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, NIRCam Short Wavelength 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. For higher precision requirements, TA is highly recommended.

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).

Starting in Cycle 4, NIRCam will offer simultaneous short wavelength (SW) grism time series, using the Dispersed Hartmann Sensor (DHS), see NIRCam Short Wavelength Grism Time Series. Depending on whether the LW grism is paired with SW grism or SW imaging, the TA will be executed using different subarrays, indicated in Figure 1 in yellow (SW imaging pairing) or cyan (SW grism pairing). The two TA share similar X-axis location, but different have different Y location, driven by the need of placing the DHS spectra towards the center of the A module, on all 4 SW detectors. Consequently, the Y-axis destination of the post-target acquisition slew will be different (see the stars symbols in 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 or cyan stars on the right in Figure 1). When F444W is used, TA places the target at the yellow or cyan star on the left of Figure 1. The positions of these field points are discussed in more detail in the dispersion section of the main grism time-series article. They are used for both subarray and full-frame observations. Note that when SW Grism is paired with LW Grism, the SW spectra will also move left or right, depending on the field point destination of the TA activity. The SW grism can be paired with different blocking filters, ensuring that, depending on the choice of the LW filter and corresponding field point, the SW spectra will fall onto the detector (see NIRCam Short Wavelength Grism Time Series). For either one of the SW imaging/grism pairings, a single TA subarray (yellow or cyan boxes 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 unique aspects of TA for grism time series are:

  • The choice of SW pairing drives the use of either one of the TA subarrays
  • Both TA subarrays are 32 × 32 pixels in size and is on the long wavelength detector on module A.
    • In the ETC the cases are not differntiated becuase the 2 TA subarrays are identical in size and hence, in frame time.
  • 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. 
  • 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.


Figure 1. Target acquisition for grism time-series observations

Depending on whether the LW grism is paired with SW grism or SW imaging, the target acquisition (TA) will be executed using different 32 × 32 pixel subarrays, indicated above in yellow (SW imaging pairing) or cyan (SW grism pairing). The 2 TA subarrays share similar X-axis location, but have different Y location, driven by the need of placing the DHS spectra towards the center of the A module, on all 4 SW detectors. Consequently, the Y-axis destination of the post-target acquisition slew will be different (star symbols). Selected grism subarrays are shown in black. The TA pointing is centered on the TA subarray (yellow or cyan squares). To ensure that the spectrum falls completely on the LW detector, the target is placed at either of 2 locations in the dispersion direction. When F277W, F322W2, or F356W are used on the LW channel, TA places the target towards the right of the LW detector (yellow or cyan stars on the right), when F444W is used the target is placed towards the left of the LW detector (yellow or cyan stars on the left).

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 = 7.0 (using F335M)
      • mK = 3.6 (using F405N)
  • Faint limit:
    • Readout pattern = DEEP8, Ngroups = 65 (19.3 s integration)
      • mK = 19.0 (using F335M)
      • mK = 15.9 (using F405N)

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.

Figure 2. TA images

These TA images were produced from sources with K band magnitudes of 3.33 to 7.33. Pixels that saturate prior to or during the second group used to create the TA image will contain no signal (modulo noise) and appear dark in the images. In this case, no more signal can accumulate between the second and third groups, leading to a group 3 and group 2 difference close to zero. This value then propagates into the final TA image. The blue box to the lower right shows the 9 × 9 pixel box used in the centroid calculations.
Figure 3 shows the centroiding accuracy of the target location algorithm versus the K band Vega Magnitude of a G2V source. These calculations were performed using simulated point sources located on a grid of subpixel locations and with several Poisson noise realizations at each location. The effect of slight offsets of the target from the nominal science field point on data quality is unknown, but may matter in cases where multiple transit observations must be combined to achieve the desired SNR.
Figure 3. Centroiding error versus source brightness

Accuracy of the target location algorithm results for NIRCam time-series and grism time-series observations versus the K band Vega magnitude of a G2V source. The accuracy is calculated for a grid of subpixel locations and Poisson noise realizations. Individual results are shown as gray points. Red points and error bars show the mean and standard deviation over all pixel phases and noise realizations at each magnitude.
Figure 4 shows the centroiding accuracy versus the number of fully saturated pixels in the scene. These are pixels that are saturated in all three of the groups used to create the TA image. The ETC also uses this definition when reporting the number of pixels that have reached "full saturation."
Figure 4. Centroiding error versus the number of fully saturated pixels.

The number of pixels on the X axis is equivalent to the number of fully saturated pixels reported by the ETC. The red x's and error bars show the mean and standard deviation of the centroiding error.
Figure 5 shows the centroiding accuracy plotted against the number of pixels that saturate in groups 2 or 3 of the three groups used to produce the TA image. This is equivalent to the number of "partially saturated" pixels reported by the ETC.
Figure 5. Centroiding error versus the number of partially saturated pixels.

The number of pixels on the X axis is equivalent to the number of partially saturated pixels reported by the ETC. The red x's and error bars show the mean and standard deviation of the centroiding error. Note that fully saturated pixels begin to occur at limits described on the ETC NIRCam Target Acquisition article.


Notable updates

  • Figure 1 updated to show LW grism pairing with SW grism or SW imaging.
Originally published