Next: Relative Quantum Efficiency as Up: Measurement Method and Analysis Previous: Pileup Corrections To Relative
Several factors contribute to the uncertainty in the relative QE measurements. Photon statistics contribute 1.4% for the 1024 individual superpixel quantum efficiency ratios, but only 0.04% to the full CCD nominal value (assuming spatially uniform QE), where the nominal value is taken to be the centroid of the Gaussian fit to the histrogram of 1024 values. The same statistical errors apply to the mean.
Reproducibility of relative QE measurements made after moving the CCDs out of and then back into the chamber is typically within 1%, but is observed to be as poor as 3% at Cu (8.05 keV), possibly due to relatively high uncertainty in the pileup factor for the reference CCD caused by a high count rate at high energy.
It should also be noted that the pileup correction parameters are generally similar from one FI CCD to another, but they do seem to be slightly different for one older reference CCD (w103c4) used to calibrate some flight CCDs. Because older electronics were used for this CCD, the depletion depth is thought to be smaller, which would exert some influence on the pileup rate. This difference has not yet been incorporated into the correction codes.
Finally, relative misalignment between flight and reference CCDs can influence low energy ratios (C, O, F) due to the non-uniformity of these sources. (Some misalignment on order 50 pixels seems evident in a few cases, despite the apparent precision achieved with the alignment system.) This is less important for higher energies, which use the commercial X-ray tube and have a flatter illumination pattern at the CCD location (primarily due to a larger source-detector separation).
A further test of accuracy of relative quantum efficiency measurements is described in section 4.7.3.