Primary Mirror Phasing with a Zernike Wavefront Sensor
The designs of a Shack-Hartmann (SHWFS) and unmodulated Pyramid Wavefront Sensor (PyWFS) lay the groundwork of the Hybrid Wavefront Sensor (HyWFS), which combines the components and strengths of both. Although both WFSs can exist separately on one bench, photon noise becomes the limiting factor when the light is split between two sensing arms and detectors. Instead, the HyWFS combines the optical components of a SHWFS and PyWFS into a single static system which produces two wavefront estimations from one image.
The optical design of the HyWFS is straightforward, using only two unique optics, a pyramid prism in the focal plane and lenslet array in the pupil plane. A typical SHWFS only measures the relative motion of spot images, while the PyWFS uses intensity maps to calculate wavefront error. Using both of these techniques with an unresolved source gives the HyWFS a unique advantage of producing two measurements for every single collected image. The final estimation is chosen strategically to yield a WFS which is highly sensitive and accurate over a large dynamic range. The layout of the HyWFS testbench is shown below.
The main advantage of the HyWFS lies in producing two wavefront estimations from a single image. Each estimation is based off typical reconstruction processes from a SHWFS and PyWFS. Both estimations require an adjustment to the HyWFS image to appear more like a conventional SHWFS or PyWFS image. By doing this, well established reconstruction and wavefront estimation techniques can be used for each mode. The final HyWFS estimation is chosen based on the overall Strehl ratio of the aberrated image.
To gain an understanding of the HyWFS response to varying strengths of turbulence, multiple simulated and bench tests have been conducted. Turbulence of the incident wavefront is constantly varying in amplitude and shape. The test shown to the right describes the strengths of the HyWFS's range, linearity, and sensitivity when used on-sky. The applied phase was scaled until the variance of the aberration was between 0.01 to 1 radian. Photon noise was present in the form of a magnitude 9.1 guide star, this value was not varied.
Results from these experiments follow the theory of the two mode response. The SHWFS mode minimizes the residual WFE present at high amplitudes of applied phase aberrations, above the PyWFS mode saturation point. Below this point, the residual WFE from the PyWFS mode correction averages an order of magnitude smaller. Switching between the modes yields a highly accurate wavefront estimation below the PyWFS mode saturation point, this will remain linear in the presence of large amplitudes of WFE.
Related Publications
Chambouleyron et al., Reconstruction methods for the phase-shifted Zernike wavefront sensor, SPIE, 2024
Salama, Guthery, et al., Towards understanding interactions between the AO system and segment co-phasing with the vector-Zernike wavefront sensor on Keck, SPIE, 2024
Salama, Guthery, et al., Keck Primary Mirror Closed-loop Segment Control Using a Vector-Zernike Wavefront Sensor, JATIS, 2024
Chambouleyron, Salama, Guthery, et al., Gemini planet imager 2.0: implementing a Zernike wavefront sensor for non-common path aberrations measurement, AO4ELT7, 2023
Wallace et al., Architecting, Implementing and Observing with a Metasurface vector Zernike wavefront sensor on the Keck Telescope, AO4ELT7, 2023
Team (Current)
Dr. Maïssa Salama, Post-doc, University of California - Santa Cruz
Dr. Vincent Chambouleyron, Post-doc, University of California- Santa Cruz
Dr. Mahawa Cisse, Post-doc, W. M. Keck Observatory
Dr. Charlotte Guthery, AO Scientist, W.M. Keck Observatory
Prof. Becky Jensen-Clem, Advisor, University of California - Santa Cruz
Dr. Jacques Delorme, AO Scientist, W. M. Keck Observatory
Dr. Maaike van Kooten, AO Scientist, Herzberg Astronomy and Astrophysics Research Centre