Narrow-band filter consisting of two aplanatic gratings

Abstract

In this paper we describe the basic principle and the resolution properties of a filter consisting of two similar concave gratings having circular grooves with variable spacing. These gratings can be fabricated by both mechanica ruling and holographic techniques. They are capable of producing an aberration-free image of a particular wavelength at the center of curvature when they are illuminated by a polychromatic beam parallel to the axis. Two such gratings having small central holes are mounted with a common axis. The centers of the two gratings are situated near the central holes. An exit pinhole is located at one of the centers of curvature. When the gratings are illuminated by a point source of white light located at the other center of curvature, an aberration-free image of a particular wavelength is formed at the exit pinhole. Although the exit pinhole has a finite diameter, light emerging from it is sufficiently monochromatic with a small bandwidth.