What contributes to the vignetting?
From an internal MPE memo by B. Aschenbach: vignetting function defined as ratio of off-axis collecting area to on-axis collecting area versus off-axis angle. The contributors to the vignetting are the mirror assembly, the magnetic deflector, the focal plane housing apertures, and the efficiency of the focal plane detectors as a function of off-axis angle. The contributions are understood for the mirror assembly (by ray tracing and ground calibration), the magnetic deflector (which affects the focal plane image for off-axis angles greater than 5 arcmin, at which point the deflector support structure is no longer in the shadow of the mirror/window support struts), and the focal plane apertures (which should be very small, as they were designed not to block photons for off-axis angles as large as 60 arcmin).
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