The defocused observations of X-IFU
The capabilities of X-IFU will be improved by using the defocusing option offered by the Movable Mirror Assembly (MMA). A defocusing of 35 mm (with respect to the nominal focal length of 12 m) will enable the observations of extremely bright Galactic sources with fluxes up to ~1 Crab, with only limited spectral resolution degradation. In this case the point spread function (PSF) of the telescope will be spread over a large number of pixels so that each pixel receives a small fraction of the overall flux.
Due to the energy dependence of the PSF shape, this mode will generate energy-dependent artefacts increasing with count rate if not analysed properly. I developed a new technique that enables the analysis of defocused observations in which the PSF varies as a function of energy. I applied this method to the Athena/X-IFU observations of bright sources, operated out of focus. I tested the method for different flux levels, exposure times, and spectral shapes.
Read more here: Kammoun et al. (2022)