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4.4 Sensitivity

The overall (OTA + COSTAR + FOC) central absolute quantum efficiency in counts photon-1 with no filters in the beam is plotted and tabulated as a function of wavelength in Figure 4.3 (and also Table 11 of the FOC Instrument Handbook, version 7.0), for the four FOC imaging and spectrographic configurations. The data represent the product of in-orbit measurements for the f/96 relay+OTA absolute quantum efficiency, and ground-based reflectance calibrations of the COSTAR mirrors for the f/48. The predicted loss of light from two reflections of MgF₂ coated aluminum COSTAR mirrors amounts to a 20% loss in the visible and a 35% loss in the ultraviolet. The loss due to the COSTAR mirrors is more than compensated by the improvement in image quality, because the encircled energy performance within a 0.1" radius has improved from 18% to

80% within the same area.

Figure 4.3: Baseline Overall Quantum Efficiency.

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