Akondi Vyas

Performance of Centroiding Algorithms at Low Light Level Conditions in Adaptive Optics

The performance metrics of different centroiding algorithms at low light level conditions were optimized in the case of a Shack Hartmann Sensor (SHS) for efficient performance of the adaptive optics system. For short exposures and low photon flux, the Hartmann spot does not have a Gaussian shape due to the photon noise which follows poissonian statistics. The centroiding estimation error was calculated at different photon levels in the case of changing spot size and shift in the spot using Monte Carlo simulations. This analysis also proves to be helpful in optimizing the SHS specifications at low light levels.

Multilayered temporally evolving phase screens based on statistical interpolation

Simulation of the dynamic effects of atmospheric turbulence assists in understanding, testing and effective implementation of adaptive optics systems. Statistical interpolation technique helps in retaining the spatial turbulence statistics when atmospheric phase screens are required to be moved by non-integer multiples of the grid spacing. We applied statistical interpolation in the simulation of temporally evolving phase screens using the multilayer model of atmospheric turbulence. A comparison of the statistical method with bilinear interpolator and random midpoint displacement method is presented. It is shown that underestimating Frieds parameter (r0) in the interpolation leads to large errors and hence it is appropriate to choose a little larger value of r0 than estimated from the phase screens.

Centroid Detection by Gaussian Pattern Matching in Adaptive Optics

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.

Experimental evaluation of centroiding algorithms at different light intensity and noise levels

In this paper, we present results from an experimental evaluation of centroiding algorithms, simple center of gravity (CoG) and weighted center of gravity (WCoG) in a Shack Hartmann wavefront sensor at different light levels and background noise conditions. CoG performs better than WCoG at good light conditions. WCoG performs better than CoG algorithm at low light level conditions and in cases of high intensity.

Efficient Minimization of Servo Lag Error in Adaptive Optics Using Data Stream Mining

Prediction of the wavefronts helps in reducing the servo lag error in adaptive optics caused by finite time delays (~ 1-5 ms) before wavefront correction. Piecewise linear segmentation based prediction is not suitable in cases where the turbulence statistics of the atmosphere are fluctuating. In this paper, we address this problem by real time control of the prediction parameters through the application of data stream mining on wavefront sensor data obtained in real-time. Numerical experiments suggest that pixel-wise prediction of phase screens and slope extrapolation techniques lead to similar improvement while modal prediction is sensitive to the number of moments used and can yield better results with optimum number of modes.

Dither-based sensor for improved consistency of adaptive optics system

The wavefront reconstruction accuracy in Shack Hartmann sensor based adaptive optics system depends on accurate centroiding and phase estimation from measured slope values. Monte Carlo simulations of vector matrix multiply and Fourier phase estimation methods show fluctuations in the value of wavefront reconstruction accuracy leading to inconsistency. In this paper, it is shown that these fluctuations can be minimized and high wavefront reconstruction accuracy can be maintained consistently by applying a dither signal on the Shack Hartmann lenslet array. The information of the dither signal to be applied can be obtained from the wavefronts of the past.

Intensity Weighted Noise Reduction in MEMS Based Deformable Mirror Images

The frames captured with a compact, high‐resolution, monochrome progressive scan CCD camera, of a continuous facesheet microelectromechanical systems (MEMS) deformable mirror (DM), include unwanted diffractive, background and readout noises; which significantly reduces the quality of imaging in many applications. Processing these images before passing them on to meet later experimental objectives can improve its performance greatly. In this paper, we propose a sequence of steps involving image intensity weighted noise removal and other smoothening techniques to minimize the noise and improve the phase production and correction capabilities of the DM.

Influence Function Measurement of Continuous Membrane Deformable Mirror Actuators Using Shack Hartmann Sensor

The characterization of a continuous membrane deformable mirror (CDM) involves the measurement of influence function, which quantifies the response of individual actuators to application of different voltages. In this paper, we present the results of influence function measurement for a 140‐actuator CDM using a Shack Hartmann sensor (SHS). The measured influence function was fitted using a Gaussian function and also decomposed into Zernike moments. The SHS‐CDM interaction matrix was also computed.

Towards Low Cost turbulence generator for AO testing: Utility, control and stability

We demonstrate and characterize an effective, statistically repeatable atmospheric turbulence generator with the aim of testing a 2m class telescope adaptive optics system in a cost effective manner.