Showing papers on "Speckle imaging published in 1984"

Estimating Fried’s parameter from a time series of an arbitrary resolved object imaged through atmospheric turbulence

Abstract: A method to obtain an estimate of Fried’s seeing parameter r0 from time series of an arbitrarily shaped, resolved structure that exhibits degradation resulting from atmospheric turbulence is presented. The basic idea is to evaluate the ratio of the observed squared modulus of the average Fourier transform and the observed average power spectrum. The theory of the method is developed, and the influence of noise on the ratio is discussed. The method has been applied to five consecutive time series of observations of solar granulation under different seeing conditions. The power spectra, which are reconstructed with appropriate theoretical modulation transfer functions, converge.

Binary stars unresolved by speckle interferometry. iii.

Abstract: The KPNO's 4-m telescope was used in 1975-1981 to determine the epochs of 1164 speckle observations for 469 unresolved, known or suspected binary stars. The data, presented in tabular form, encompass visual binaries with eccentric orbits, occultation binaries, astrometric binaries, Hyades stars of known or suspected duplicity, and many long period spectroscopic binaries.

Simultaneous measurement of slope and curvature with a three-aperture speckle shearing interferometer.

Abstract: A simple three-aperture speckle shearing arrangement is proposed for simultaneous measurement of slope and curvature of the specimen. The double exposed shearogram of the laser-illuminated specimen yields five distinct diffraction halos when subjected to Fourier filtering. The first-order halos on either side of the central halo give slope fringes of the specimen, together with relatively broad low-visibility curvature fringes. The second-order halos provide pure slope fringes with a sensitivity double that of the slope fringes obtained from the first-order halo.

Electronic speckle pattern interferometry using a CO2 laser

Abstract: The advantages and problems of using electronic speckle pattern interferometry (ESPI) at longer (infra-red) wavelengths are discussed. The adjustment and use of an ESPI set-up with a CO 2 laser and pyroelectric vidicon are described and illustrated by some experimental results.

Use of holographic optical elements in speckle metrology: part 2.

Abstract: Use of a two hololens imaging system in speckle metrology is studied in detail. Theoretical analysis and experimental investigations reveal that an almost diffraction-limited performance in speckle metrology can be achieved over the entire object field that can be imaged by this hololens configuration.

Single-aperture speckle shearing interferometry with a single grating

Abstract: A simple method for measuring the displacement-slope distribution is proposed, and its usefulness is verified experimentally. In this method we use a single-aperture recording system with only one grating. The grating forms a spatial carrier of high spatial frequency inside speckles in the image and also shears the images. The sheared images are doubly recorded on the photoplate before and after deformation of the object. Clear fringes are obtained by filtering this plate. This method gives us fringes indicating the slope distribution. It is verified experimentally that these fringes indicate the slope distribution with sufficient accuracy.

Theory of speckle-correlation measurements using nonlinear detectors

Abstract: A general and explicit formula is derived for the intensity-correlation dependence of the average of an arbitrary function of two partially correlated speckle intensities. The average is expressed as a power series in the intensity correlation with coefficients given by the corresponding average for uncorrelated intensities. Specialized formulas are derived for the autocorrelation of nonlinearly recorded speckle patterns and for real-time speckle correlation with a nonlinearly recorded reference mask. An explicit closed-form formula is obtained for speckle correlation by intensity addition on a nonlinear detector. The theory is applied to analyze exponential and hard-limiting nonlinearities.

The Differential Speckle Interferometer

Abstract: We describe a new technique called "Differential Speckle Interferometry" (DSI) which uses simultaneous narrow band images of astronomical objects to study their structure. Simultaneous specklegrams of red supergiant and giant stars taken in the hydrogen lines and in the nearby continuum allow us to reconstruct the image of the extended chromospheres of these stars at resolutions of 100 nanoradians and better. We describe the instrumentation, analysis techniques, and results related to DSI.© (1984) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Speckle interferometric methods of measuring small out-of-plane displacements

Abstract: We present two new methods for measuring small out-of-plane displacement. These methods are descendents of Leendertz's method in speckle interferometry but provide fringes of good contrast.

Speckle shear interferometry

Abstract: A number of speckle shear methods and their applications to ndt are discussed. Experiments using multiple apertures with custom-made optical elements have shown that in-plane displacement, slope and curvature patterns can be obtained by filtering through appropriate haloes.

Digital Speckle Pattern Interferometry (DSPI) Using A 100X100 Imaging Array

Abstract: Digital speckle pattern interferometry (DSPI) is a variation of electronic speckle pattern interferometry (ESPI). Both methods use the same basic techniques for nondestructive testing, but DSPI processes speckle pattern data digitally in a computer instead of using analog electronics to enhance fringe contrast. Both testing methods are an outgrowth of holography and speckle interferometry. The system described here uses a Reticon 100x100 diode array camera with an integration time of 5 ms, instead of a television camera, coupled to an HP-9836C computer. The use of digital methods provides flexibility in measurement and processing. Results of measuring both static and dynamic object movement show high-contrast fringes with electronic noise at 1/500 of the dynamic range. A new technique for testing vibrating objects has been developed that significantly in-creases fringe visibility. It involves subtracting a reference frame containing only self-interference terms and no cross-interference term from a time-averaged data frame of the object vibration. This reference frame is created by vibrating a reference mirror at a high amplitude while the object is at rest. Using software written in assembly language, the maximum frame rate is 3 processed frames/sec. Trade-offs of using TV systems with analog processing vs diode arrays with digital processing are discussed.

Multiplexing in Speckle Shear Interferometry

Abstract: Split-lens speckle shear interferometry has been used to obtain slope data from loaded members. The information is usually recorded in the form of double-exposure records on a photographic plate, and only one set of data is recorded on a plate. A number of methods for storing multiple data on a single photographic plate and later retrieving them are discussed, and experimental results are presented.

Speckle interferometry of Hipparcos link stars – III

Abstract: A third list of stars is presented which have been tested by speckle interferometry for use in the Hubble Space Telescope link between the Hipparcos astrometric reference frame and the extragalactic VLBI reference frame. Structural information on angular scales of 0.15-1.2 arcsec for 34 Southern Hemisphere stars is reported from observations made with the Imperial College Speckle Interferometer mounted on the Mount Stromlo 1.9-m telescope. Twenty-four percent of the stars (8 out of the 34) show evidence of multiplicity, in agreement with previous observations in this program.

Effect of scintillation noise in heterodyne speckle photogrammetry.

Abstract: Heterodyne interferometry provides an accurate method of measuring specklegram halo fringes for strain analysis; however, when fringe contrast is less than unity, the uncorrelated portions of the fields generate a background scintillation that is a random function of specklegram and detector positions. In practice, this scintillation effect establishes the actual limits on the utility of heterodyne phase measurements, regardless of the accuracy with which they can be made. This effect has been analyzed for heterodyne holography, and these results are applied here to the derivation of a fundamental relationship between image resolution, gage length, strain resolution, and parameters related to fringe contrast and the readout system.

Phase estimation with few photons.

Abstract: A new algorithm is presented for phase tracking faint low-contrast optical fringes using a proper image photon counter. Simulations show that the tenacity of the new algorithm manifestly surpasses what has been conventionally assumed to be the photon statistical limit. Furthermore, the new algorithm is very quick and easy to compute. Results using actual stellar observations renew hopes for speckle imaging at low light levels.

Correlation properties of heterodyned dynamic speckle in a two-lens imaging system and its application to two-dimensional velocity measurements

Abstract: The space–time correlation function and its space- and time-integration effects of dynamic speckle in a two-lens imaging system superimposed upon a coherent laser light are analytically deduced. From considerations of the specific cases, two methods to perform two-dimensional velocity measurements in real time are proposed: a fringe method and a moment method. As a main result, it was found that the moment method has better spatial resolution than the fringe method.

Fringe sharpening and information coupling in speckle shear interferometry.

Abstract: Fringe sharpening is required to obtain a higher accuracy of measurement. Nonlinear recording and consequently filtering at the higher-order halos for image formation give sharpened fringes in the image. Furthermore, if the apertures during recording are properly arranged, cross-frequency orders appear, which when used for filtering provide moire fringes of higher contrast due to intercoupling.

Vibration of flutes studied by electronic speckle pattern interferometry

Abstract: This paper describes how electronic speckle pattern interferometry can be used to observe in real time the wall vibrations of simple flutes excited in realistic playing conditions. Dynamic interferograms were created by driving the phase-modulating mirror by means of the amplified sound signal picked up by a microphone. Some preliminary results are given.

The importance of random phase for image reconstruction from frequency offset Fourier data

Abstract: Motivated by the ability of synthetic aperture radar and related imaging systems to produce images of suprisingly high quality, we consider the problem of reconstructing the magnitude of a complex signal f from samples of the Fourier transform of f located in a small region offset from the origin. It is shown that high quality speckle reconstructions are possible so long as the phase of f is highly random. In this case the quality of the reconstruction is insensitive to the location of the known Fourier data, and edges at all orientations are reproduced equally well. Computer examples are presented demonstrating these attributes.

Speckle interferometry image reconstruction from the Fourier transform phase.

Abstract: A method suggested earlier for the restoration of speckle images from the average phase of the spectrum is described and investigated in detail. Much attention is concentrated on the reconstruction ambiguity that arises because of inexact knowledge of the spectrum phase, uncertainty about the position and the size of the object, and the probable presence of symmetric factors in the spectrum. Several algorithms that permit unambiguous object image reconstruction (to within arbitrary symmetric factors) have been developed. Results of model calculations for simple one- and two-dimensional images are given.

Investigations of High Resolution Imaging through the Earth's Atmosphere Using Speckle Interferometry

Abstract: : Speckle interferometry is a post-detection image processing technique for producing diffraction limited data using large astronomical telescopes in the presence of Earth's turbulent atmosphere which otherwise limits the imaging resolution The Steward Observatory digital video speckle interferometry system consists of subsystems for a) video digitization of intensified, magnified, focal-plane images, b) Fourier transform (or autocorrelation) signal processing, and c) image analysis to produce the final interpreted results Speckle interferometry has now been shown capable of yielding diffraction images for objects as faint as visual magnitude 16 Objectives of present developments are to improve a) the resolution (by using the Multiple Mirror Telescope with its 6 9m optical aperture), b) the accuracy of the derived results (by implementing better photon detection and recording devices), and c) the efficiency and speed of the reduction algorithms which extract the information (by means of high- speed digital signal-processing hardware) The work includes a growing effect in speckle image reconstruction using a combination of amplitude processing, various phase retrieval methods, holographic methods, and shift-and-add methods The techniques have been applied to observations of Earth-orbiting satellites, asteroids, Pluto/Charon, binary stars, bright supergiant stars, active galactic nuclei, and quasi-stellar objects (QSOs) Originator-supplied keywords: Speckle interferometry; Image reconstruction; Optical interferometry, Phased optical arrays, MMT, Diffraction limited imaging, Speckle imaging

Speckle interferometry, speckle masking, speckle spectroscopy and speckle frame selection

Abstract: High-resolution speckle imaging is one of the most fascinating possibilities of a Very Large Telescope (VLT) Various speckle methods can yield a resolution of about 001" with a 8-m to 10-m VLT As exiting as the resolution is the limiting magnitude of the speckle methods The limiting magnitude is extremely seeingdependent The following limiting magnitudes are possible with speckle interferometry, speckle masking and speckle spectroscopy: 4" seeing: limiting magnitude 15 2" seeing: limiting magnitude 175 1" seeing: limiting magnitude 20 (!)