Image formation in multilayers optics: The Schwartzchild objective

The construction of efficient X-ray optics has been hampered in the past by the strong aberrations implicit in the use of grazing optics. This is in turn due to the necessity of working below the critical angle for total external reflection. For most materials this (grazing) angle falls in the range of a few degrees. The advent of Layered Synthetic Microstructures (LSM) has opened up new possibilities, as mirrors with high reflectivities at large incidence angles in the soft X-ray region, particularly below the Carbon edge, have been demonstrated. 1:1 imaging with LSM deposited on a curved Silicon substrate has been obtained. Designs based on two or more mirrors, in particular of the Schwartzschild objective type (SO), appear possible and have been already proposed. The proposed use is in a scanning microscope mount, where the X-rays are brought to a fine focus on axis and the sample is scanned across it under computer control. The resulting image is a two dimensional map of either the transmission or of some secondary process. The SO are normally diffraction limited even at wavelengths as short as 50 Angstroms. The authors' discuss in detail some results and its effect on the image formation of the SO, as a test case. It appears that for the range of angles encountered in these objectives the influence of phase shifts is modest and should not degrade the image formation, while the non-uniform absorption on the primary may present serious problems in the case of high-power sources