Illumination Pattern

Before calculating the K:L ratio, it is necessary to check that the ICM source has not moved with respect to the focal plane. While the FI chips are flat, that is they show virtually no variations in QE or spectral response across their surface, the BI chips do have noticable spatial variations. If the area illuminated by the ICM changed from measurement to measuremnt, we might see variations in the K:L ratio. Figure 4.112 shows the illumination pattern of the ICM source on the focal plane. The map was made by selecting Mn K $\alpha _{1,2}$ events. Data was binned into 16 pixel$\times$16 pixel cells to smooth the Poisson fluctuations The chips were then normalized with respect to the maximum cell values. The spatial variations in the BI chip S3 are clearly visible as a general reduction in detection efficiency near the Quad A/Quad B boundary and the small fluctuations over Quads B & C. As long as the illumination pattern remains stable with time, however, these spatial variations will be normalized out of K:L ration calculation. Figure 4.113 shows the normalized contours of the Mn K $\alpha _{1,2}$ events from three different measurements: XRCF-I calibration from 1997 June, ISIM-TV measurements in 1998 January, and AXAF-TV measurements in 1998 May. Due to a lack of sufficient low temperature HST data, there are no intensity contours for S2 from AXAFTV. Nonetheless, it is clear that ICM position is stable with respect to the focal plane.

  

Figure 4.112: Illumination pattern of Mn K $\alpha _{1,2}$ events from the ICM as measured at XRCF-I