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Like many telescopes around the world, JWST was tasked to observe the double asteroid system Didymos / Dimorphos during the impact of NASA’s Double Asteroid Rendezvous Test (DART) spacecraft on Dimorphos on Sept 28, 2022. At the date of the test, the target asteroids were moving with a rate of ~110 milli-arcseconds per second across the JWST field of view, almost four times the nominal design rate for JWST’s moving target tracking mode. To help ensure that some data was acquired, eleven different observations were acquired with different guide stars, seven with Guider 1 and four with Guider 2. The large number of observations and the high motion rate provided an unprecedented opportunity to evaluate FGS performance across approximately half the field of view of both guiders. The DART tracking data provides an opportunity to examine guider performance across a wide area of the 5μm cutoff H2RG HgCdTe arrays employed in the guiders, providing a means to measure the systematic offset in the centroid as the PSF moves across, and is sampled by, the pixel grid. Systematic offsets in the centroid are of interest because they could create undesirable offsets in the pointing of the science instruments. An FFT analysis of the centroid time-series was used to identify the spatial scales and magnitudes of these centroid offsets. The expected systematic offset magnitude in the centroid due to the pixel sampling scale should be on the order of ~2 milli-arcseconds, with a corresponding variation in the count rate of 6 to 8%. The observed Guider 2 performance meets both expectations, while the results for Guider 1 show significantly larger variations in the reported count rate, likely due to the greater degree of ‘cross-hatching’ on the Guider 1 detector array. Despite these significantly larger count rate variations, the systematic offset variations observed on Guider 1 were found to be similar Guider 2 and thus consistent with expectations. This somewhat surprising result may be due to charge migration effectively blurring the detected point spread function.
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