  We present the first on-sky segmented primary mirror closed-loop piston
control using a Zernike wavefront sensor (ZWFS) installed on the Keck II
telescope. Segment co-phasing errors are a primary contributor to contrast
limits on Keck and will be necessary to correct for the next generation of
space missions and ground-based extremely large telescopes (ELTs), which will
all have segmented primary mirrors. The goal of the ZWFS installed on Keck is
to monitor and correct primary mirror co-phasing errors in parallel with
science observations. The ZWFS is ideal for measuring phase discontinuities
such as segment co-phasing errors and is one of the most sensitive WFS, but has
limited dynamic range. The vector-ZWFS at Keck works on the adaptive optics
(AO) corrected wavefront and consists of a metasurface focal plane mask which
imposes two different phase shifts on the core of the point spread function
(PSF) to two orthogonal light polarizations, producing two pupil images. This
design extends the dynamic range compared with the scalar ZWFS. The primary
mirror segment pistons were controlled in closed-loop using the ZWFS, improving
the Strehl ratio on the NIRC2 science camera by up to 10 percentage points. We
analyze the performance of the closed-loop tests, the impact on NIRC2 science
data, and discuss the ZWFS measurements.
