Example: H-IAI-CR-1.001

As a test case, we chose the count rate linearity test H-IAI-CR-1.001, sampling the on-axis in-focus PSF at Al K$\alpha$ (1.486 keV). This test is purported by the CMDB to have the lowest flux of any Phase H ACIS test; since we are unable to simulate pile-up at present, this test is the best comparitor due to its minimal pile-up. Even so, some pile-up is present, as illustrated by Figure 6.12. At Al K$\alpha$, most of the photons produce single-pixel events, so pile-up acts to suppress mainly the height of the central pixel in the PSF peak. The height of this central pixel is the main difference between the simulated and real PSF for this test, as shown below. So even modest pile-up acts to degrade the spatial resolution of AXAF - a fact that should not be lost by the user in the rush to worry about spectral degradation due to pile-up.
 

Figure 6.12:  The spectrum of the PSF spot in the XRCF test H-IAI-CR-1.001.
No grade filtering was applied, in an effort to illustrate the pile-up
present in this test.  The data were spatially filtered to include only
a 10 X10 pixel region around the PSF centroid, so spurious
cosmic ray events do not contribute strongly to this spectrum.  A
light curve shows the ACIS count rate to be approximately 2.5 counts/sec.
Even at such a modest flux (by XRCF standards), the 2-photon and 3-photon
pile-up peaks are discernable.  This spectrum also shows that the
source spectrum is quite clean, so this example analysis was carried out
with no grade or spectral filtering.