Light-trap design using multiple reflections and solid-angle attenuation: application to a spaceborne electron spectrometer.

The design and performance of a new light trap for a spaceborne electron spectrometer are described. The light trap has a measured photon-rejection ratio of 2 x 10(-11), allowing only one in 5 x 10(10) incident photons to reach the sensitive area of the instrument. This rejection is more than sufficient because the ambient ultraviolet in Earth orbit requires a rejection no better than 10(-8) to maintain the photon interference to less than 10 count/s. The light trap uses triple reflections to keep most of the light passing through the entrance slit away from the sensitive area of the spectrometer. However, because of electron-optic requirements, the edge of one of the metallic electrodes falls within the field of view of the sensitive area, allowing double-reflection photon paths to reach the sensitive area. Assuming diffuse reflectance r, the author shows that the photon rejection can be written as epsilon = G(2)r(2) + G(3)r(3) with G(3) approximately 10 times larger than G(2). Both coefficients depend only on the internal surface geometry; G(2) represents the electrode edge reflections (second-order) and G(3) represents the triple-reflection (third-order) paths. As shown by the analysis and measurements taken at two different values of r, the rejection is controlled by triple reflections if r > 0.08. It is shown that the average reflectance of all the internal surfaces must be less than 0.006, which is consistent with the data on the black coating applied to all surfaces. The analysis makes it possible to compare the photon contributions of each of the internal reflecting areas and to estimate the effective scattering width of the metallic electrode edge.