Imaging extrasolar planets by stellar halo suppression in separately-corrected color bands

Extra-solar planets have not been imaged directly with existing ground or space telescopes because they are too faint to be seen against the halo of the nearby bright star. Most techniques being explored to suppress the halo are achromatic, with separate correction of diffraction and wavefront errors. Residual speckle structure may be subtracted by differencing images taken through narrowband filters, but photon noise remains and ultimately limits sensitivity. Here we describe two ways to take advantage of narrow bands to reduce speckle photon flux and to obtain better control of systematic errors. Multiple images are formed in separate color bands of 5-10% bandwidth, and recorded by coronagraphic interferometers equipped with active control of wavefront phase and/or amplitude. In one method, a single deformable pupil mirror is used to actively correct both diffraction and wavefront components of the halo. This yields good diffraction suppression for complex pupil obscuration, with high throughput over half the focal plane. In a second method, the coronagraphic interferometer is used as a second stage after conventional apodization. The halo from uncontrollable residual errors in the pupil mask or wavefront is removed by destructive interference made directly at the detector focal plane with an "anti-halo", synthesized by spatial light modulators in the reference arm of the interferometer. In this way very deep suppression may be achieved by control elements with greatly relaxed, and thus achievable, tolerances. In both examples, systematic errors are minimized because the planet imaging cameras themselves also provide the error sensing data.Comment: Accepted by ApJ

Extragalactic Fields Optimized for Adaptive Optics

In this paper we present the coordinates of 67 55' x 55' patches of sky which have the rare combination of both high stellar surface density (>0.5 arcmin^{-2} with 13<R<16.5 mag) and low extinction (E(B-V)<0.1). These fields are ideal for adaptive-optics based follow-up of extragalactic targets. One region of sky, situated near Baade's Window, contains most of the patches we have identified. Our optimal field, centered at RA: 7h24m3s, Dec: -1deg27'15", has an additional advantage of being accessible from both hemispheres. We propose a figure of merit for quantifying real-world adaptive optics performance, and use this to analyze the performance of multi-conjugate adaptive optics in these fields. We also compare our results to those that would be obtained in existing deep fields. In some cases adaptive optics observations undertaken in the fields given in this paper would be orders of magnitude more efficient than equivalent observations undertaken in existing deep fields.Comment: 28 pages, 15 figures, 1 table; accepted for publication in PAS

Modern optical astronomy: technology and impact of interferometry

The present `state of the art' and the path to future progress in high spatial resolution imaging interferometry is reviewed. The review begins with a treatment of the fundamentals of stellar optical interferometry, the origin, properties, optical effects of turbulence in the Earth's atmosphere, the passive methods that are applied on a single telescope to overcome atmospheric image degradation such as speckle interferometry, and various other techniques. These topics include differential speckle interferometry, speckle spectroscopy and polarimetry, phase diversity, wavefront shearing interferometry, phase-closure methods, dark speckle imaging, as well as the limitations imposed by the detectors on the performance of speckle imaging. A brief account is given of the technological innovation of adaptive-optics (AO) to compensate such atmospheric effects on the image in real time. A major advancement involves the transition from single-aperture to the dilute-aperture interferometry using multiple telescopes. Therefore, the review deals with recent developments involving ground-based, and space-based optical arrays. Emphasis is placed on the problems specific to delay-lines, beam recombination, polarization, dispersion, fringe-tracking, bootstrapping, coherencing and cophasing, and recovery of the visibility functions. The role of AO in enhancing visibilities is also discussed. The applications of interferometry, such as imaging, astrometry, and nulling are described. The mathematical intricacies of the various `post-detection' image-processing techniques are examined critically. The review concludes with a discussion of the astrophysical importance and the perspectives of interferometry.Comment: 65 pages LaTeX file including 23 figures. Reviews of Modern Physics, 2002, to appear in April issu

Effects of lithographic roughness and sidewall slope on the optical performance of polymer rectangular waveguides: Modeling

Fabrication of polymer, multimode, channel waveguides have gained much attention recently because of usefulness in replacing fiber in backplane interconnects. Waveguides are more desirable because they are able to achieve greater bandwidth, they are immune to electromagnetic interference and they consume less power compared to traditional electrical communication links. They are however subject to insertion losses due to fabrication defects such as the waveguide sidewall roughness and sidewall slope. This requires fabrication tolerances that are on the micron level. Determining the acceptable minimum tolerances required in the fabrication process in terms of the overall channel link budget is critical for determining how precise of fabrication methods must be utilized. © 2010 IEEE

Three Hundred and Sixty Degree Real-Time Monitoring of 3-D Printing Using Computer Analysis of Two Camera Views

Prosumer (producing consumer)-based desktop additive manufacturing has been enabled by the recent radical reduction in 3-D printer capital costs created by the open-source release of the self-replicating rapid prototype (RepRap). To continue this success, there have been some efforts to improve reliability, which are either too expensive or lacked automation. A promising method to improve reliability is to use computer vision, although the success rates are still too low for widespread use. To overcome these challenges an open source low-cost reliable real-time optimal monitoring platform for 3-D printing from double cameras is presented here. This error detection system is implemented with low-cost web cameras and covers 360 degrees around the printed object from three different perspectives. The algorithm is developed in Python and run on a Raspberry Pi3 mini-computer to reduce costs. For 3-D printing monitoring in three different perspectives, the systems are tested with four different 3-D object geometries for normal operation and failure modes. This system is tested with two different techniques in the image pre-processing step: SIFT and RANSAC rescale and rectification, and non-rescale and rectification. The error calculations were determined from the horizontal and vertical magnitude methods of 3-D reconstruction images. The non-rescale and rectification technique successfully detects the normal printing and failure state for all models with 100% accuracy, which is better than the single camera set up only. The computation time of the non-rescale and rectification technique is two times faster than the SIFT and RANSAC rescale and rectification technique

Signal-to-Noise Comparison of Deconvolution From Wave Front Sensing With Traditional Linear and Speckle Image Reconstruction

It is well known that atmospheric turbulence severely degrades the performance of ground based imaging systems. Techniques to overcome the effects of the atmosphere have been developing at a rapid pace over the last 10 years. These techniques can be grouped into two broad categories: pre-detection and post detection techniques. A recent newcomer to the post detection scene is &quot;deconvolution from wave front sensing&quot; (DWFS). DWFS is a post-detection image reconstruction technique that makes use of one feature of pre-detection techniques. A WFS is used to record the wave front phase distortion in the pupil of the telescope for each short exposure image. The additional information provided by the WFS is used to estimate of the system&apos;s point spread function (PSF). The PSF is then used in conjunction with the ensemble of short exposure images to obtain an estimate of the object intensity distribution via deconvolution. With the addition of DWFS into the suite of possible post detection imag..

Constrained least-squares estimation in deconvolution from wave-front sensing

We address the optimal processing of astronomical images using the deconvolution from wave-front sensing technique (DWFS). A constrained least-squares (CLS) solution which incorporates ensemble average DWFS data is derived using Lagrange minimization. The new estimator requires DWFS data, noise statistics, OTF statistics, and a constraint. The constraint can be chosen such that the algorithm selects a conventional regularization constant automatically. No ad hoc parameter tuning is necessary. The algorithm uses an iterative Newton-Raphson minimization to determine the optimal Lagrange multiplier. Computer simulation of a 1 m telescope imaging through atmospheric turbulence is used to test the estimation scheme. CLS object estimates are compared with those processed via manual tuning of the regularization constant. The CLS algorithm provides images with comparable resolution and is computationally inexpensive, converging to a solution in less than 10 iterations. © 1998 Elsevier Science B.V. All rights reserved

Methods for modeling multimode waveguides with abrupt changes in propagation axis

Characterization and performance estimates for polymer, step-index, multi-mode, channel waveguides requires simulation of light propagation in models requiring large numbers of spatial samples using the method of finite-difference time-domain methods (FDTD). As noted in [1], full 3-D solutions for multi-mode waveguides with a cross-section of tens of microns are nontrivial. In many instances, designers are forced to use a ray-tracing model to characterize the channel. However, ray-tracing is not intended to characterize spectral and modal properties. Approximation of Helmholtz equation for slowly varying field in the propagation plane, termed beam propagation methods (BPM), has proved efficient for modeling optical waveguides [2-4]; however, these are primarily intended for modeling waveguides with slowly varying axes of propagation. © 2010 IEEE

Reconstruction of turbulence-degraded images using the vector Wiener filter

Adaptive optics and speckle imaging are common methods for improving Fourier domain signal-to-noise ratio (SNR) in astronomical images. These techniques may benefit from linear processing to deconvolve blurring due to the attenuation of high spatial frequencies in the compensated images. Typical linear deconvolution methods require an explicit estimate of the random atmospheric-optical system point spread function or optical transfer function (OTF). In addition, a priori knowledge of the object class and noise are not used in an optimal manner. We apply a vector Wiener filter to photon-limited images degraded by atmospheric turbulence to demonstrate the potential advantages of optimal deconvolution processing. This filter incorporates model-based information about object, OTF, and noise. To our knowledge, this is the first application of complete OTF correlation statistics to the reconstruction of turbulence-degraded images. Computer simulation of binary star images show the vector Wiener filter provides superior reconstructions when compared to the traditional scalar Wiener filter for non-wide-sense stationary objects. Much of this performance improvement can be attributed to variance reduction in the noisy Fourier data at spatial frequencies where the mean OTF is severely attenuated. However, vector Wiener filter performance is substantially degraded with respect to both mean square error and mean square phase error at spatial frequencies where the OTF SNR is less than unity. ©1998 Society of Photo-Optical Instrumentation Engineers

Far field beam shaping and steering using phase retrieval-based wavefront control

Electro-optical (EO) sensors on future aerospace systems will require extensive beam delivery control. While both beam shaping and steering have been demonstrated in the past, simultaneous demonstration of these techniques (for a common aperture) has not. We demonstrate the use of phase-retrieval based algorithms which has been previously used for simultaneous beam shaping and steering in the far field. Experimental results using a segmented, phase-only, liquid crystal wavefront control device are presented. © 2000 SPIE-The International Society for Optical Engineering