Adaptive optics for high-contrast imaging: pyramid sensor versus spatially filtered Shack—Hartmann sensor

Abstract

A comparison between pyramid‐based and spatially filtered Shack–Hartmann‐based high‐order adaptive optics (AO) is presented in the framework of 8‐m‐class and extremely large telescopes (ELTs). We first show, with end‐to‐end simulations of an 8‐m AO assisted telescope, how each sensor deals with the aliasing error, which may be the dominant error source at high flux. Then, focusing on photon noise error propagation, we study the field dependence of the sensitivity gain provided by the pyramid sensor with respect to the Shack–Hartmann sensor. From this analysis, we investigate, with an analytical model, visible correction on ELTs for detection of Earth‐like exoplanets.

Adaptive Optics for Astronomy

“…The Pyramid WFS, however, offers the flexibility to adjust the modulation amplitude and hence the sensitivity of the sensor to the observing conditions. In the extreme case of no modulation at all, the Pyramid WFS very much behaves like an interferometer and measures the phase directly rather than its slope (Verinaud et al 2005).…”

Planet Formation Instrument for the Thirty Meter Telescope

“…It therefore ignores the increase in the noise propagator for high-order S-H systems due to reconstructing the wavefront. The Zernike phase contrast sensor, the pyramid wave-front sensor (unmodulated) and the four-bin interferometer all act as interferometers and have a nearly flat spectral response to noise, however, the Shack-Hartmann wave-front sensor has a photon noise propagation power spectral density proportional to d -2 f -2 sinc -2 (df), with d being the sub-aperture size [Verinaud 2005]. A reduction of 1.5 in the RMS error of the Shack-Hartmann sensor was observed in certain cases using a weighted center of gravity algorithm [Nicolle 2004].…”

“…It has been demonstrated to be very robust and does not suffer from chromatic effects. With new techniques applied to these wave-front sensors, such as spatial filtering to prevent aliasing [Poyneer 2004] and matched filtering or weighted center of mass algorithms [Nicolle 2004] to reduce photon noise on the centroiding (provided read noise is negligible), ShackHartmann wave-front sensors have been shown to compare favorably to pyramid sensors [Verinaud 2005]. However, the wavefront reconstruction process means that the ShackHartmann sensor always measures low-order modes less effectively than the other sensors described here.…”

Two-sided pyramid wavefront sensor in the direct phase mode

“…They can in principle be used in slope mode to bootstrap to a high Strehl ratio regime and then operate in a direct phase sensing mode, which has a much lower wavefront reconstruction error for low photon fluxes. The pyramid wave-front sensor has been compared in both regimes to the spatially filtered Shack-Hartmann wavefront sensor 4 . The pyramid wave-front sensor in the direct phase mode acts similarly in many ways to a white-light interferometer, but even though its operation in this mode is interferometric in nature, it acts fundamentally different from familiar interferometers such as the Mach-Zehnder interferometer.…”