Advanced coronagraphs

The vortex coronagraph was originally designed for telescopes with clear, circular apertures (Mawet et al. 2005, Foo et al. 2005). In this case, the suppression of a distant on-axis point source is theoretically perfect. However, the contrast performance of a vortex coronagraph is limited on telescopes with non-circular apertures or in the presence of aperture obstructions, such as a secondary mirror, spider support structures, and gaps between mirror segments. To remedy this, we are exploring advanced optical designs that provide improved contrast on telescopes with complicated or arbitrary apertures. Our approach is to introduce optical elements with amplitude and/or phase that is optimized for the shape of the telescope aperture.

Apodizers are semi-transparent optical elements that may be placed in a pupil plane to improve the starlight suppression of a vortex coronagraph. A ring-apodizer (Mawet et al. 2013) provides ideal contrast in the presence of a central obscuration. For more complicated systems, we are exploring the use of real-valued Zernike amplitude pupil functions (Ruane et al. 2015a). Finite sums of Zernike polynomials provide ideal contrast, though the number of polynomials that may be used is limited by the charge of the vortex phase mask. Moreover, complex coefficients yield complicated phase components, which may be difficult to manufacture.

Focal plane phase correctors are also being explored. Improved contrast may be achieved by use of phase-only optical elements that correct the phase in the focal plane. Though, these designs tend to lead to degraded throughput for off-axis sources (Ruane et al. 2015a).

Lyot-plane phase masks modify the on-axis point spread function after the coronagraph to yield a dark hole in the starlight at the image plane where dim companions may be detected (Ruane et al. 2015aRuane et al. 2015b).