High-resolution bandpass x-ray imaging with crystal reflectors: overcoming geometric aberrations

TL;DR

This paper revisits the imaging of specular reflectors, deriving aberration limits and demonstrating that ellipsoidal crystal geometries enable high-resolution, polychromatic x-ray imaging with reduced aberrations compared to traditional designs.

Contribution

It extends the theoretical understanding of aberration limits in x-ray imaging and introduces ellipsoidal crystal geometries for improved high-magnification, polychromatic x-ray imaging.

Findings

Ellipsoidal crystal imagers outperform toroidal ones in image quality.

Aberration limits depend on aperture size and incidence angle, especially near backscattering.

Polychromatic imaging is feasible with ellipsoidal crystals, reducing higher-order aberrations.

Abstract

The imaging problem of a specular reflector is revisited. Retaining terms through second order in the reflector surface expansion, we derive the form of the aberration-limiting aperture for arbitrary magnification assuming no bandwidth limitations. A permissible relative aperture size of the reflector is limited by a set relative aberration tolerance and scales with the tangent of the central glancing angle of incidence. These limiting aberrations become practically insignificant near backscattering. The results extend to x-ray diffracting crystals in symmetric Bragg geometry shaped as an ellipsoid of revolution. This geometry permits polychromatic imaging for hard x-rays over a bandwidth defined by the accepted range of Bragg angles, thereby suppressing aberrations of higher orders. We assess ellipsoidal crystal imagers using ray tracing simulations for two high-magnification designs with Bragg angles far from and close to backscattering. In both cases the ellipsoidal crystals produce images of higher quality compared to those formed by equivalent toroidal crystal imagers.