The Off-Axis Point Response Function

PRF measurements were obtained at the XRCF at three energies and at several off-axis angles between 4-12 arcmin with ACIS-I and 4-9 arcmin with ACIS-S. The spot size growth with off-axis angle and energy is shown in Figure 12. The 70% encircled energy radii (EER(70%)) are shown as circles and the 90% encircled energy radii (EER(90%)) are shown as triangles. Filled points indicate ACIS-S measurements and open points indicate ACIS-I measurements. Several trends are seen in Figure 12. First, at 3.5 keV EER(90%) increases linearly between 4-12 arcmin from about 9 pixels to about 34 pixels. The slopes are similar for the ACIS-I and ACIS-S data, but the ACIS-I spot is systematically smaller at a given off-axis angle compared to the ACIS-S spot. Similar trends are seen for EER(70%), but the spot size is smaller and grows more slowly with radius compared to EER(90%). Although there is less data at 5.2 keV and 6.4 keV, similar trends with radius are seen. Apart from the degraded performance (larger spot size) of the ACIS-S data seen in the 3.5 keV panel of Figure 12, these properties primarily reflect the mirror performance. The degraded off-axis performance of the ACIS-S is roughly as expected from the mismatch between the focal and detector surfaces.

  

Figure 12: ACIS-I Off-axis 70% and 90% encircled energy radii