of the target.
- After the onboard TA process completes, a slew moves the target from the center of the TA subarray to one of the 2 science field points indicated in Figure 1, as described above.
Target acquisition saturation and sensitivity limits
See also: NIRCam Bright Source Limits
The time-series TA subarray frame time is 0.015 s. It is recommended that users choose a TA exposure time that achieves a 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. Approximate F335M and F405N saturation and sensitivity limits for NIRCam grism time series, and time-series TA, are summarized below. Limits are for a G2V star and are given in Vega magnitudes. Users should use the Exposure Time Calculator (ETC) to estimate appropriate TA integration times for their targets.
- Bright limit:
- Readout pattern = RAPID, Ngroups = 3 (0.06 s integration)
- mK = 7.1 (using F335M)
- mK = 3.9 (using F405N)
- Readout pattern = RAPID, Ngroups = 3 (0.06 s integration)
- Faint limit:
- Readout pattern = DEEP8, Ngroups = 65 (19.3 s integration)
- mK = 18.9 (using F335M)
- mK = 16.0 (using F405N)
- Readout pattern = DEEP8, Ngroups = 65 (19.3 s integration)
The bright limit for TA through the F405N filter is approximately 3.5–4.0 magnitudes brighter, depending on the spectrum of the target. The figures and discussion below are intended to help observers determine whether to accept some saturation in their TA images acquired using F335M, or to switch to the F405N filter in order to achieve higher TA accuracy at the expense of longer TA integration time.
When the TA integration saturates the accuracy of the onboard centroiding algorithm is degraded. Modeling has been performed to characterize the effects of saturation, and the results are summarized below. The immediate effect of saturation is that the core of the observed PSF appears dark in the image used by the target location algorithm, as seen in Figure 2. The modeling indicates that centroiding accuracy degrades gradually as saturation increases, as summarized in Figure 3.