Towards understanding interactions between the AO system and segment co-phasing with the vector-Zernike wavefront sensor on Keck
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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.
本研究利用矢量泽尼克波前传感器（vector-Zernike wavefront sensor, vZWFS），在凯克望远镜上首次实现了主镜分段闭环控制，并将NIRC2科学相机的斯特列尔比最高提升10个百分点。分段共相误差是制约凯克望远镜对比度极限的主要因素，同时也是分段式极大望远镜与未来航天任务中必须校正的误差源。凯克望远镜上的自适应光学后置vZWFS系统的设计目标，是在开展科学观测的同时，并行监测并校正分段共相误差。vZWFS是测量相位不连续性的理想方案，也是灵敏度最高的波前传感器（wavefront sensor, WFS）之一，但其动态范围有限。本研究中的凯克vZWFS采用超表面掩模对正交偏振光施加两种不同的相移，并将光束分为两个瞳面图像，从而拓展了其动态范围。本研究报道了vZWFS的闭环共相性能，以及为厘清自适应光学系统与分段共相之间相互作用所开展的初步研究工作。本研究还对比了两种望远镜共相方案的自适应光学性能：一种是当前凯克望远镜采用的通过对齐分段边缘实现共相的方案，另一种是vZWFS采用的基于分段平均相位的共相方案。本研究将结合高对比度成像应用场景对实验结果展开讨论。
