Novel Methods in Computational Imaging with Applications in Remote Sensing

This dissertation is devoted to novel computational imaging methods with applications in remote sensing. Computational imaging methods are applied to three distinct applications including imaging and detection of buried explosive hazards utilizing array radar, high resolution imaging of satellites in geosynchronous orbit utilizing optical hypertelescope arrays, and characterization of atmospheric turbulence through multi-frame blind deconvolution utilizing conventional optical digital sensors. The first application considered utilizes a radar array employed as a forward looking ground penetrating radar system with applications in explosive hazard detection. A penalized least squares technique with sparsity-inducing regularization is applied to produce imagery, which is consistent with the expectation that objects are sparsely populated but extended with respect to the pixel grid. Additionally, a series of pre-processing steps is demonstrated which result in a greatly reduced data size and computational cost. Demonstrations of the approach are provided using experimental data and results are given in terms of signal to background ratio, image resolution, and relative computation time. The second application involves a sparse-aperture telescope array configured as a hypertelescope with applications in long range imaging. The penalized least squares technique with sparsity-inducing regularization is adapted and applied to this very different imaging modality. A comprehensive study of the algorithm tuning parameters is performed and performance is characterized using the Structure Similarity Metric (SSIM) to maximize image quality. Simulated measurements are used to show that imaging performance achieved using the pro- posed algorithm compares favorably in comparison to conventional Richardson-Lucy deconvolution. The third application involves a multi-frame collection from a conventional digital sensor with the primary objective of characterizing the atmospheric turbulence in the medium of propagation. In this application a joint estimate of the image is obtained along with the Zernike coefficients associated with the atmospheric PSF at each frame, and the Fried parameter r0 of the atmosphere. A pair of constraints are applied to a penalized least squares objective function to enforce the theoretical statistics of the set of PSF estimates as a function of r0. Results of the approach are shown with both simulated and experimental data and demonstrate excellent agreement between the estimated r0 values and the known or measured r0 values respectively

Direct imaging with highly diluted apertures. I. Field of view limitations

Future optical interferometric instrumentation mainly relies on the availability of an efficient cophasing system: once available, what has so far postponed the relevance of direct imaging with an interferometer will vanish. This paper focuses on the actual limits of snapshot imaging, inherent to the use of a sparse aperture: the number of telescopes and the geometry of the array impose the maximum extent of the field of view and the complexity of the sources. A second limitation may arise from the beam combination scheme. Comparing already available solutions, we show that the so called hypertelescope mode (or densified pupil) is ideal. By adjusting the direct imaging field of view to the useful field of view offered by the array, the hypertelescope makes an optimal use of the collected photons. It optimizes signal to noise ratio, drastically improves the luminosity of images and makes the interferometer compatible with coronagraphy, without inducing any loss of useful field of view.Comment: 13 pages, 6 figures, accepted for publication in MNRAS. Full-resolution version available at http://www.obs-hp.fr/~lardiere/publi/2006-Lardiere-MNRAS.pd

Hypertelescopes: The Challenge of Direct Imaging at High Resolution

This book is a collection of 19 articles which reflect the courses given at the Collège de France/Summer school “Reconstruction d'images − Applications astrophysiques“ held in Nice and Fréjus, France, from June 18 to 22, 2012. The articles presented in this volume address emerging concepts and methods that are useful in the complex process of improving our knowledge of the celestial objects, including Earth

Formation, Simulation and Restoration of Hypertelescopes Images

This book is a collection of 19 articles which reflect the courses given at the Collège de France/Summer school “Reconstruction d'images − Applications astrophysiques“ held in Nice and Fréjus, France, from June 18 to 22, 2012. The articles presented in this volume address emerging concepts and methods that are useful in the complex process of improving our knowledge of the celestial objects, including Earth

Discretized aperture mapping with a micro-lenses array for interferometric direct imaging

Discretized Aperture Mapping (DAM) appears as an original filtering technique easy to play with existing adaptive optics (AO) systems. In its essential DAM operates as an optical passive filter removing part of the phase residuals in the wavefront without introducing any difficult-to-align component in the Fourier conjugate of the entrance pupil plane. DAM reveals as a new interferometric technique combined with spatial filtering allowing direct imaging over a narrow field of view (FOV). In fact, the entrance pupil of a single telescope is divided into many sub-pupils so that the residual phase in each sub-pupil is filtered up to the DAM cut-off frequency. DAM enables to smooth the small scale wavefront defects which correspond to high spatial frequencies in the pupil plane and to low angular frequencies in the image plane. Close to the AO Nyquist frequency, such pupil plane spatial frequencies are not well measured by the wavefront sensor (WFS) due to aliasing. Once bigger than the AO Nyquist frequency, they are no more measured by the WFS due to the fitting limit responsible for the narrow AO FOV. The corresponding image plane angular frequencies are not transmitted by DAM and are useless to image small FOVs, as stated by interferometry. That is why AO and DAM are complementary assuming that the DAM cut-off frequency is equal to the AO Nyquist frequency. Here we describe the imaging capabilities when DAM is placed downstream an AO system, over a convenient pupil which precedes the scientific detector. We show firstly that the imaging properties are preserved on a narrow FOV allowing direct imaging throughout interferometry. Then we show how the residual pupil plane spatial frequencies bigger than the AO Nyquist one are filtered out, as well as the residual halo in the image is dimmed

Imaging stellar surfaces with intensity interferometry

Context. Intensity interferometry was invented and used by R.Hanbury Brown and R.Q.Twiss in the 1960's to measure stellar angular diameters. Its main advantage over conventional interferometry is that it enables very long baselines and is insensitive to poor seeing. However, because it requires very large light collectors, it was never pursued further. The Cherenkov Telescope Array (CTA) is a new upcoming facility that will detect rapid flashes of optical Cherenkov light induced by extraterrestrial gamma-rays. Its large telescopes could very well be used part-time for intensity interferometry. With its 2 km maximum baseline, it could image surfaces of hot stars at an unprecedented sub-milliarcsecond resolution. Aim. To experimentally simulate intensity interferometry in the laboratory with an array analogous to the planned CTA. Methods. Small pinhole apertures were illuminated by experimentally produced light with appropriate quantum statistics to simulate stars. High-speed single-photon counting avalanche diode detectors mounted on laboratory telescopes made up the array, enabling more than 100 baselines. A digital data processor was used to calculate the spatial coherence of the stars. Results. Intensity interferometry was successfully performed for stars of different sizes and shapes. With all the baselines available, it was possible to reconstruct two-dimensional maps of the spatial coherence required for image restoration. Conclusions. The results experimentally demonstrated the validity and potential of a multi-telescope array similar to the CTA for stellar surface imaging.Stjärnorna på himlen syns vara små eftersom de är avlägsna objekt, solar på enorma avstånd. Den närmaste stjärnan är Alfa Centauri på ett avstånd av 4,4 ljusår, cirka 41 miljon miljoner kilometer. Solen är den enda stjärna vars yta vi kan se i detalj medan andra stjärnor är så avlägsna att de inte ens i de största teleskopen syns som mer än små ljusa prickar. De skarpaste bilder som i dag erhålls av himmelsobjekt fås med så kallade interferometrar. Dessa är anläggningar där flera teleskop kopplas ihop för att bilda ett gemensamt större instrument. Kraftfullast bland dessa är Europeiska Sydobservatoriets interferometer i Chile och dess amerikanska motsvarighet i Kalifornien. Med dessa har man lyckats avbilda ett fåtal stora stjärnor. Någon visade sig inte vara rund utan kraftigt avplattad eftersom den snurrar jättesnabbt kring sin axel. Andra stjärnor kan tänkas ha andra former eller kan bestå av flera stjärnor i omloppsbanor tätt kring varandra. Att se stjärnor som utsträckta objekt kan lära oss mycket om dem men också om vår egen stjärna, solen. De stjärnor som hittills kunnat avbildas är jättestjärnor, mycket större än solen, och det finns tusentals ljusa stjärnor som fortfarande bara kan ses som prickar. Bildskärpan i en interferometer bestäms av avståndet mellan de teleskop som ingår i anläggningen: större avstånd ger bättre skärpa. Fastän man sedan länge drömt om att länka teleskop över många kilometrar, är det ännu inte möjligt över mer än ett par hundra meter. Begränsningarna sätts av kraven på extrem precision i hur ljuset mellan teleskopen måste kombineras, samt av luftoron i jordens atmosfär. En annan teknik, så kallad intensitets-interferometri, tillåter längre avstånd mellan teleskopen och därmed en högre bildskärpa. Metoden innebär att det synliga ljuset i teleskopet omvandlas till elektroniska signaler som överförs i kablar utan att störas av luftens turbulens. Nackdelen är att viss information går förlorad, vilket gör det svårare att återskapa bilder av himmelsobjekten. Dessutom kräver denna teknik mycket ljus och därför också stora teleskop. Genom en historisk tillfällighet uppförs nu en anläggning med sådana stora teleskop, CTA, ”Cherenkov Telescope Array”, för ett helt annat huvudändamål, att observera gammastrålning från världsrymden. När energirika gammastrålar tränger in i jordens atmosfär, skapas partiklar som utsänder blixtar av blåaktigt ljus, så kallad Tjerenkovstrålning. Eftersom denna är mycket ljussvag, måste teleskopen vara både stora och många. Teleskopens prestanda råkar motsvara vad som krävs för intensitets-interferometri och möjligheten till denna tillämpning har uppmärksammats inom projektet. Teleskopen kommer att ligga på avstånd upp till ett par kilometrar vilket möjliggör en bildskärpa som är storleksordningen bättre än med dagens anläggningar. Detta kommer att möjliggöra avbildning av främst stjärnor som är hetare än solen (tekniken fungerar bäst för varmare stjärnor). Möjligen kommer man till och med att kunna se silhuetter av planeter när de syns passera över stjärnskivan! Eftersom tekniken aldrig använts med modern digital elektronik, måste metoderna utvecklas och testas innan observationer i full skala kan påbörjas. Detta är vad som gjorts i detta examensarbete. Många små teleskop sattes upp i ett laboratorium i ett mönster motsvarande det kommande CTA. Med denna installation mättes olika konstgjorda stjärnor. Efter analys av mätningarna, kunde storlek och form på de olika ”stjärnorna” bestämmas och det kunde experimentellt visas att teorin fungerade. Detta är första gången som avbildande intensitets-interferometri genomförts för astronomiskt relevanta objekt. Med denna teknik torde det bli möjligt att erhålla bilder av stjärnytor när CTA kommer i drift någon gång kring år 2020

PAINTER: a spatio-spectral image reconstruction algorithm for optical interferometry

International audienceAstronomical optical interferometers sample the Fourier transform of the intensity distribution of a source at the observation wavelength. Because of rapid perturbations caused by atmospheric turbulence, the phases of the complex Fourier samples (visibilities) cannot be directly exploited. Consequently, specific image reconstruction methods have been devised in the last few decades. Modern polychromatic optical interferometric instruments are now paving the way to multiwave-length imaging. This paper is devoted to the derivation of a spatio-spectral ("3D") image reconstruction algorithm, coined PAINTER (Polychromatic opticAl INTErferometric Reconstruction software). The algorithm relies on an iterative process, which alternates estimation of polychromatic images and of complex visibilities. The complex visibilities are not only estimated from squared moduli and closure phases, but also differential phases, which helps to better constrain the polychromatic reconstruction. Simulations on synthetic data illustrate the efficiency of the algorithm and in particular the relevance of injecting a differential phases model in the reconstruction

Remote-Sensing Image Classification Based on an Improved Probabilistic Neural Network

This paper proposes a hybrid classifier for polarimetric SAR images. The feature sets consist of span image, the H/A/α decomposition, and the GLCM-based texture features. Then, a probabilistic neural network (PNN) was adopted for classification, and a novel algorithm proposed to enhance its performance. Principle component analysis (PCA) was chosen to reduce feature dimensions, random division to reduce the number of neurons, and Brent’s search (BS) to find the optimal bias values. The results on San Francisco and Flevoland sites are compared to that using a 3-layer BPNN to demonstrate the validity of our algorithm in terms of confusion matrix and overall accuracy. In addition, the importance of each improvement of the algorithm was proven

Human habitats: prospects for infrastructure supporting astronomy from the moon

Instrumentatio