Improvement of Shack–Hartmann wave-front sensor measurement for extreme adaptive optics

Abstract

The development of high-performance adaptive optics systems requires the optimization of wave-front sensors (WFSs) working in the high-order correction regime. We propose a new method to improve the wave-front slope estimation of a Shack-Hartmann WFS in such a regime. Based on a detailed analysis of the different errors in the slope estimation with a classical centroid and with the new method, the gain in terms of wave-front-sensing accuracy in both the detector and the photon noise regimes is stressed. This improvement is proposed without major system disruption.

Planet Formation Instrument for the Thirty Meter Telescope

“…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]. This algorithm requires a larger number of CCD pixels per subaperture and so zero read noise detectors are required to take full advantage of this technique.…”

“…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.…”

Shack-Hartmann spot dislocation map determination using an optical flow method

“…However, the results obtained from this technique are not accurate in Hartmann patterns affected by noise, background illumination and spot modulating signals. In the past, several approaches have been proposed to accurately obtain the Hartmann centroids in these adverse situations, such as, the weighted centre of gravity [3], the iteratively weighted centroiding [4] and matched filter approaches [5]. These methods can obtain accurate results in presence of noise, but they are not appropriate in cases where the Hartmann pattern is affected by background illumination and/or a varying spot modulation or contrast signal [6,7].…”

“…This method is robust against strong noise, background illumination and spot modulating signals. Observe that any of the algorithms presented above [3][4][5][6][7] will not obtain accurate wavefront error reconstruction results if some of the detected spot centroids are not accurately located as these methods do not introduce any prior knowledge about the smoothness of the wavefront.…”

Adaptive wavefront control with asynchronous stochastic parallel gradient descent clusters

“…5 With the conventional phaseconjugation-based wavefront control approach, a key bottleneck for these extreme-AO systems is the time delay between the wavefront sensor's data measurements and the completion of control signals computation. Typically, the time required for the wavefront sensor data processing increases proportionally to the square of the number of wavefront corrector (AO mirror) actuators.…”