Nicholas J. Wooder

Observation of the enhancement of coherence by backscattering through turbulence

Abstract We present results of an experiment in which a plane mirror is coherently illuminated and viewed through a turbulent cell. Using a two-pinhole aperture in the pupil plane for both illumination and viewing, we have obtained time-averaged images containing fringes. This gives qualitative confirmation of theoretical predictions claiming that the enhanced back-scattering effect may be used to image deterministic objects.

USE OF POLARIZATION IN INTERFEROMETRY AFTER DOUBLE PASSAGE THROUGH TURBULENCE

We report what are to our knowledge the first experimental results of coherence enhancement that use polarization to separate coherent and incoherent paths.

Horizontal scintillation detection and ranging C n 2 (z) estimation

The strength of optical turbulence, Cn2(z), 2-3 m above ground level, was measured as a function of distance along a 1.23-km path by the simultaneous capture of the scintillation from two infrared laser sources. The data collected differ in a number of important aspects from the normal vertical scintillation detection and ranging (SCIDAR) data in astronomy. The SCIDAR inversion method for the horizontal path problem is outlined and demonstrated on experimental data collected from three field trials.

Results from SCIDAR experiments

We demonstrate that by using the remote turbulence profiling method of scintillation detection and ranging (SCIDAR) one is able to determine both the optical strength of turbulence and the velocity of the turbulent layers, as a function of altitude. The technique of generalized SCIDAR, which allows low level turbulence to be characterized, is also outlined and demonstrated. Preliminary results obtained from SCIDAR observing runs at La Palma, Canary Is., are discussed.

Use of double passage and spatial filtering in imaging through turbulence

Abstract It has been predicted theoretically that the combination of spatial filtering together with double-passage illumination and viewing of a deterministic object leads to an improvement in image quality. The results of experimental investigations of this effect are presented.

Progress report on sodium laser monitoring for laser guide star adaptive optics

Abstract Laser guide star adaptive optics will be implemented at all major observatories in the next decade. In this paper, we describe an experiment to monitor the short and long timescale variations of the altitude profile of mesospheric sodium. This experiment will provide information that will help in the specification and design of the laser guide star system that we hope to install at the William Herschel Telescope and commission in 2003/4.

Comparison of shift-and-add and bispectrum image reconstruction methods for astronomy in the near infrared

Using the results of photometry applied to speckle image reconstructions of the triple stellar system LHS1070 in the near infrared, we compare quantitatively the performance of shift-and-add and bispectrum image reconstruction methods and find that comparable results can be obtained. It is also found that, within this regime, compensation of the bispectrum transfer function is necessary to prevent telescope aberrations from appearing within bispectrum image reconstructions. The effective point-spread functions of these aberrations have been calculated with phase data obtained from the stellar calibration source.

The region of two-point coherence enhancement after backscattering through turbulence

Experimental observations of the backscattered intensity distribution after double passage through turbulence using a polarisation interferometer are described and discussed. The existence of regions in which there is an enhancement of coherence is demonstrated and explained.

DOUBLE PASSAGE IMAGING OF PARTIALLY-COHERENT OBJECTS THROUGH TURBULENCE - EXPERIMENTAL RESULTS

It has been shown in previous work that double passage imaging combined with spatial filtering leads to an improvement in the quality of the time-averaged image obtained from coherently reflecting objects (mirrors) which are placed in turbulence. In this paper, the results of experimental investigation of the imaging of partially coherent objects are presented. We have demonstrated that double passage imaging leads to an improvement in image quality only for objects whose correlation scale is larger than the coherence radius of the illuminating radiation.

Measurements of scintillation over a 17.55-km horizontal path

An extensive set of measurements of scintillation over a 17.55 km path have been made using point sources at wavelengths of 633 nm and 10.6 micrometers , and using an extended thermal source at 3 - 5 micrometers and 8 - 12 micrometers . The basic data consists of normalized variances, probability histograms and normalized autocorrelation functions of intensity. The main aim was to product a set of data that might be used as inputs to models for scintillation. The measurements, as expected, showed a very large range of observed fluctuations, with a highest recorded normalized variance at 633 nm of approximately equals 34 and an average value of 4.8 (averaged over 130 data sets), with a standard deviation of 4.1. The probability histograms have been fitted using log- normal, exponential, log-normally modulated exponential and K distributions. As a general rule, the log-normal model gives a good fit in a large number of cases. Power spectra and correlations functions were measured and show the expected trends with wavelength, with average correlation times (defined in the text) in the range 10 msec (visible) to 68 msec (CO2).