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We describe the commissioning of a new electron–ion coincidence spectrometer used to diagnose the photon beam from a plane grating monochromator beamline at the ASTRID2 synchrotron radiation facility. The spectrometer allows determination of the absolute photon intensity by calibration to the photoabsorption cross sections of known gases, such as the rare gases Ar, Kr, and Xe presented here. The spectrometer operates at very low pressure (∼10−8–10−9 mbar) and—due to the coincidence electron–ion detection scheme—the detector efficiencies can be determined routinely; hence, the spectrometer can be recalibrated swiftly. By variation of a single potential of the spectrometer, the content of higher order radiation in the monochromatized synchrotron radiation can be analyzed. The layout and operation of the synchrotron radiation beamline at ASTRID2 and its additional photon diagnostic units are additionally described. [ABSTRACT FROM AUTHOR]