Grazing incidence and multilayer X-ray optical systems

The development of X-ray optics for astrophysical investigations in the 40-100 keV energy range is extremely important. In this energy range, a focusing system is necessary to resolve crowded regions, to improve sensitivity, and to provide the deep sky images necessary to make the next great step forward in this field. Such a step was ably demonstrated by the Einstein and ROSAT observatories. These systems used gracing incidence optics, and, as is well known, the critical angle of reflectivity decreases linearly with energy for ordinary metal surfaces which adversely impacts on the design of a focusing system for higher energy X-rays. At least 3 parameters are negatively affected: (a) the field of view is decreased; (b) the projected area of an individual mirror element is decreased; and, (c) the fecal length for a fixed diameter system is increased. In order to counter these effects, mirrors coated with multilayers have been designed. It is theoretically possible to increase the grazing angle by coating the mirror surface with a graded d-spacing. The ability to produce a coated mirror with close to theoretical performance is, however, technically challenging. We describe our approach to the fabrication of a system designed for the 40-100 keV range that is based on electroforming technology. We also describe some of the general considerations that must be taken into account when fabricating a viable mirror.