Comparative study of manufacturing techniques for coronagraphic binary pupil masks: masks on substrates and free-standing masks

TL;DR

This study compares manufacturing methods for binary pupil masks used in coronagraphs, evaluating masks on substrates and free-standing masks through experiments, and finds high-contrast performance suitable for various telescopic applications.

Contribution

It provides a direct comparison of different manufacturing techniques for binary pupil masks, including on substrates and free-standing, with experimental contrast measurements.

Findings

Average contrasts: 8.4×10^{-8} for masks on BK7 glass

Free-standing copper masks achieved 1.2×10^{-7} contrast

No significant correlation between mask properties and contrast

Abstract

We present a comparative study of the manufacture of binary pupil masks for coronagraphic observations of exoplanets. A checkerboard mask design, a type of binary pupil mask design, was adopted, and identical patterns of the same size were used for all the masks in order that we could compare the differences resulting from the different manufacturing methods. The masks on substrates had aluminum checkerboard patterns with thicknesses of 0.1/0.2/0.4/0.8/1.6$\mu$m constructed on substrates of BK7 glass, silicon, and germanium using photolithography and chemical processes. Free-standing masks made of copper and nickel with thicknesses of 2/5/10/20$\mu$m were also realized using photolithography and chemical processes, which included careful release from the substrate used as an intermediate step in the manufacture. Coronagraphic experiments using a visible laser were carried out for all the masks on BK7 glass substrate and the free-standing masks. The average contrasts were 8.4$\times10^{-8}$, 1.2$\times10^{-7}$, and 1.2$\times10^{-7}$ for the masks on BK7 substrates, the free-standing copper masks, and the free-standing nickel masks, respectively. No significant correlation was concluded between the contrast and the mask properties. The high contrast masks have the potential to cover the needs of coronagraphs for both ground-based and space-borne telescopes over a wide wavelength range. Especially, their application to the infrared space telescope, SPICA, is appropriate.