348 research outputs found
Weak Lensing from Space I: Instrumentation and Survey Strategy
A wide field space-based imaging telescope is necessary to fully exploit the
technique of observing dark matter via weak gravitational lensing. This first
paper in a three part series outlines the survey strategies and relevant
instrumental parameters for such a mission. As a concrete example of hardware
design, we consider the proposed Supernova/Acceleration Probe (SNAP). Using
SNAP engineering models, we quantify the major contributions to this
telescope's Point Spread Function (PSF). These PSF contributions are relevant
to any similar wide field space telescope. We further show that the PSF of SNAP
or a similar telescope will be smaller than current ground-based PSFs, and more
isotropic and stable over time than the PSF of the Hubble Space Telescope. We
outline survey strategies for two different regimes - a ``wide'' 300 square
degree survey and a ``deep'' 15 square degree survey that will accomplish
various weak lensing goals including statistical studies and dark matter
mapping.Comment: 25 pages, 8 figures, 1 table, replaced with Published Versio
Focus-extension by depth-encoded synthetic aperture in Optical Coherence Tomography
We present a novel method to extend the depth-of-focus of Optical Coherence Tomography (OCT). OCT is an interferometric imaging technique that provides depth-resolved scattering information. The axial resolution in OCT is provided by the coherence gate and is invariant over the full image depth. The lateral resolution is determined by the beam parameters such as wavelength and numerical aperture. The Rayleigh range determines the depth range over which the lateral resolution can be maintained. The lateral resolution is often sacrificed to maintain relatively long Rayleigh range. In this study, we propose to use a depth-encoded synthetic aperture detection scheme to extend the depth range over which a sharp focus can be maintained beyond the Rayleigh range. An annular phase plate is inserted into the light path in the sample arm, which gives rise to three separate images in a single B-scan, corresponding to three different optical path length encoded apertures. These three images are coherently summed after phase-manipulation to reconstruct a new image with a lateral resolution that is maintained over a five times larger depth range. © 2013 Optical Society of America
Resonant Adaptive Mirrors
Deformable mirrors (DMs) are integrated into adaptive optical (AO) systems to compensate
for wavefront aberrations. These aberrations degrade the image resolution of telescopes,
microscopes, ophthalmoscopes, and optical coherent tomographs. The objective of
the DM in these applications is to compensate for wavefront aberrations. Continuous and
segmented DMs utilize a variety of mechanisms such as electrostatic, piezoelectric, and
electromagnetic actuation. Micro-electromechanical systems (MEMS) DMs have the advantages
of low cost, low power consumption, and high electrode density. As the electrode
count increases, the possibility of the desired modes corresponding to the Zernike modes
appearing increases. However, the complexity of the static actuation also increases.
In ophthalmology, fth order Zernike modes are used to categorize the aberrations
induced by the human eye. These aberrations would degrade the image resolution of the
retina during laser scanning. Therefore, a dynamically continuous DMs were developed and
actuated at a natural frequency corresponding to the desired Zernike mode. The actuations
would drive the mirror plate to deform into the shape of the desired mode. Multiple
modes corresponding to low- and high-order Zernike modes were obtained. Resonant DMs
exploit the dynamic ampli cation available at natural frequency's in order to reduce voltage
and power requirements. This will also reduce the requirements for spatial control of
individual electrodes' voltage. However, the use of circular mirror plates to create the
electromechanical modes has led to the appearance of degenerate modes (pairs of almost
identical modes with closely spaced frequencies). Electrostatic elds were designed to
separate those modes and help break coupling between them. The elds employ selectively,
actuating some of the electrodes under the DM while grounding the rest. An AC voltage
was applied to selective scheme of electrodes in order to induced mode shapes that are
corresponding to the Zernike modes. This design relies on a new technique which uses
pulsed laser scanning instead of continuous laser scanning.
The proposed DM was designed and fabricated using a Micra-GEM fabrication process.
Simulations using the nite element method (FEM) software COMSOL were used
in order to determine the natural frequencies and mode shapes, and to separate degenerate
modes natural frequencies by applying electrostatic elds that increase the di erence
between them. Characterization of the DM was conducted using laser Doppler vibrometer
to identify the mode shapes and its natural frequencies experimentally. The stroke
measurements of the target DM were shown as a function of frequency and amplitude. In
addition, RMS error measurements were used as a comparison between DM modes and
there corresponding Zernike mode.
The aim of this research was to over come the in
uence function due to mechanical
coupling in the continuous DMs. In
uence function requires di erent voltages that apply
to electrode scheme. Therefore, static actuation of the DMs rely on a complex driving
circuits. Resonant DMs eliminate the e ect of the in
uence function by triggering the
mirror via its natural frequencies. They reduce the number of red electrode scheme by
applying single voltage to the electrodes. As a result, they reduce the complexity of the
driving circuits that require to control its shape. This research requires a new technique of
using a pulsed laser instead of a continuous laser for the proposed DM. This may lead to
manipulation of the optical laser signal using the mirror as a part of the signaling process.
This should be completed by synchronizing the frequencies of both the DM and the laser
to produce a high resolution image of the retina
Modeling and applications of the focus cue in conventional digital cameras
El enfoque en cámaras digitales juega un papel fundamental tanto en la calidad de la imagen como en la percepción del entorno. Esta tesis estudia el enfoque en cámaras digitales convencionales, tales como cámaras de móviles, fotográficas, webcams y similares. Una revisión rigurosa de los conceptos teóricos detras del enfoque en cámaras convencionales muestra que, a pasar de su utilidad, el modelo clásico del thin lens presenta muchas limitaciones para aplicación en diferentes problemas relacionados con el foco. En esta tesis, el focus profile es propuesto como una alternativa a conceptos clásicos como la profundidad de campo. Los nuevos conceptos introducidos en esta tesis son aplicados a diferentes problemas relacionados con el foco, tales como la adquisición eficiente de imágenes, estimación de profundidad, integración de elementos perceptuales y fusión de imágenes. Los resultados experimentales muestran la aplicación exitosa de los modelos propuestos.The focus of digital cameras plays a fundamental role in both the quality of the acquired images and the perception of the imaged scene. This thesis studies the focus cue in conventional cameras with focus control, such as cellphone cameras, photography cameras, webcams and the like. A deep review of the theoretical concepts behind focus in conventional cameras reveals that, despite its usefulness, the widely known thin lens model has several limitations for solving different focus-related problems in computer vision. In order to overcome these limitations, the focus profile model is introduced as an alternative to classic concepts, such as the near and far limits of the depth-of-field. The new concepts introduced in this dissertation are exploited for solving diverse focus-related problems, such as efficient image capture, depth estimation, visual cue integration and image fusion. The results obtained through an exhaustive experimental validation demonstrate the applicability of the proposed models
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