Centroid Detection by Gaussian Pattern Matching in Adaptive Optics

Abstract

Shack Hartmann wavefront sensor is a two dimensional array of lenslets which is used to detect the incoming phase distorted wavefront through local tilt measurements made by recording the spot pattern near the focal plane. Wavefront reconstruction is performed in two stages -(a) image centroiding to calculate local slopes, (b) formation of the wavefront shape from local slope measurement. Centroiding accuracy contributes to most of the wavefront reconstruction error in Shack Hartmann sensor based adaptive optics system with readout and background noise. It becomes even more difficult in atmospheric adaptive optics case, where scintillation effects may also occur. In this paper we used a denoising technique based on thresholded Zernike reconstructor to minimize the effects due to readout and background noise. At low signal to noise ratio, this denoising technique can be improved further by taking the advantage of the shape of the spot. Assuming a Gaussian pattern for individual spots, it is shown that the centroiding accuracy can be improved in the presence of strong scintillations and background.

Stars’ Centroid Determination Using PSF-Fitting Method

“…So far, the literature [15][16][17][18] has described PSF as useful for processing and enhancing the quality of already obtained images. The novelty of the solution described in this paper is using this method for analysing the images of the guiding star in real time, during acquisition of long exposure images of astronomical objects.…”

“…Mathematically simplest methods are based on computing the centre of mass [11] and the weighted centre of mass [12][13][14] of a star, respectively. A Gaussian pattern matching [15] is also used in the centroid detection. Very popular centroid detection algorithms employ turbulence prediction methods adopting Zernike polynomials [16] and a principal component analysis with Kalman filtering [17].…”

Advanced Methods for Improving the Efficiency of a Shack Hartmann Wavefront Sensor

“…In this process, the features that do not follow a Gaussian like structure are eliminated. This technique is more suitable to use along with the CoG and IWC methods (Vyas et al, 2010a).…”

“…In the case of NGS, a Gaussian weighting function is assumed and for the case of LGS, the weighting function used is a standard reference of the spot pattern without noise, by assuming the most probabilistic simple sodium layer profile. The thresholded Zernike reconstructor based centroiding has been numerically tested for the case of NGS and LGS based sensing (Vyas et al, 2010a;.…”

An efficient algorithm for determining positions of astronomical objects in the Deep Sky Object pictures

“…The simplest consists in determining the centers of gravity of stars [8] and its derivatives such as weighted centers of gravity, iteratively weighted centers of gravity or iterative weighted centroid [9][10][11]. Gaussian pattern matching is also used [12]. These methods are also preceded by calculations predicting the atmospheric turbulence (temperature oscillations and the influence of wind) by the Zernike polynomials [13], or Principal Component Analysis with Kalman filtering [14].…”