Upgrading the MMT AO system with a Near-Infrared Pyramid Wavefront Sensor

There are long existing limitations of the sky coverage of astronomical Adaptive Optics (AO) systems that use natural guide stars (NGSes) as reference sources. In this work, we present numerical simulations and lab test results of an optical NGS double roof prisms wavefront sensor and an upgrade plan for the near-infrared (NIR) pyramid wavefront sensor (PWFS) MMT AO system covering the wavelength range from 0.85-1.8 mu m. The potential increase of sky coverage benefits from the gain in sensitivity of the PWFS in a system compared with the optical Shack-Hartmann wavefront sensor (SHWFS), using IR avalanche photodiode (APD) array with extremely low readout noise (at sub-electron level) at a high frame rate (over 1kHz). This upgraded system will access a larger portion of the sky by looking at fainter, redder reference stars. We use AO simulations to show the expected limiting magnitude gain of NIR PWFS compared with the existing optical SHWFS. The sky coverage will increase by 11 times at the Galactic plane and by 6 times at the North Galactic Pole when compared to traditional optical WFSes. This novel WFS will also enable observations of the dust obscured plane of the Galaxy, where the optical light of most stars is more extincted. Due to the difficulties of the manufacture of pyramid prisms, we demonstrate an optical lab test with a set of double roof prisms. We will upgrade to SAPHIRA where the set of double roof prisms would be used for NIR detection and an achromatic pyramid prism for future lab tests. We evaluate the overall performance of the PWFS on our lab AO bench, present captured micro-pupil images and demonstrate the wavefront reconstruction. We plan to implement this system at MMT and carry out on-sky tests in 2019.