Fredrick M. Cady

Towards true imaging by wideband speckle interferometry

Abstract Taking a sequence of speckle images of an astronomical object, the brightest point in each image is shifted to the centre of image space and all images are superimposed. This process, which has been simulated under severe seeing conditions in an optical laboratory, is found to resolve the true image nearly to the diffraction limit, even when the speckle images are formed in white light. It is suggested that the process is suitable for imaging faint astronomical objects with large optical telescopes.

Speckle processing gives diffraction-limited true images from severely aberrated instruments

Speckle images of spatially incoherent objects viewed under severe seeing conditions are formed in the optical laboratory (organized for simulations of stellar speckle interferometry). The brightest pixel in each speckle image is shifted to the center of image space, and the translated image is added to all other speckle images that have been similarly processed. A recognizable diffraction-limited version of the true image of the object results, even when the imaging instrument is defocused such that, under perfect seeing conditions, the Airy disk is spread over an area comparable with that covered by a typical speckle image.

Design review of a high-accuracy UV to near-IR scatterometer

A unique broadband scatterometer has been designed, built and tested for NASA Goddard. One use of the instrument will be to accurately measure the BRDF of calibration standards used by shuttle astronauts in experiments to measure atmospheric ozone. BRDF accuracy is better than 1% except for angles of incidence and scatter greater than 80 degrees. The source employs a high intensity xenon arc and programmable monochromator that allows measurements to be made anywhere from 0.23 to 0.9 micrometers over adjustable bandwidths as small as four nanometers. The goniometer allows out-of-plane measurements to be made in either transmission or reflection from horizontal samples.

Design Review Of A Complete Angle Scatter Instrument

An instrument has been designed and built to measure plane of incidence scatter at multiple wavelengths including .6328μm, from reflective, transmissive, specular, diffuse, flat and curved optics. An extensive software package accompanies the instrument and is used both to control the measurement process and to analyze the measurement data. Techniques employed in the design and development of this instrument are described. An error analysis for the measured BSDF is provided and calibration of the instrument is discussed.

BRDF Error Analysis

A Bidirectional reflectance distribution function (BRDF) error analysis is given. Errors in measuring each of the components of BRDF are analyzed and a total RMS error calculation is given. A computerized error analysis allows each of the terms to be tested for their effect on the total error. This shows that different error mechanisms contribute in different measurement regimes.

Reduction Of Instrument Signature In Near Angle Scatter Measurements

The light scattered by optical components is a sensitive indication of component surface and bulk quality. Angular scatter measurement as an indication of component quality has been reported by a number of investigators since the early 1970s [1-4]. Most of these measurements are limited to angles outside a near angle cone centered about the reflected (or transmitted) specular beam. Measurements are more difficult at near angles because the instrument (scatterometer) source optics create scatter near the specular input beam that is superimposed on the sample scatter. This source scatter is often referred to as instrument signature and its presence must be considered when near angle scatter measurements are taken. The minimum measurable angle depends on the difference between instrument signature and sample scatter. Usually these two are of comparable size somewhere in the region of 0.50 to 50 from the specular beam. Inside this limit, sample scatter is referred to as near, small or low angle scatter. Outside this limit, sample scatter is referred to as far, large or high angle scatter. Because the scattered light density increases rapidly as the specular beam is approached, reduction of near angle scatter is a design concern for many modern optical systems. At least four investigators have reported scatter measurements in the near angle region [5-8], using various techniques to reduce or account for instrument signature.

Design review of a multiwavelength, three-dimensional scatterometer

A design review of a five wavelength, laser-based, full reflection hemispherical BRDF instrument is given. A source box design which provides full polarization control of incident radiation is described. The receiver design uses a periscope mirror which allows near (< 1 .5°) retroreflection measurements with full received polarization control. Replacement of the periscope mirror with a beam splitter pellicle allows 0° retroreflection measurements. The goniometer design shown allows the measurement of full reflection hemisphere scatter with transmission measurements possible. A new software control approach which allows a user to define scans in scatter space without being concerned with mechanical axes motion is described.

Measurement Of Low Angle Scatter

Abstract. A low angle scatter instrument (LASI) has been designed and built at Montana State University under contract to the Los Alamos National Labora-tory. The instrument is capable of measuring scatter at 0.1 ° from the specular beam on most samples and at 0.01 on high scatter samples. Samples may be either reflective or transmissive components. LASI consists of a small aperture detector (100 Am nominal) that is scanned through a focused beam under computer control. The system measures light intensity over approximately nine orders of magnitude. Because the instrument produces some scatter, scans are made with and without the sample present. Following a normalization process, the two data sets are compared to determine the scatter due to the sample alone. Extensive software is used to control the data-taking process, analyze the data, and present the results in graphical form. In this paper a review of the instrument is presented, rather than an extensive survey of scattering data. The basic design problems involved in making these types of instruments are discussed, and procedures for taking measurements are given. We conclude that the major constraint on this type of measurement is scatter from the instrument itself. The difficulties involved in obtaining optics with small amounts of low angle scatter emphasize the need for this type of instrumentation.

Separation And Measurement Of Surface Scatter And Volume Scatter From Transparent Optics

A prototype instrument has been developed that separates light scattered from transparent optics into light scattered from the surface and light scattered from the bulk. The instrument also calculates the loss coefficient for transparent materials. The method has been combined with a raster scanning technique to produce a sensitive method of detecting component contamination in the presence of surface scatter.

Speckle Processing, Shift-And-Add, And Compensating For Instrument Aberrations

Abstract. The formation of speckle images and the more practically important aspects of their statistics are reviewed. The ways in which fixed aberrations can be absorbed within randomly fluctuating ones are detailed. The processing methods of Labeyrie and Knox-Thompson are briefly described. The shift-and-add principle and its extensions are treated in some detail, and the results of optical laboratory simulations are presented. Experimental precautions found to be particularly important are emphasized.