The future of high angular resolution x-ray astronomy

The angular resolution of Chandra is close to the practical limit of grazing incidence telescopes due to the difficulty of imparting an accurate figure and smooth surface to mirror substrates whose physical area is over two orders of magnitude larger than their effective area. However, important scientific objectives lie beyond the reach of Chandra and all future missions being planned by the space agencies. By transmitting X-rays diffractive and refractive optics are not subject to the same limitations and have a superior diffraction limit. A Fresnel zone plate can be paired with a refractive lens such that their intrinsic chromatic aberrations cancel to 1st order at a specific energy. The result is a limited but significant energy band where the resolution is a milli arc second or better, for example, at 6 keV. Chromatic aberration can be corrected to 2nd order by separating the diffractive and refractive elements. This configuration allows a resolution of a few micro arc seconds. The optics are very light weight but have extremely long focal lengths resulting in a requirement for very long distance formation flying between optics and detector spacecraft, and small fields of view. Opacity of the refractive element imposes a lower limit upon the X-ray energy of about a few keV.