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

摘要

We present a comparative study of the manufacture of binary pupil masks forcoronagraphic observations of exoplanets. A checkerboard mask design, a type ofbinary pupil mask design, was adopted, and identical patterns of the same sizewere used for all the masks in order that we could compare the differencesresulting from the different manufacturing methods. The masks on substrates hadaluminum checkerboard patterns with thicknesses of 0.1/0.2/0.4/0.8/1.6$\mu$mconstructed on substrates of BK7 glass, silicon, and germanium usingphotolithography and chemical processes. Free-standing masks made of copper andnickel with thicknesses of 2/5/10/20$\mu$m were also realized usingphotolithography and chemical processes, which included careful release fromthe substrate used as an intermediate step in the manufacture. Coronagraphicexperiments using a visible laser were carried out for all the masks on BK7glass substrate and the free-standing masks. The average contrasts were8.4$\times10^{-8}$, 1.2$\times10^{-7}$, and 1.2$\times10^{-7}$ for the masks onBK7 substrates, the free-standing copper masks, and the free-standing nickelmasks, respectively. No significant correlation was concluded between thecontrast and the mask properties. The high contrast masks have the potential tocover the needs of coronagraphs for both ground-based and space-bornetelescopes over a wide wavelength range. Especially, their application to theinfrared space telescope, SPICA, is appropriate.