Using notch filter masks for high-contrast imaging of extra solar planets

Abstract

We present the first lab experiments using a notch filter mask, a coronagraphic mask that dims the light from an on-axis source while passing off-axis light unscathed. The notch filter mask is essentially an optimized Lyot coronagraph that diffracts all of the light from the central object into a small ring that can be blocked by a Lyot stop. Notch filter masks provide a high throughput, very high contrast alternative to traditional Lyot coronagraphs. These masks, like all methods for achieving high contrast, require a high amount of accuracy in design to be successful. Nanofabrication techniques can meet these design challenges; with the first notch filter mask prototype fabricated with .25 μm precision using an e-beam lithography machine. When placed in a test bed, initial results show that 10-5 contrast is achieved at 3λ/Δ and 10-6 at ~8λ/Δ with a throughput of 27%. The coronagraph rejects light from the point source's peak by at least 4 orders of magnitude despite leakage of light through the mask. We speculate on the as-is performance of such a mask in the Hubble Space Telescope.