Vector Zernike wavefront sensor on the Santa Cruz Extreme AO Lab (SEAL) testbed

We present a description of the lab setup and first measurements from the vector-Zernike wavefront sensor (WFS) on the Santa Cruz Extreme AO Lab (SEAL) testbed. A goal of SEAL is to test and compare high-contrast imaging techniques for large telescopes with segmented primary mirrors. We focus on being a daytime lab for the W. M. Keck Observatory, which has a Zernike WFS already installed. SEAL consists of three deformable mirrors (one segmented and a woofer/tweeter system) and five wavefront sensors (Shack-Hartmann, two Pyramid WFSs, FAST focal plane, and vector-Zernike). The vector-Zernike WFS simultaneously measures phase and amplitude aberrations and is well-suited for phasing segmented primary mirrors as well as general-purpose low-order wavefront sensing. The setup consists of a liquid-crystal mask placed in the focal plane to cause a +/-pi/2 phase offset on the core of the point spread function (PSF), which then interferes with the remaining PSF. The circularly polarized light is split into two intensity pupil images, which are used to reconstruct the wavefront phase and amplitude. We use the segmented deformable mirror, which simulates the Keck segmented primary mirror, to introduce piston errors and correct them using the vector-Zernike WFS.