Laboratory testing of coronagraphs for future space telescopes on the Caltech high contrast spectroscopy testbed for segmented telescopes (HCST) (Conference Presentation)

Abstract

Imaging Earth-like exoplanets with future space telescopes will require a coronagraph instrument that is capable of creating a dark zone in the starlight at the image plane that is ten orders of magnitude fainter than the off-axis image of the host star. What is more, the coronagraph must simultaneously provide a stable dark zone and high throughput over the angular separations that correspond to habitable zones around nearby Sun-like stars (~10-100 milliarcseconds). Since the pupils of most large-aperture space telescope architectures are likely to be obstructed by secondary mirrors, spider support structures, and gaps between mirror segments, the coronagraph optics must also be specially tailored to passively suppress starlight diffracted from the obstructions and discontinuities in the telescope pupil. Here, we demonstrate an apodized vortex coronagraph optimized for an off-axis segmented telescope on the new High Contrast Spectroscopy Testbed for Segmented Telescopes (HCST) at Caltech. The coronagraph consists of a microdot apodizer, a liquid crystal vortex phase mask in the focal plane, and a Lyot stop. The microdot apodizer is an AR-coated glass window with 10um gold microdots to be used in reflection around lambda=800nm. We describe the HCST optical system; the apodizer optimization, fabrication, and metrology procedures; and present end-to-end testbed results of the coronagraph coupled with a 32x32 Boston Micromachines deformable mirror for wavefront control. We aim to achieve a dark zone 10^-7 times fainter than the simulated host star over a wavelength range of 800±40nm in Spring 2018. Finally, we will outline future plans to demonstrate coronagraph concepts for centrally obscured telescopes.