Diffraction-Limited Optics and Techniques

The potential diffraction-limited angular resolution of an X-ray telescope with aperture diameter D = 1 m operating at 1 keV is Δθ ≈ λ∕D = 260 μ arc seconds, but to achieve such an impressive performance requires X-ray optics with unprecedented precision and quality. Fourier optics analysis is used to explore the geometry and accuracy of X-ray optics needed to meet this goal. Conventional grazing incidence X-ray telescopes could, in principle, work at the diffraction limit in the soft X-ray band 0.1–10 keV, but the technology and metrology required to produce the optics doesn’t currently exist. X-ray lenses are viable at energies greater than ∼10 keV, are relatively easy to make, and can provide imaging at a resolution of 3 μ arc seconds or better, but the separation of the lens and detector must be very large, typically ∼105 km. X-ray interferometry has the potential to provide diffraction-limited imaging at ultrahigh angular resolutions of 100 μ arc seconds or better, and currently available technology is good enough to manufacture the mirrors required. However, designing a practical interferometer which fits within a reasonable envelope and that has sufficient collecting area to deliver such a performance is a major challenge.