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)
K sat (G2V)
|PageWithExcerpt||MR:APT GUI footnote|
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.
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 a dataset of simulated point sources located within a grid of subpixel locations and with several Poisson noise realizations at each location. The gray points show the accuracy of the calculated centroid compared for all observations. The red points and error bars show the mean accuracy and standard deviation 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 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 5 shows the centroiding accuracy plotted against the number of pixels that saturate in groups 2 or 3 of the 3 groups used to produce the TA image. This is equivalent to the number of "partially saturated" pixels reported by the ETC.
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. 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.