Douglas Hope

Multiframe blind deconvolution for imaging in daylight and strong turbulence conditions

We describe results from new computational techniques to extend the reach of large ground-based optical telescopes, enabling high resolution imaging of space objects under daylight conditions. Current state-of-the-art systems, even those employing adaptive optics, dramatically underperform in such conditions because of strong turbulence generated by diurnal solar heating of the atmosphere, characterized by a ratio of telescope diameter to Fried parameter as high as 70. Our approach extends previous advances in multi-frame blind deconvolution (MFBD) by exploiting measurements from a wavefront sensor recorded simultaneously with high-cadence image data. We describe early results with the new algorithm which may be used with seeing-limited image data or as an adjunct to partial compensation with adaptive optics to restore imaging to the diffraction limit even under the extreme observing conditions found in daylight.

Recurrence quantification analysis as a post-processing technique in adaptive optics high contrast imaging

Recurrence Quantification Analysis (RQA) is a non-linear time series analysis technique widely employed in many different research fields. Among the many applications of this method, it has been shown that it can be successfully employed in the detection of small signals embedded into noise. In this work we explore the possibility of using the RQA in astronomical high contrast imaging, for the detection of faint objects nearby bright sources in very high frame rate (1 KHz) data series. For this purpose, we used a real 1 kHz image sequence of a bright star, acquired with the SHARK-VIS forerunner at LBT. Our results show excellent performances in terms of detection contrasts even with a very short data sequence (a few seconds). The use of RQA in astronomical high contrast imaging is discussed in light of the possible science applications and with respect to other techniques like, for example, the angular differential imaging (ADI) or the Speckle-Free ADI (SFADI).

High-resolution imaging through strong atmospheric turbulence

We propose the use of an aperture diverse imaging system for high-resolution imaging through strong atmospheric turbulence. The system has two channels. One channel partitions the aperture into a set of annular apertures that provide a set of images of the target at different spatial resolutions. The other channel feeds an imaging Shack-Hartmann wavefront sensor with a small number of sub-apertures. The combined imagery from this setup is processed using a blind restoration algorithm that captures the inherent temporal correlations in the observed atmospheric wave fronts. This approach shows significant promise for providing high-fidelity imagery for observations acquired through strong atmospheric turbulence. The approach also allows for the separation of the phase perturbations from different layers of the atmosphere. This characteristic offers potential for the accurate restoration of images with fields of view substantially larger than the isoplanatic angle.

Design of a space-based infrared imaging interferometer

Abstract. Present space-based optical imaging sensors are expensive. Launch costs are dictated by weight and size, and system design must take into account the low fault tolerance of a system that cannot be readily accessed once deployed. We describe the design and first prototype of the space-based infrared imaging interferometer (SIRII) that aims to mitigate several aspects of the cost challenge. SIRII is a six-element Fizeau interferometer intended to operate in the short-wave and midwave IR spectral regions over a 6×6  mrad field of view. The volume is smaller by a factor of three than a filled-aperture telescope with equivalent resolving power. The structure and primary optics are fabricated from light-weight space-qualified carbon fiber reinforced polymer; they are easy to replicate and inexpensive. The design is intended to permit one-time alignment during assembly, with no need for further adjustment once on orbit. A three-element prototype of the SIRII imager has been constructed with a unit telescope primary mirror diameter of 165 mm and edge-to-edge baseline of 540 mm. The optics, structure, and interferometric signal processing principles draw on experience developed in ground-based astronomical applications designed to yield the highest sensitivity and resolution with cost-effective optical solutions. The initial motivation for the development of SIRII was the long-term collection of technical intelligence from geosynchronous orbit, but the scalable nature of the design will likely make it suitable for a range of IR imaging scenarios.

A novel lightweight Fizeau infrared interferometric imaging system

Aperture synthesis imaging techniques using an interferometer provide a means to achieve imagery with spatial resolution equivalent to a conventional filled aperture telescope at a significantly reduced size, weight and cost, an important implication for air- and space-borne persistent observing platforms. These concepts have been realized in SIRII (Space-based IR-imaging interferometer), a new light-weight, compact SWIR and MWIR imaging interferometer designed for space-based surveillance. The sensor design is configured as a six-element Fizeau interferometer; it is scalable, light-weight, and uses structural components and main optics made of carbon fiber replicated polymer (CFRP) that are easy to fabricate and inexpensive. A three-element prototype of the SIRII imager has been constructed. The optics, detectors, and interferometric signal processing principles draw on experience developed in ground-based astronomical applications designed to yield the highest sensitivity and resolution with cost-effective optical solutions. SIRII is being designed for technical intelligence from geo-stationary orbit. It has an instantaneous 6 x 6 mrad FOV and the ability to rapidly scan a 6x6 deg FOV, with a minimal SNR. The interferometric design can be scaled to larger equivalent filled aperture, while minimizing weight and costs when compared to a filled aperture telescope with equivalent resolution. This scalability in SIRII allows it address a range of IR-imaging scenarios.

Tomographic wave-front sensing with a single guide star

Adaptive optics or numerical restoration algorithms that restore high resolution imaging through atmospheric turbulence are subject to isoplanatic wave-front errors. Mitigating those errors requires that the wave-front aberrations be estimated within the 3D volume of the atmosphere. Present techniques rely on multiple beacons, either natural stars or laser guide stars, to probe the atmospheric aberration along different lines of sight, followed by tomographic projection of the measurements onto layers at defined ranges. In this paper we show that a three-dimensional estimate of the wave-front aberration can be recovered from measurements by a single guide star in the case where the aberration is stratified, provided that the telescope tracks across the sky with non-uniform angular velocity. This is generally the case for observations of artificial earth-orbiting satellites, and the new method is likely to find application in ground-based telescopes used for space situational awareness.

Atmospheric tomography for artificial satellite observations with a single guide star.

Estimation of wavefront errors in three dimensions is required to mitigate isoplanatic errors when using adaptive optics or numerical restoration algorithms to recover high-resolution images from blurred data taken through atmospheric turbulence. Present techniques rely on multiple beacons, either natural stars or laser guide stars, to probe the atmospheric aberration along different lines of sight, followed by tomographic projection of the measurements. In this Letter, we show that a three-dimensional estimate of the wavefront aberration can be recovered from measurements by a single guide star in the case where the aberration is stratified, provided that the telescope tracks across the sky with nonuniform angular velocity. This is generally the case for observations of artificial Earth-orbiting satellites, and the new method is likely to find application in ground-based telescopes used for space situational awareness.

Fast Tomographic Reconstruction of Atmospheric Turbulence from Micro-Lens Imagery

Data acquired from a micro-lens array is used to obtain a 3-D reconstruction of the wave front. The problem is modeled as a large-scale inverse problem, which can be efficiently solved using iterative optimization techniques.