Showing papers on "Speckle imaging published in 1990"

Deconvolution from wave-front sensing: a new technique for compensating turbulence-degraded images

Abstract: A new technique of high-resolution imaging through atmospheric turbulence is described. As in speckle interferometry, short-exposure images are recorded, but in addition the associated wave fronts are measured by a Hartmann–Shack wave-front sensor. The wave front is used to calculate the point-spread function. The object is then estimated from the correlation of images and point-spread functions by a deconvolution process. An experimental setup is described, and the first laboratory results, which prove the capabilities of the method, are presented. A signal-to-noise-ratio calculation, permitting a first comparison with the speckle interferometry, is also presented.

Three-dimensional correlation coefficient of speckle intensity for rectangular and circular apertures

Abstract: The average three-dimensional size of speckles produced by the illumination of optically rough surfaces with coherent laser light is determined from the autocorrelation function of intensity in space. Analytical and numerical results are given for rectangular and circular diffuser apertures. Important special cases are discussed and compared with available data from the literature. Several of the derived properties of the speckle patterns are stated for the first time and could lead to a greater accuracy of different speckle-interferometric measurement techniques.

Contouring by electronic speckle pattern interferometry employing dual beam illumination

Abstract: In this paper we extend and study the method for generating contours of diffuse objects employing a dual beam illumination coupled with electronic speckle pattern interferometry. The sensitivity and the orientation of the contour planes are analyzed. A novel method for tilting the planes of contours and experimental results incorporating phase shifting and fringe analysis are also presented. The theoretical and the experimental results show good agreement.

Triple-correlation subplane reconstruction of photon-address stellar images.

Abstract: The performance of a photon-address, subplane implementation of the triple-correlation (TC) algorithm is evaluated for application to near-real-time, stellar speckle imaging at low-light levels. A simple least-squares relaxation algorithm for recovering object phase from the bispectrum is proposed and found to be consistently better than the usual recursive method. Photon-address speckle data from six simulated objects of different degrees of complexity, and from the binary stars β Del and μ Ori, were used in this study. For real-time applications for which computational efficiency is critical, the relaxed two-plane TC algorithm offers excellent performance and rugged-ness with respect to object complexity.

The rotating envelope of the hot star Gamma Cassiopeiae resolved by optical interferometry.

Abstract: CERTAIN hot stars, belonging to the Be class, may have an envelope of hydrogen gas, possibly in the form of a rotating disk1 or spheroidal shell2. Other models involve elliptical rings3 or close binary systems where the Roche lobe of the companion is filled with hydrogen2. The angular size of these features is too small for direct detection by conventional telescopes, and attempts to resolve the structure using speckle interferometry (a technique that restores the diffraction-limited resolution otherwise degraded or spoiled by the atmosphere) have failed. The various models of the structure of the hydrogen envelope are based on spectroscopic data, together with polarization and variability measurements. After a century of spectroscopic observations, which showed considerable but little-understood variations, the hydrogen envelope of the star gamma Cassiopeiae was angularly resolved by the prototype interferometer I2T in 1986 (ref. 4). Here we report observations from its successor GI2T, which show further high-resolution details of the hydrogen envelope. The data clearly show the envelope in rotation and approximately fit a disk model. Thus, the GI2T yields optical information capable of constraining astrophysical models on a milliarcsecond scale.

Imaging of coherently illuminated objects through turbulence: plane-wave illumination

Abstract: The average spectrum and energy spectrum of the short-exposure image intensity are evaluated for the case of an object illuminated by a monochromatic plane wave and viewed through atmospheric turbulence, assuming a complex Gaussian model for the turbulence. In general, the average image energy spectrum contains diffraction-limited information about the complex amplitude reflectivity of the object, but, unlike in the case of incoherent imaging, it is generally not possible to extract this information in a straightforward manner. This is illustrated by a computer simulation example of imaging a double-point object in incoherent and coherent illumination. When the object is optically rough and it is physically possible to average over an object ensemble (e.g., the object rotates slightly), then the image energy spectrum is more simply related to the object energy spectrum. The possibilities for diffraction-limited object reconstruction are discussed, and it is pointed out that the image bispectrum does not provide diffraction-limited imaging, in contrast to the case of incoherent imaging.

High resolution telescope including an array of elemental telescopes aligned along a common axis and supported on a space frame with a pivot at its geometric center

Abstract: A large effective-aperture, low-cost optical telescope with diffraction-limited resolution enables ground-based observation of near-earth space objects. The telescope has a non-redundant, thinned-aperture array in a center-mount, single-structure space frame. It employes speckle interferometric imaging to achieve diffraction-limited resolution. The signal-to-noise ratio problem is mitigated by moving the wavelength of operation to the near-IR, and the image is sensed by a Silicon CCD. The steerable, single-structure array presents a constant pupil. The center-mount, radar-like mount enables low-earth orbit space objects to be tracked as well as increases stiffness of the space frame. In the preferred embodiment, the array has elemental telescopes with subaperture of 2.1 m in a circle-of-nine configuration. The telescope array has an effective aperture of 12 m which provides a diffraction-limited resolution of 0.02 arc seconds. Pathlength matching of the telescope array is maintained by a electro-optical system employing laser metrology. Speckle imaging relaxes pathlength matching tolerance by one order of magnitude as compared to phased arrays. Many features of the telescope contribute to substantial reduction in costs. These include eliminating the conventional protective dome and reducing on-site construction activities. The cost of the telescope scales with the first power of the aperture rather than its third power as in conventional telescopes.

Remarkable algebraic structures of phase-closure imaging and their algorithmic implications in aperture synthesis

Abstract: To reveal some new results of a study concerning phase-closure imaging, first we introduce three key operators: the cophasing operator A, the phase-aberration operator B, and the phase-closure operator C. We then show that the generalized inverses of these operators are equal to their (Hilbert space) adjoints divided by the number of pupil pinholes. This remarkable property, which can be stated in terms of backprojection, plays an essential part in the understanding and the treatment of the inverse problems of aperture synthesis. The notion of backprojection is illustrated in a geometrical manner. As an example of applications we present a new self-calibration algorithm for solving the phase-restoration problem in radio imaging. The solution of this deconvolution problem is obtained without phase unwrapping by means of backprojection mechanisms. The implications of these structures in speckle imaging are also examined. Whenever possible, nonredundant configurations should be preferred. The main developments of our approach concern, in particular, the very-long-baseline array and the interferometric mode of the very large telescope.

Spatial interferometry in optical astronomy

Abstract: A bibliographic guide is presented to publications of spatial interferometry techniques applied to optical astronomy. Listings appear in alphabetical order, by first author, as well as in specific subject categories listed in chronological order, including imaging theory and speckle interferometry, experimental techniques, and observational results of astronomical studies of stars, the Sun, and the solar system.

A simplified system for digital speckle interferometry

Abstract: A video speckle interferometer has been demonstrated which uses a spherical surface of small radius at the centre of the camera aperture to simplify the alignment of the reference beam. In combination with a fast microcomputer, this gives a compact and relatively inexpensive system which is able to display phase-stepped interferograms of good quality with a repetition rate approaching one second. The software for phase stepping and unwrapping has novel features and can be applied to other types of interferometer.

Electronic speckle-pattern interferometry using single-mode fibers and active fringe stabilization.

Abstract: Optical fibers are used to provide object and reference illumination in the context of the electronic speckle-pattern interferometry technique. An all-fiber Michelson interferometer is used, with an electronic servo and a piezoelectric phase modulator, to obtain compensation for environmentally induced phase coupling into the fiber arms.

Modified shift-and-add speckle imaging algorithm

Abstract: A simple but powerful improvement of the shift-and-add method for the restoration of astronomical images is presented. In this modified algorithm, the shift vector and weight of a specklegram are estimated by computing the cross correlation between the specklegram and the current average before shifting the specklegram and updating the average. In this sense, all the information contained in the specklegrams is used for maximizing the coherence during the averaging process. Numerical simulations indicate that the suggested modification widens the applicability of the shift-and-add method to extended objects without the appearance of ghost images.

The multi-telescope telescope: a cost-effective approach to fiber-fed spectroscopy

Abstract: A spectroscopic facility is being developed to extend the Center for High Angular Resolution Astronomy program in speckle interferometry to new areas of stellar astrophysics. The facility is based on a multitelescope telescope (MTT) and a fiber-optic-fed spectrograph. The MTT is a one-meter light-collecting telescope with a primary mirror made up of nine 33.3-cm telescope mirrors. The mirrors will focus light into separate optical fibers, avoiding the loss of light associated with the dead space in conventional fiber bundles. It is estimated that the system will have an overall optical efficiency of about 2.7 percent, corresponding to a limiting magnitude of 9.4 for 1000 sec exposure, S/N = 100, and a resolution of 0.2 A/pixel. The possible applications of the system are discussed, including spectroscopic observations of binaries and nonradial pulsation in Be stars.

Experimental validation of extended image reconstruction using bispectral speckle interferometry

Abstract: Results will be presented from a horizontal path imaging experiment in which we used a 0.5 meter telescope focused on a target located at a range of 1 .2 km. The targets included various extended objects from simple binary letters to extended representations of satellites with grey scale and size variations. Imaging at a center wavelength of 0.7 microns, we found an atmospheric degradation factor of Dir0 = 17, on average. We used a slow read-rate bare CCD detector and thus had to effectively deal with additive noise in the speckle measurements. Our image reconstruction algorithms are based on the use of the complex bispectrum and we have demonstrated diffraction-limited imaging down to light levels approaching a few photons per speckle per resolution area. We have paid careful attention to the effects of additive noise on the reconstruction process and shown that they can be adequately overcome.© (1990) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In-Plane Strain Measurement By Digital Phase Shifting Speckle Interferometry

Abstract: A digital phase shifting speckle interferometric technique for measuring the 3D displacement vector field and the in-plane strain components of a deformed object is presented. The displacement components at each point on the surface of the object are calculated by subtracting the measured phases before and after deformation. Accurate phase measurement in each detector point is achieved by using reference beam phase shifting, fast digitizing of the interference patterns and image processing techniques. The 3D displacement vector can be calculated from measurements of the displacement components along four sensitivity vectors. Application of a special gradient filter enables the determination of the in-plane strain components. Special purpose hardware allows a complete measurement of the phase change modulo 2n on a 512*512 grid every 240 ms. The measuring system and its performance will be discussed and results of measurements of the in-plane strain components in a simple experimental aluminium object are shown. For small loads the estimated inaccuracy amounts to 0.3 listrain r.m.s..

Bispectral-based optimization algorithms for speckle imaging

Abstract: We present results of a horizontal path imaging experiment using a 0.5-m telescope focused on a target 1.2 km away. We consider an extended representation of a satellite with grey scale and size variations. We imaged at 0.7 µm and found an average atmospheric degradation factor of D/ro = 17. We used a slow read-rate, bare CCD detector; thus, we had to deal with additive measurement noise. Our image reconstruction algorithms are based on the complex bispectrum, and we have demonstrated diffraction-limited imaging down to light levels approaching a few photons per speckle per resolution area. We have paid careful attention to the effects of additive noise on the reconstruction process and have shown that they can be adequately overcome. We present some new algorithms based on conjugate gradients and least squares. We demonstrate that these algorithms can improve image quality.

High time-resolution imaging with the MAMA detector systems

Abstract: Current uses of the MAMA detector which utilize the photon time-tagging capabilities of these detectors are reported These applications currently include image stabilization by means of post-processing corrections of platform drift and speckle interferometry The initial results of a sounding rocket experiment to obtain UV images of NGC 6240 and results from speckle interferometry of Neptune's moon Triton are presented

Probability imaging of the bright double star ζ Aqr in the infrared

Abstract: The technique of probability imaging is applied to the bright, well resolved, infrared binary star ζ Aqr. It is shown how the second probability density function (pdf) contains all pertinent information to reconstruct the binary system with no ambiguity. Relations between the second pdf and other techniques (speckle masking technique and fork algorithm) are briefly indicated. An analysis of the behaviour of the second pdf under additive signal-independent nose is undertaken. As expected from the general theory of signal processing, the second pdf of the noise is found to blur the second pdf of the signal by means of a convolution relation.

Simulating speckle interferometry

Abstract: This paper describes a simulator we have developed to model speckle imaging and resconstruction of astronomical objects. The simulator was designed as a tool in the development of new signal and image processing techniques for our high resolution imaging research. It has been found to accurately replicate the speckle imaging process, and can be used to predict experimental results under various environmental conditions. The simulator is described in detail, including the modeling of atmospheric turbulence effects, the generation of speckle images, and the simulation of the telescope and image detection processes.

Composite spectra. Paper 5 : orbital elements for 30 systems.

Abstract: A comprehensive survey of bright composite-spectrum binaries in the northern sky has provided so many radial-velocity data that orbits can now be determined for many objects whose orbits were hitherto unknown or else insecure or actually erroneous. Elements are given for the orbits of 30 such objects, thereby more than doubling the number of composite-spectrum binaries with known orbits

Speckle reduction in synthetic-aperture-radar imagery

Abstract: Speckle appearing in synthetic-aperture-radar images degrades the information contained in these images. Speckle noise can be suppressed by adapted local processing techniques, permitting the definition of statistical parameters inside a small window centered on each pixel of the image. Two processing algorithms are examined; the first one uses the intensity as a variable, and the second one works on a homomorphic transformation of the image intensity. A statistical model for speckle noise that takes into account correlation in multilook imagery has been used to develop these processing algorithms. Several experimental results of processed Seasat-A synthetic-aperture-radar images are discussed.

Image reconstruction from the bispectrum using an iterative algorithm and applications of the method to astronomical objects

Abstract: The atmosphere of the earth restricts the resolution of conventional astrophotography to~1 arc- sec. Much higher resolution can be obtained by interferometric speckle techniques. Bispectrum analysis1-3 (also called speckle masking) of many speckle interferograms (short-exposure photographs; exposure time ~0.05sec) yields diffraction-limited images with, for example, 0.02arcsec resolution for a 5-m telescope. After the first processing steps of speckle masking the object bispectrum O(3)(u,v) is obtained up to the cut-off frequency of the telescope.

Surface and interior stress intensity factor measurement by a random speckle method

Abstract: An embedded random speckle method is developed to measure the displacement, both on the surface and interior, around a crack tip of a four-point bend specimen made of Plexiglas From the displacement measurements in different planes across the thickness, stress intensity factors are evaluated assuming either a plane stress or a plane strain condition The results are compared with theoretical and other experimental values The variation of SIF across the thickness clearly demonstrates the existence of the effect of free surface

Objective prism speckle spectroscopy and wideband projection speckle spectroscopy

Abstract: The angular resolution of conventional astronomical spectroscopy is limited to about 1 aresec by the turbulent atmosphere. Much higher resolution can be obtained by (a) objective prism speckle spectroscopy1, which yields diffraction-limited objective prism spectra, and by (b) wideband projection speckle spectroscopy1, which yields spectrally dispersed 1-dimensional projections of the object, i. e., object/spectrum reconstructions O(x,X) or O(x,y,X).

Recovery of Diffraction-Limited Object Autocorrelations from Astronomical Speckle Interferograms using the CLEAN Algorithm

Abstract: We present a new technique for processing speckle interferometric data is presented, which uses the CLEAN algorithm, originally developed for the removal of the effects of incomplete spatial frequency coverage in aperture synthesis radio maps. Since Labeyrie first noted in 1970 that the autocorrelation of a speckle-gram preserves information up to the diffraction limit of an optical telescope, a number of different techniques have been applied to recover this information, usually by Fourier deconvolution of the average power spectrum of the specklegrams with a similar spectrum for a known point source. An alternative is to deconvolve the average autocorrelation of the specklegrams directly, rather than the power spectrum; this is where CLEAN, which has been shown to be a powerful tool in deconvolution problems, may be used. Also, because of the immunity of CLEAN to gaps in the spatial frequency coverage of the power spectrum, deconvolution is robust under conditions where regions of low signal-to-noise ratio i the raw speckle data effectively introduce such gaps. We find that CLEAN is straightforward to apply, and yields results that exceed the quality of recoveries made using at least one other existing technique. Diffraction-limited and near-diffraction-limited results are presented, using photon-noise-limited specklegrams taken with the Hale 5-m telescope at Palomar under a variety of seeing conditions.

Speckle-interferometric camera for displacement measurements

Abstract: A simple setup of standard optical elements comparable to a shearographic camera can be used to record speckle interferograms with a fast lens. Rigid-body translations of the object are compensated for by a reference mirror attached to the object; the method requires only moderate stability and resolution of the storage medium (film). Interferogram reconstruction is possible with white light. Hence the method unites advances of different holographic and speckle-interferometric setups.