Line spread function degradation in photon-counting detectors

An investigation of the degradation of the line spread function due to electron scattering in a micro-channel plate intensifier employing a semi-transparent photocathode and proximity focusing is reported. Data obtained using the Anglo-Australian Telescope (AAT) are used to measure emission line broadening and infilling of absorption features in astronomical spectra, due to scattering in the micro-channel plate intensifier. A comparison with a magnetically focused four-stage cascade intensifier is made. Values are obtained with the two intensifiers for equivalent width and residual infilling of a totally saturated absorption feature. The micro-channel plate intensifier is found to scatter into the wings of emission lines a larger fraction of the input signal than the cascade intensifier, amounting to 6.5% of the total signal. A model of the line spread function degradation observed in the micro-channel plate intensifier, based on electron scattering in the input proximity focused lens, is also presented. The modified line spread function obtained from this model is found to be a remarkably good fit to the data obtained at the AAT, and the values for infilling of absorption line features calculated from the model are close to those observed in the spectra. Finally, suggestions are made as to how the design of future micro-channel plate intensifiers can be modified to reduce scattering from the input micro-channel plate.