X-ray characterization of eROSITA mirror shells using out-of-focus images

摘要

Test campaign of mirror shells for eROSITA, to be launched in 2011, has been performed at the PANTER X-ray test facility. The results of the campaign are adequately described in our companion paper. In this paper, we focus on the detailed analysis of intra- and extrafocal images to investigate possible outcome of the out-of-focus measurements in the framework of the eROSITA mirror test programme. The images taken at several out-of-focus positions are ring-shaped, that provide crucial keys to the understanding of the optics performance. With those out-of-focus rings, we can easily examine where the deformation occurs on the shell. Conceivable error causes of the image degradation are examined quantitatively. Properties of the in-focus image can be predicted by an interpolation of the out-of-focus rings, a kind of "Hartmann test". Performing Hartmann test, we can estimate how large portion of the image degradation can be attributed to the out-of-roundness error which is mainly due to fluctuations in taper angles. The reconstructed image using Hartmann test appears similar to the core of actual in-focus image. We also pursue the possibility to evaluate the contributions of slope errors using the width of the radial profiles of the out-of-focus rings. These diagnostic techniques with the out-of-focus measurements should be useful for future measurements, not only for the eROSITA mirror but also for other missions (e.g. XEUS optics), where the images are only available at out-of-focus positions in PANTER facility (i.e. impossible to take in-focus images) because of the significantly long focal lengths.