22 July 2016 Revisiting static modulation in pyramid wavefront sensing
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Abstract
The Pyramid Sensor (PS) is based on the Focault knife-edge test, yielding then, in geometrical approximation, only the sign of the wavefront slope. To provide linear measurements of the wavefront slopes the PS relies on a technique known as modulation, which also plays a central role to improve the linear range of the pyramid WFS, very small in the nonmodulated case. In the main PS using modulation so far, this task is achieved by moving optical components in the WFS, increasing the complexity of the system. An attractive idea to simplify the optical and mechanical design of a pyramid WFS is to work without any dynamic modulation.

This concept was only merely described and functionally tested in the framework of MAD, and subsequently, with a holographic diffuser. The latter produce a sort of random distribution of the light coming out from the pupil plane, leading to sort of inefficient modulation, as most of the rays are focused in the central region of the light diffused by such device. The bi-dimensional original grating is, in contrast, producing a well defined deterministic distribution of the light onto a specifically shaped pattern. A crude option has been already discussed as a possibility, and it is here generalized to holographic plates leading to various distribution of lights, including a circle whose diameter would match the required modulation pattern, or more cost effective approaches like the one of a square pattern. These holographic diffusers would exhibit also zero-th and high order patterns and the actual size of the equivalent modulation would be linearly wavelength dependent, leading to colour effects that requires a careful handling in order to properly choose the right amount of equivalent modulation.
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L. Marafatto, R. Ragazzoni, D. Vassallo, M. Bergomi, F. Biondi, J. Farinato, D. Greggio, D. Magrin, V. Viotto, "Revisiting static modulation in pyramid wavefront sensing", Proc. SPIE 9912, Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation II, 99122Q (22 July 2016); doi: 10.1117/12.2232987; https://doi.org/10.1117/12.2232987
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