The small, on-axis spot size adds a degree of difficulty to the study of the wings (r >~5 arcsec), as only ~1 in 100 photons reach beyond a radius of 5 arcsec. High photon fluxes and azimuthal averaging are required to obtain a good signal. An example of a surface brightness profile for an Fe-Ka target is shown in Figure 10. The incident flux of Fe-Ka photons was roughly 250 counts per second (corrected for QE), and the frametime of 0.122 sec provided for a fluence of ~30 counts per frame. At this rate the core of the spot is saturated, strong pileup occurs over the central several pixels, and the shape of the PRF is distorted.
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Ten tests at several energies between 0.7-8 keV were conducted to
measure the wings of the PRF. Based on SAOSAC simulations, the
azimuthally averaged, radial surface brightness profile is expected to
decrease in the power law fashion
.
We therefore measured the power law index alpha(E) using least squares
linear fits in log-log space to the surface brightness profiles of
each image. The fit to the Fe-Ka E=6.4 keV data for test
H-IAS-PW-1.043 is shown in Figure 10
with the linear fit superposed. We find a power law profile of
µ ~ r-1.3 between 10 and 15 pixels. The annular
region between 10-15 pixels was chosen to avoid pileup in the inner
region, and to avoid poor statistics at the edges of the subarrays
where the density of events and the area are diminished.
A plot of power law index alpha(E) as a function of energy is shown in Figure 11. We find a smooth, quadratic increase in alpha(E) with increasing energy, indicating broadening wings and flattening profiles with increasing energy. This trend presumably reflects the increasing dominance of shell 6 with increasing energy.
