The point-spread function of fiber-coupled area detectors

The point‐spread function (PSF) of a fiber‐optic taper‐coupled CCD area detector was measured over five decades of intensity using a 20 µm X‐ray beam and ∼2000‐fold averaging. The `tails' of the PSF clearly revealed that it is neither Gaussian nor Lorentzian, but instead resembles the solid angle subtended by a pixel at a point source of light held a small distance (∼27 µm) above the pixel plane. This converges to an inverse cube law far from the beam impact point. Further analysis revealed that the tails are dominated by the fiber‐optic taper, with negligible contribution from the phosphor, suggesting that the PSF of all fiber‐coupled CCD‐type detectors is best described as a Moffat function.