Diffractive-refractive X-ray optics for very high angular resolution X-ray astronomy

The 0.5 arcsec angular resolution of the Chandra X-ray Observatory is possibly the best that a grazing incidence telescope with substantial collecting area will ever attain. We describe a telescope consisting of a diffractive and refractive doublet that transmits rather than reflects X-rays. Therefore, its angular resolution is relatively insensitive to figure errors and surface roughness, the factors that limit grazing incidence telescopes. When the focal length of the refractive component is minus twice that of the diffractive the chromatic aberration that is inherent in the diffractive and refractive components individually will compensate each other within a limited but not insignificant energy band. We consider a system with a long focal length, about 5000 km, to allow the refractive lens to be thin and transmit X-rays. The long focal length requires very long distance formation flying between an optics spacecraft and a detector spacecraft that must be aligned to better than a few centimeters. Solar orbit is a likely site. We simulate an imaging system where chromatic aberration is corrected to first order at 6 keV. The angular resolution is expected to be better than 0.3 miliarcsec within a 10% bandwidth. The components are lightweight, and can probably be made in a machine shop.