Study On Properties Of The Sinusoidal Phase Mask Coronagraph

In the field of detecting extrasolar planets and stars, traditional telescopes can’t be used indirect imaging of extrasolar planets. The main reason is that the planet is much fainter than thenearby star. One efficient approach employs a stellar coronagraph (coronagraph for short).Coronagraph is a kind of optical device which can obtain high contrast image. In a series ofcoronagraphs, the phase mask coronagraph draws much attention because of its simple structure,high image quality, and ultra high angular resolution. So far various phase mask coronagraphshave been proposed.The paper systematically introduces optical system and theoretical derivation of phase maskcoronagraphs. We focus on the design principles of wide-band phase mask coronagraph and firstpropose the sinusoidal phase mask coronagraph. The phase mask has double periods in theangular direction. Each period has alternatively sinusoidal and uniform functions. We not onlypresent the analytical derivation in detail, but also check the performance of sinusoidal phasemask coronagraph through numerical simulation using Matlab2-D Fast Fourier Transform, andcompare it with traditional vortex phase mask coronagraph and four-quadrant phase maskcoronagraph. It is shown that the sinusoidal phase mask gives an extinction of the star light in thedesigned wavelength. With a1.1λ0/d IWA, it gives an ultra high angular resolution. Even moreimportant, we find that this mask has a better performance in chromatism than the others in thevicinity of designed wavelength. Low chromatism property allows coronagraphs to work inbroad bandwidth, and it is the biggest advantage of sinusoidal phase mask coronagraph.Broadband property significantly reduces imaging time of detecting extrasolar planets, andimproves the imaging efficiency and quality.