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 Fried's 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.
Simulations of optical wavefronts propagating through the atmosphere are widely used in the design and testing of
adaptive optics systems. Phase screens are defined by their spatial and temporal statistics. In many applications,
a controlled production of phase is necessary. A linear combination of normalized Zernike polynomials can be
used for the generation of phase screens through the computation of Zernike moments following Kolmogorov
turbulence spectrum. In this paper, a technique for controlled production of normalized phase screens using a
known Fried's parameter, r0 is proposed by taking the advantage of the fact that with increasing radial index (n)
of Zernike polynomials, the spatial frequency increases. We arrived at an empirical relation between the index
interval of Zernike polynomials and r0. At large value of 'n', there is saturation in the minimum achievable r0
value.
Centroiding inaccuracies contribute to most of the wavefront reconstruction error in a Shack Hartmann sensor
based adaptive optics system. These errors primarily occur due to the presence of photon noise, readout noise,
finite background and strong scintillations. Elongation of the spots in the case of large apertures while using Laser
guide star as reference makes the situation further worse. A denoising procedure based on thresholded Zernike
reconstructor and pattern matching is suggested in this paper to largely overcome these problems. Individual
spot pattern images are reconstructed using Zernike polynomials and matched with ideal spot pattern without
distortion to arrive at accurate local centroid positions.
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.
Real-time measurement of effective wind speed helps in continuous monitoring of the temporal bandwidth in
adaptive optics systems. In this paper, we propose a simple and efficient method for estimating wind speed
from wavefront sensor data. A small portion from the center of a wavefront arriving at time 't' is mapped at all
possible positions on phase screens coming later than 't' by a time n × Δtint (Δtint-integration time, n-integer)
to form correlation maps. Wind speed and direction is estimated statistically via the calculation of the vectors
formed by joining the central portion and the position of maximum correlation on the correlation maps. Number
of such realizations to be considered to arrive at an accurate estimate of wind speed was optimized to reduce
computing time and increase accuracy.
A hybrid centroiding technique involving Iteratively Weighted Center of Gravity (IWCoG) algorithm and correlation
technique for a Laser Guide Star (LGS) based Shack Hartmann wavefront sensor is proposed. A simple
method for simulating LGS elongated spots with photon noise and read out noise is demonstrated. The problems
associated with IWCoG are addressed (a) Error saturation is minimized by adding random numbers iteratively
to centroid positions, (b) non uniform convergence of Centroid Estimation Error (CEE) is reduced by using
the hypothesis that the iteration number with maximum correlation between the weighting function and the
actual spot image function is the iteration with minimum error, (c) convergence rate is improved by shifting the
weighting function to the point of maximum intensity in first iteration. The novelty of the algorithm is tested
by comparing with other centroiding algorithms.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.