Towards understanding interactions between the AO system and segment co-phasing with the vector-Zernike wavefront sensor on Keck

We demonstrated the first on-sky primary mirror segment closed-loop control on Keck using a vector-Zernike wavefront sensor (vZWFS) and improved the Strehl ratio on the NIRC2 science camera by up to 10 percentage points. Segment co-phasing errors are a primary contributor to Keck contrast limits and will be necessary to correct for the segmented Extremely Large Telescopes and future space missions. The goal of the post-AO ZWFS on Keck is to monitor and correct segment co-phasing errors in parallel with science observations. The ZWFS is ideal for measuring phase discontinuities and is one of the most sensitive WFSs, but has limited dynamic range. The Keck vZWFS consists of a metasurface mask imposing two different phase shifts to orthogonal polarizations, split into two pupil images, extending its dynamic range. We report on the vZWFS closed-loop co-phasing performance and early work towards understanding the interactions between the AO system and segment phasing. We also discuss a comparison of the AO performance when phasing the telescope by aligning segment edges, as is currently done at Keck, compared aligning to average phase over the segments, as is done by the vZWFS. We discuss the results in the context of high-contrast imaging.