Research

Abstract: In the development of laser guide star (LGS) technology, Vertical External Cavity Surface Emitting Lasers (VECSELs) are attractive because of their simplicity and compactness. VECSELs operating at 1178 nm may readily be frequency doubled to the sodium D2 resonance at 589 nm. The output power of VECSELs can be scaled to multiple tens of watts by expanding the spot size, or “lateral scaling”. It can also be scaled by using multiple VECSEL devices in one cavity, or “longitudinal scaling”. In the case of longitudinal scaling, at least one VECSEL device must be at a fold of the cavity. This typically causes problems in longitudinal mode stability. In this paper, we present the results of successful experiments to demonstrate a technique to deliver stable single-frequency operation of a multi-VECSEL cavity at 1178 nm. The new technology paves the way for sodium guide star lasers delivering tens of watts that are more compact and substantially cheaper per watt than any existing technology.

STABLE NARROW-LINE VECSEL OPERATION FOR SODIUM GUIDE STAR GENERATION


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FAST TOMOGRAPHIC RECONSTRUCTION OF ATMOSPHERIC TURBULENCE FROM MICRO-LENS IMAGERY


MULTI-FRAME BLIND DECONVOLUTION FOR IMAGING IN DAYLIGHT AND STRONG TURBULENCE CONDITIONS


Abstract: We consider using data acquired from a micro-lens array through which multiple images of the full field-of-view of an astronomical target are formed to attempt to reconstruct the 3-D wave front for the observations. This opens the door for both a beacon-less wave front sensor and imaging of fields-ofview substantially larger than the isoplanatic angle. The reconstruction problem can be modeled as a large-scale linear inverse problem, but standard algorithms used for 3-D computed tomography (CT) reconstruction cannot be applied because measured data are only taken from limited angular range, leaving entire regions of the frequency space un-sampled. However, we show that there is substantial structure in the mathematical model that can be exploited to obtain a robust algorithm that is amenable to efficient implementations.

Abstract: 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.

We use numerical simulations to study the prospective performance of a dual channel imaging system for observing space-based targets through strong atmospheric turbulence: one channel of the system employs aperture diversity and the other an imaging Shack-Hartmann wave-front sensor. The raw images acquired by this setup are processed using a blind restoration algorithm that captures the inherent temporal correlations in the observed phases. This approach, which strengthens the synergy between the image acquisition and post-processing steps, shows strong potential for providing high-resolution imagery at high levels of atmospheric turbulence. The approach may also allow for the separation of the phase perturbations from different layers of the atmosphere and offers promise for accurate restoration of images with fields of view substantially larger than the isoplanatic angle.

HIGH-RESOLUTION IMAGING THROUGH STRONG ATMOSPHERIC TURBULENCE AND OVER WIDE FIELDS-OF-VIEW


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GROUND-LAYER ADAPTIVE OPTICS WITH MULTIPLE LASER GUIDE STARS


RESOLVED OBSERVATIONS OF GEOSTATIONARY SATELLITES FROM THE 6.5 M MMT


Abstract: We report the first closed-loop results obtained from an adaptive optics system with multiple laser guide beacons. The system is mounted on the 6.5 m MMT in Arizona, and is designed to explore advanced altitude-conjugated techniques for wide-field image compensation. Five beacons are made by Rayleigh scattering of laser beams at 532 nm integrated over a range from 20 to 29 km by dynamic refocus of the telescope optics. The return light is analyzed by a unique Shack-Hartmann sensor that places all five beacons on a single detector, with electronic shuttering to implement the beacon range gate. The wavefront sensor divides the 6.5 m telescope pupil into 60 subapertures, and wavefront correction is applied with the telescope's unique deformable secondary mirror. The first application of the system is to correct boundary-layer turbulence, resulting in image quality of 0.2 arcsec in the near infrared bands from 1.2 to 2.5 µm. In this mode we do not try to reach the diffraction limit of the 6.5 m aperture, but instead aim for improved seeing over a field of view much larger than the isoplanatic patch. In this paper we present images of the central 2 arcmin region of the globular cluster M13 in the halo of the Milky Way, and an open cluster, where correction is almost uniform across the full field. The system has particular scientific application to extragalactic survey work, typically done in dark fields where guide stars are very faint, and where large samples of objects are required.

Abstract: We report observations of a number of geostationary spacecraft recorded in the J, H, and Ks bands (centered around 1.2 µm, 1.6 µm, and 2.2 µm) at the 6.5 m MMT telescope in January 2015. With adaptive optics, the satellites were resolved at close to the diffraction limit in each of the wavebands. True color images may be recovered from the multiple wavebands, while the large aperture allows accurate photometric calibration with excellent time resolution of even small, faint objects in these distant orbits. Of note are our observations of solar panels, which can only be satisfactorily imaged in bands longer than their cut-off wavelengths. Since the cut-off is generally in the neighborhood of 1.5 – 2 µm, the panels will only be well resolved by telescopes larger than 4 m. In one case observed at the MMT, solar panels were seen to span approximately 24 m, twice the extent described in published data.

Abstract: Current ground-based resolved imaging of resident space objects is mostly limited to dawn and dusk, thus severely restricting the timing of observations. The possibility of daylight imaging represents an advance that would increase the amount of sky accessible for space surveillance operations while enormously relaxing timing restrictions on data collections. Imaging in full daylight represents a challenge because of the level of atmospheric turbulence noise, high sky background, and the damaging effects of Rayleigh scattering on the signal-to-noise ratio. An important part of overcoming this challenge is the collection of wave-front sensor (WFS) measurements contemporaneous with the focal plane imagery. These measurements are used both to estimate frame-by-frame point-spread functions and to estimate the number and velocities of turbulent layers in a frozen flow model of the atmosphere. [...]

IMAGE RESTORATION FROM SODIUM GUIDE STAR OBSERVATIONS IN DAYLIGHT


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HIGH-RESOLUTION SPACE SITUATIONAL AWARENESS IMAGING USING CARBON FIBER TELESCOPES


HIGH RESOLUTION IMAGING OF SATELLITES IN DAYLIGHT


Abstract: High-resolution imagery of satellites from ground-based telescopes plays an important role in space situational awareness (SSA). Current approaches generally rely on large aperture telescopes equipped with adaptive optics. The cost and operational demands of such systems place severe limitations on worldwide surveillance capability. Coverage of the sky is restricted to the regions that happen to be above the small number of geographical locations where facilities exist. However, recent advances in carbon fiber reinforced polymer (CFRP) mirrors offer the potential for field-deployable telescopes with an aperture diameter of 1 m or larger for ground-based imaging of space objects. Concave CFRP mirrors are made from convex mandrels, which are challenging to fabricate. Therefore, a major cost driver of a CFRP telescope is the optical quality of the mandrel. Here we show, using both numerical simulations and real data, that a telescope with significant optical aberration can be used for high-resolution imaging if the telescope is equipped with a wavefront sensor (WFS) and the recorded images and WFS data are processed with an appropriate image restoration algorithm. We anticipate that the maturation of the technology to manufacture telescopes at this size from light-weight replicated components, at a cost considerably lower than conventional telescopes, will address a growing demand for SSA data.

Abstract: Ground-based imaging of satellites during the daytime represents a formidable challenge due the strong turbulence induced noise in the imagery and the high background noise. Two important approaches for overcoming the problem of imaging through this strong turbulence include aperture partitioning and the collection of wavefront measurements for use in image restoration post-processing. The aperture partitioning enables a reduction in the turbulence induced noise in the recorded imagery, while the wavefront measurements can be used to constrain a frozen flow estimate of the wave front. Together the WFS measurements and FFM enable the recovery of high spatial frequencies in the wave front, which leads to higher fidelity estimates of the PSFs necessary for estimating the recovered image of the satellite. Improvements in image restoration due to the aperture partitioning will be demonstrated by comparing image restoration algorithms that used WFS data with imagery acquired using a filled aperture versus imagery acquired using an aperture partitioning scheme. This comparison will be made using imagery acquired in daylight using a 3m-class telescope.

Abstract: The Nonlinear Curvature Wavefront Sensor (nlCWFS), first proposed by Guyon,[1] determines wavefront shape from images of a reference beacon in a number of planes between the pupil and focal plane of a telescope. We describe a new algorithm that rapidly recovers the low-order aberrations accurately enough to allow practical use of the nlCWFS in an adaptive optics (AO) system. The algorithm was inspired by refractive strong scintillation in the interstellar me dium[2] , which behaves similarly to near-pupil linear curvature focusing, but over larger scales. The refractive component is extracted from the speckled images by binning with the lowest-order aberrations being additionally estimated through the use of first and second distribution moments. The linearity of the refractive scintillation process allows us to use a reconstructor matrix to compute an estimate of the pupil wavefront. [...]

A FAST WAVEFRONT RECONSTRUCTOR FOR THE NONLINEAR CURVATURE WAVEFRONT SENSOR


Image
Read Article

ADVANCES IN POLARIZED REMOTE ACOUSTIC IMAGING


IMAGE REGISTRATION FOR DAYLIGHT ADAPTIVE OPTICS


Abstract: Small surface vibrations imparted from both internal and external driving acoustics, such as speech, machinery, or structural modes result in observable changes to the degree of linear polarization from light reflected off common materials such as windows, painted surfaces, and metals. We show that by passively sampling these oscillations, optical sensors measuring linear polarization can extract these signals and return them as both infrasound and audible sound. In this work we explore the physical signal characteristics and we additionally demonstrate a prototype instrument and its experimental results. From these results, we show simple sinusoids captured to provide a basis for SNR metrics. Additionally, we show more complex acoustic signals such as recorded human speech that were recovered from the vibrations of these polarizing surfaces.

Abstract: Daytime use of adaptive optics (AO) at large telescopes is hampered by shot noise from the bright sky background. Wave-front sensing may use a sodium laser guide star observed through a magneto-optical filter to suppress the background, but the laser beacon is not sensitive to overall image motion. To estimate that, laser-guided AO systems generally rely on light from the object itself, collected through the full aperture of the telescope. Daylight sets a lower limit to the brightness of an object that may be tracked at rates sufficient to overcome the image jitter. Below that limit, wave-front correction on the basis of the laser alone will yield an image that is approximately diffraction limited but that moves randomly. I describe an iterative registration algorithm that recovers high-resolution long-exposure images in this regime from a rapid series of short exposures with very low signal-to-noise ratio. The technique takes advantage of the fact that in the photon noise limit there is negligible penalty in taking short exposures, and also that once the images are recorded, it is not necessary, as in the case of an AO tracker loop, to estimate the image motion correctly and quickly on every cycle. The algorithm is likely to find application in space situational awareness, where high-resolution daytime imaging of artificial satellites is important.

Abstract: To determine the influence of the environment on star formation, we need to study the process in the extreme conditions of massive young star clusters ( 104 solar masses) near the centre of our own Galaxy. Observations must be carried out in the near infrared because of very high extinction in visible light within the Galactic plane. We need high resolution to identify cluster members from their peculiar motions , and because most such clusters span more than 19, efficient observation demands a wide field of view. There is at present no space-based facility that meets all these criteria. Ground-based telescopes can in principle make such observations when fitted with ground-layer adaptive optics (GLAO), which removes the optical aberration caused by atmospheric turbulence up to an altitude of 500 m . A GLAO system that uses multiple laser guide stars has been developed at the 6.5-m MMT telescope, in Arizona. [...]

A GROUND-LAYER ADAPTIVE OPTICS SYSTEM WITH MULTIPLE LASER GUIDE STARS


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REMOTE ACOUSTIC IMAGING OF GEOSYNCHRONOUS SATELLITES


DESIGN OF A SPACE-BASED INFRARED IMAGING INTERFEROMETER


Abstract: Identification and characterization of orbiting objects that are not spatially resolved are challenging problems for traditional remote sensing methods. Hyper temporal imaging, enabled by fast, low-noise electro-optical detectors is a new sensing modality which may allow the direct detection of acoustic resonances on satellites enabling a new regime of signature and state detection. Detectable signatures may be caused by the oscillations of solar panels, high-gain antennae, or other on-board subsystems driven by thermal gradients, fluctuations in solar radiation pressure, worn reaction wheels, or orbit maneuvers. Herein we present the first hyper-temporal observations of geosynchronous satellites. Data were collected at the Kuiper 1.54-meter telescope in Arizona using an experimental dual-channel imaging instrument that simultaneously measures light in two orthogonally polarized beams at sampling rates extending up to 1 kHz. In these observations, we see evidence of acoustic resonances in the polarization state of satellites. The technique is expected to support object identification and characterization of on-board components and to act as a discriminant between active satellites, debris, and passive bodies.

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 costeffective 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.

Abstract: 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.

ATMOSPHERIC TOMOGRAPHY FOR ARTIFICIAL SATELLITE OBSERVATIONS WITH A SINGLE GUIDE STAR


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DAYLIGHT OPERATION OF A SODIUM LASER GUIDE STAR FOR ADAPTIVE OPTICS WAVEFRONT SENSING


A COMPREHENSIVE APPROACH TO HIGH-RESOLUTION DAYLIGHT IMAGING FOR SSA


Abstract: We report contrast measurements of a sodium resonance guide star against the daylight sky when observed through a tuned magneto-optical filter (MOF). The guide star was created by projection of a laser beam at 589.16 nm into the mesospheric sodium layer and the observations were made with a collocated 1.5-m telescope. While MOFs are used with sodium light detecting and ranging systems during the day to improve the signalto-noise ratio of the measurements, they have not so far been employed with laser guide stars to drive adaptive optics (AO) systems to correct atmospherically induced image blur. We interpret our results in terms of the performance of AO systems for astronomy, with particular emphasis on thermal infrared observations at the next generation of extremely large telescopes now being built.

Abstract:High resolution daytime imaging of resident space objects (RSO) from the ground is presently severely challenging. At visible wavelengths, where diffraction-limited resolution is the highest before the atmosphere becomes opaque in the UV, shot noise from the bright background degrades the information that may be recovered from RSO imagery. Total exposure times must be limited in order to avoid motion blur induced either by the object’s intrinsic rotation or simply by its orbital motion over the site. Fundamentally, then, one cannot collect enough light from the object to achieve adequate signal-to-noise ratio (SNR) in the presence of very high noise before the apparent shape of the object has changed. To overcome this limitation, we propose in this paper a suite of techniques which we believe will collectively enable high-resolution imaging during daylight [...]

Abstract: We demonstrate that high-resolution imaging through strong atmospheric turbulence can be achieved by acquiring data with a system that captures short exposure (“speckle”) images using a range of aperture sizes and then using a bootstrap multi-frame blind deconvolution restoration process that starts with the smallest aperture data. Our results suggest a potential paradigm shift in how we image through atmospheric turbulence. No longer should image acquisition and post processing be treated as two independent processes: they should be considered as intimately related.

HIGH-RESOLUTION SPECKLE IMAGING THROUGH STRONG ATMOSPHERIC TURBULENCE


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Research

Abstract: In the development of laser guide star (LGS) technology, Vertical External Cavity Surface Emitting Lasers (VECSELs) are attractive because of their simplicity and compactness. VECSELs operating at 1178 nm may readily be frequency doubled to the sodium D2 resonance at 589 nm. The output power of VECSELs can be scaled to multiple tens of watts by expanding the spot size, or “lateral scaling”. It can also be scaled by using multiple VECSEL devices in one cavity, or “longitudinal scaling”. In the case of longitudinal scaling, at least one VECSEL device must be at a fold of the cavity. This typically causes problems in longitudinal mode stability. In this paper, we present the results of successful experiments to demonstrate a technique to deliver stable single-frequency operation of a multi-VECSEL cavity at 1178 nm. The new technology paves the way for sodium guide star lasers delivering tens of watts that are more compact and substantially cheaper per watt than any existing technology.

STABLE NARROW-LINE VECSEL OPERATION FOR SODIUM GUIDE STAR GENERATION


Image
Read Article

FAST TOMOGRAPHIC RECONSTRUCTION OF ATMOSPHERIC TURBULENCE FROM MICRO-LENS IMAGERY


MULTI-FRAME BLIND DECONVOLUTION FOR IMAGING IN DAYLIGHT AND STRONG TURBULENCE CONDITIONS


Abstract: We consider using data acquired from a micro-lens array through which multiple images of the full field-of-view of an astronomical target are formed to attempt to reconstruct the 3-D wave front for the observations. This opens the door for both a beacon-less wave front sensor and imaging of fields-ofview substantially larger than the isoplanatic angle. The reconstruction problem can be modeled as a large-scale linear inverse problem, but standard algorithms used for 3-D computed tomography (CT) reconstruction cannot be applied because measured data are only taken from limited angular range, leaving entire regions of the frequency space un-sampled. However, we show that there is substantial structure in the mathematical model that can be exploited to obtain a robust algorithm that is amenable to efficient implementations.

Abstract: 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.

Abstract: We use numerical simulations to study the prospective performance of a dual channel imaging system for observing space-based targets through strong atmospheric turbulence: one channel of the system employs aperture diversity and the other an imaging Shack-Hartmann wave-front sensor. The raw images acquired by this setup are processed using a blind restoration algorithm that captures the inherent temporal correlations in the observed phases. This approach, which strengthens the synergy between the image acquisition and post-processing steps, shows strong potential for providing high-resolution imagery at high levels of atmospheric turbulence. The approach may also allow for the separation of the phase perturbations from different layers of the atmosphere and offers promise for accurate restoration of images with fields of view substantially larger than the isoplanatic angle.

HIGH-RESOLUTION IMAGING THROUGH STRONG ATMOSPHERIC TURBULENCE AND OVER WIDE FIELDS-OF-VIEW


Image
Read Article

GROUND-LAYER ADAPTIVE OPTICS WITH MULTIPLE LASER GUIDE STARS


RESOLVED OBSERVATIONS OF GEOSTATIONARY SATELLITES FROM THE 6.5 M MMT


Abstract: We report the first closed-loop results obtained from an adaptive optics system with multiple laser guide beacons. The system is mounted on the 6.5 m MMT in Arizona, and is designed to explore advanced altitude-conjugated techniques for wide-field image compensation. Five beacons are made by Rayleigh scattering of laser beams at 532 nm integrated over a range from 20 to 29 km by dynamic refocus of the telescope optics. The return light is analyzed by a unique Shack-Hartmann sensor that places all five beacons on a single detector, with electronic shuttering to implement the beacon range gate. The wavefront sensor divides the 6.5 m telescope pupil into 60 subapertures, and wavefront correction is applied with the telescope's unique deformable secondary mirror. The first application of the system is to correct boundary-layer turbulence, resulting in image quality of 0.2 arcsec in the near infrared bands from 1.2 to 2.5 µm. In this mode we do not try to reach the diffraction limit of the 6.5 m aperture, but instead aim for improved seeing over a field of view much larger than the isoplanatic patch. In this paper we present images of the central 2 arcmin region of the globular cluster M13 in the halo of the Milky Way, and an open cluster, where correction is almost uniform across the full field. The system has particular scientific application to extragalactic survey work, typically done in dark fields where guide stars are very faint, and where large samples of objects are required.

Abstract: We report observations of a number of geostationary spacecraft recorded in the J, H, and Ks bands (centered around 1.2 µm, 1.6 µm, and 2.2 µm) at the 6.5 m MMT telescope in January 2015. With adaptive optics, the satellites were resolved at close to the diffraction limit in each of the wavebands. True color images may be recovered from the multiple wavebands, while the large aperture allows accurate photometric calibration with excellent time resolution of even small, faint objects in these distant orbits. Of note are our observations of solar panels, which can only be satisfactorily imaged in bands longer than their cut-off wavelengths. Since the cut-off is generally in the neighborhood of 1.5 – 2 µm, the panels will only be well resolved by telescopes larger than 4 m. In one case observed at the MMT, solar panels were seen to span approximately 24 m, twice the extent described in published data.

Abstract: Current ground-based resolved imaging of resident space objects is mostly limited to dawn and dusk, thus severely restricting the timing of observations. The possibility of daylight imaging represents an advance that would increase the amount of sky accessible for space surveillance operations while enormously relaxing timing restrictions on data collections. Imaging in full daylight represents a challenge because of the level of atmospheric turbulence noise, high sky background, and the damaging effects of Rayleigh scattering on the signal-to-noise ratio. An important part of overcoming this challenge is the collection of wave-front sensor (WFS) measurements contemporaneous with the focal plane imagery. These measurements are used both to estimate frame-by-frame point-spread functions and to estimate the number and velocities of turbulent layers in a frozen flow model of the atmosphere. [...]

IMAGE RESTORATION FROM SODIUM GUIDE STAR OBSERVATIONS IN DAYLIGHT


Image
Read Article

HIGH-RESOLUTION SPACE SITUATIONAL AWARENESS IMAGING USING CARBON FIBER TELESCOPES


HIGH RESOLUTION IMAGING OF SATELLITES IN DAYLIGHT


Abstract: High-resolution imagery of satellites from ground-based telescopes plays an important role in space situational awareness (SSA). Current approaches generally rely on large aperture telescopes equipped with adaptive optics. The cost and operational demands of such systems place severe limitations on worldwide surveillance capability. Coverage of the sky is restricted to the regions that happen to be above the small number of geographical locations where facilities exist. However, recent advances in carbon fiber reinforced polymer (CFRP) mirrors offer the potential for field-deployable telescopes with an aperture diameter of 1 m or larger for ground-based imaging of space objects. Concave CFRP mirrors are made from convex mandrels, which are challenging to fabricate. Therefore, a major cost driver of a CFRP telescope is the optical quality of the mandrel. Here we show, using both numerical simulations and real data, that a telescope with significant optical aberration can be used for high-resolution imaging if the telescope is equipped with a wavefront sensor (WFS) and the recorded images and WFS data are processed with an appropriate image restoration algorithm. We anticipate that the maturation of the technology to manufacture telescopes at this size from light-weight replicated components, at a cost considerably lower than conventional telescopes, will address a growing demand for SSA data.

Abstract: Ground-based imaging of satellites during the daytime represents a formidable challenge due the strong turbulence induced noise in the imagery and the high background noise. Two important approaches for overcoming the problem of imaging through this strong turbulence include aperture partitioning and the collection of wavefront measurements for use in image restoration post-processing. The aperture partitioning enables a reduction in the turbulence induced noise in the recorded imagery, while the wavefront measurements can be used to constrain a frozen flow estimate of the wave front. Together the WFS measurements and FFM enable the recovery of high spatial frequencies in the wave front, which leads to higher fidelity estimates of the PSFs necessary for estimating the recovered image of the satellite. Improvements in image restoration due to the aperture partitioning will be demonstrated by comparing image restoration algorithms that used WFS data with imagery acquired using a filled aperture versus imagery acquired using an aperture partitioning scheme. This comparison will be made using imagery acquired in daylight using a 3m-class telescope.

Abstract: The Nonlinear Curvature Wavefront Sensor (nlCWFS), first proposed by Guyon,[1] determines wavefront shape from images of a reference beacon in a number of planes between the pupil and focal plane of a telescope. We describe a new algorithm that rapidly recovers the low-order aberrations accurately enough to allow practical use of the nlCWFS in an adaptive optics (AO) system. The algorithm was inspired by refractive strong scintillation in the interstellar me dium[2] , which behaves similarly to near-pupil linear curvature focusing, but over larger scales. The refractive component is extracted from the speckled images by binning with the lowest-order aberrations being additionally estimated through the use of first and second distribution moments. The linearity of the refractive scintillation process allows us to use a reconstructor matrix to compute an estimate of the pupil wavefront. [...]

A FAST WAVEFRONT RECONSTRUCTOR FOR THE NONLINEAR CURVATURE WAVEFRONT SENSOR


Image
Read Article

ADVANCES IN POLARIZED REMOTE ACOUSTIC IMAGING


IMAGE REGISTRATION FOR DAYLIGHT ADAPTIVE OPTICS


Abstract: Small surface vibrations imparted from both internal and external driving acoustics, such as speech, machinery, or structural modes result in observable changes to the degree of linear polarization from light reflected off common materials such as windows, painted surfaces, and metals. We show that by passively sampling these oscillations, optical sensors measuring linear polarization can extract these signals and return them as both infrasound and audible sound. In this work we explore the physical signal characteristics and we additionally demonstrate a prototype instrument and its experimental results. From these results, we show simple sinusoids captured to provide a basis for SNR metrics. Additionally, we show more complex acoustic signals such as recorded human speech that were recovered from the vibrations of these polarizing surfaces.

Abstract: Daytime use of adaptive optics (AO) at large telescopes is hampered by shot noise from the bright sky background. Wave-front sensing may use a sodium laser guide star observed through a magneto-optical filter to suppress the background, but the laser beacon is not sensitive to overall image motion. To estimate that, laser-guided AO systems generally rely on light from the object itself, collected through the full aperture of the telescope. Daylight sets a lower limit to the brightness of an object that may be tracked at rates sufficient to overcome the image jitter. Below that limit, wave-front correction on the basis of the laser alone will yield an image that is approximately diffraction limited but that moves randomly. I describe an iterative registration algorithm that recovers high-resolution long-exposure images in this regime from a rapid series of short exposures with very low signal-to-noise ratio. The technique takes advantage of the fact that in the photon noise limit there is negligible penalty in taking short exposures, and also that once the images are recorded, it is not necessary, as in the case of an AO tracker loop, to estimate the image motion correctly and quickly on every cycle. The algorithm is likely to find application in space situational awareness, where high-resolution daytime imaging of artificial satellites is important.

Abstract: To determine the influence of the environment on star formation, we need to study the process in the extreme conditions of massive young star clusters ( 104 solar masses) near the centre of our own Galaxy. Observations must be carried out in the near infrared because of very high extinction in visible light within the Galactic plane. We need high resolution to identify cluster members from their peculiar motions , and because most such clusters span more than 19, efficient observation demands a wide field of view. There is at present no space-based facility that meets all these criteria. Ground-based telescopes can in principle make such observations when fitted with ground-layer adaptive optics (GLAO), which removes the optical aberration caused by atmospheric turbulence up to an altitude of 500 m . A GLAO system that uses multiple laser guide stars has been developed at the 6.5-m MMT telescope, in Arizona. [...]

A GROUND-LAYER ADAPTIVE OPTICS SYSTEM WITH MULTIPLE LASER GUIDE STARS


Image
Read Article

REMOTE ACOUSTIC IMAGING OF GEOSYNCHRONOUS SATELLITES


DESIGN OF A SPACE-BASED INFRARED IMAGING INTERFEROMETER


Abstract: Identification and characterization of orbiting objects that are not spatially resolved are challenging problems for traditional remote sensing methods. Hyper temporal imaging, enabled by fast, low-noise electro-optical detectors is a new sensing modality which may allow the direct detection of acoustic resonances on satellites enabling a new regime of signature and state detection. Detectable signatures may be caused by the oscillations of solar panels, high-gain antennae, or other on-board subsystems driven by thermal gradients, fluctuations in solar radiation pressure, worn reaction wheels, or orbit maneuvers. Herein we present the first hyper-temporal observations of geosynchronous satellites. Data were collected at the Kuiper 1.54-meter telescope in Arizona using an experimental dual-channel imaging instrument that simultaneously measures light in two orthogonally polarized beams at sampling rates extending up to 1 kHz. In these observations, we see evidence of acoustic resonances in the polarization state of satellites. The technique is expected to support object identification and characterization of on-board components and to act as a discriminant between active satellites, debris, and passive bodies.

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 costeffective 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.

Abstract: 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.

ATMOSPHERIC TOMOGRAPHY FOR ARTIFICIAL SATELLITE OBSERVATIONS WITH A SINGLE GUIDE STAR


Image
Read Article

DAYLIGHT OPERATION OF A SODIUM LASER GUIDE STAR FOR ADAPTIVE OPTICS WAVEFRONT SENSING


A COMPREHENSIVE APPROACH TO HIGH-RESOLUTION DAYLIGHT IMAGING FOR SSA


Abstract: We report contrast measurements of a sodium resonance guide star against the daylight sky when observed through a tuned magneto-optical filter (MOF). The guide star was created by projection of a laser beam at 589.16 nm into the mesospheric sodium layer and the observations were made with a collocated 1.5-m telescope. While MOFs are used with sodium light detecting and ranging systems during the day to improve the signalto-noise ratio of the measurements, they have not so far been employed with laser guide stars to drive adaptive optics (AO) systems to correct atmospherically induced image blur. We interpret our results in terms of the performance of AO systems for astronomy, with particular emphasis on thermal infrared observations at the next generation of extremely large telescopes now being built.

Abstract:High resolution daytime imaging of resident space objects (RSO) from the ground is presently severely challenging. At visible wavelengths, where diffraction-limited resolution is the highest before the atmosphere becomes opaque in the UV, shot noise from the bright background degrades the information that may be recovered from RSO imagery. Total exposure times must be limited in order to avoid motion blur induced either by the object’s intrinsic rotation or simply by its orbital motion over the site. Fundamentally, then, one cannot collect enough light from the object to achieve adequate signal-to-noise ratio (SNR) in the presence of very high noise before the apparent shape of the object has changed. To overcome this limitation, we propose in this paper a suite of techniques which we believe will collectively enable high-resolution imaging during daylight [...]

Abstract: We demonstrate that high-resolution imaging through strong atmospheric turbulence can be achieved by acquiring data with a system that captures short exposure (“speckle”) images using a range of aperture sizes and then using a bootstrap multi-frame blind deconvolution restoration process that starts with the smallest aperture data. Our results suggest a potential paradigm shift in how we image through atmospheric turbulence. No longer should image acquisition and post processing be treated as two independent processes: they should be considered as intimately related.

HIGH-RESOLUTION SPECKLE IMAGING THROUGH STRONG ATMOSPHERIC TURBULENCE


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STABLE NARROW-LINE VECSEL OPERATION FOR SODIUM GUIDE STAR GENERATION


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Abstract: In the development of laser guide star (LGS) technology, Vertical External Cavity Surface Emitting Lasers (VECSELs) are attractive because of their simplicity and compactness. VECSELs operating at 1178 nm may readily be frequency doubled to the sodium D2 resonance at 589 nm. The output power of VECSELs can be scaled to multiple tens of watts by expanding the spot size, or “lateral scaling”. It can also be scaled by using multiple VECSEL devices in one cavity, or “longitudinal scaling”. In the case of longitudinal scaling, at least one VECSEL device must be at a fold of the cavity. This typically causes problems in longitudinal mode stability. In this paper, we present the results of successful experiments to demonstrate a technique to deliver stable single-frequency operation of a multi-VECSEL cavity at 1178 nm. The new technology paves the way for sodium guide star lasers delivering tens of watts that are more compact and substantially cheaper per watt than any existing technology.

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FAST TOMOGRAPHIC RECONSTRUCTION OF ATMOSPHERIC TURBULENCE FROM MICRO-LENS IMAGERY


Abstract: We consider using data acquired from a micro-lens array through which multiple images of the full field-of-view of an astronomical target are formed to attempt to reconstruct the 3-D wave front for the observations. This opens the door for both a beacon-less wave front sensor and imaging of fields-ofview substantially larger than the isoplanatic angle. The reconstruction problem can be modeled as a large-scale linear inverse problem, but standard algorithms used for 3-D computed tomography (CT) reconstruction cannot be applied because measured data are only taken from limited angular range, leaving entire regions of the frequency space un-sampled. However, we show that there is substantial structure in the mathematical model that can be exploited to obtain a robust algorithm that is amenable to efficient implementations.

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MULTI-FRAME BLIND DECONVOLUTION FOR IMAGING IN DAYLIGHT AND STRONG TURBULENCE CONDITIONS


Abstract: 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.

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HIGH-RESOLUTION IMAGING THROUGH STRONG ATMOSPHERIC TURBULENCE AND OVER WIDE FIELDS-OF-VIEW


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We use numerical simulations to study the prospective performance of a dual channel imaging system for observing space-based targets through strong atmospheric turbulence: one channel of the system employs aperture diversity and the other an imaging Shack-Hartmann wave-front sensor. The raw images acquired by this setup are processed using a blind restoration algorithm that captures the inherent temporal correlations in the observed phases. This approach, which strengthens the synergy between the image acquisition and post-processing steps, shows strong potential for providing high-resolution imagery at high levels of atmospheric turbulence. The approach may also allow for the separation of the phase perturbations from different layers of the atmosphere and offers promise for accurate restoration of images with fields of view substantially larger than the isoplanatic angle.

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GROUND-LAYER ADAPTIVE OPTICS WITH MULTIPLE LASER GUIDE STARS


Abstract: We report the first closed-loop results obtained from an adaptive optics system with multiple laser guide beacons. The system is mounted on the 6.5 m MMT in Arizona, and is designed to explore advanced altitude-conjugated techniques for wide-field image compensation. Five beacons are made by Rayleigh scattering of laser beams at 532 nm integrated over a range from 20 to 29 km by dynamic refocus of the telescope optics. The return light is analyzed by a unique Shack-Hartmann sensor that places all five beacons on a single detector, with electronic shuttering to implement the beacon range gate. The wavefront sensor divides the 6.5 m telescope pupil into 60 subapertures, and wavefront correction is applied with the telescope's unique deformable secondary mirror. The first application of the system is to correct boundary-layer turbulence, resulting in image quality of 0.2 arcsec in the near infrared bands from 1.2 to 2.5 µm. In this mode we do not try to reach the diffraction limit of the 6.5 m aperture, but instead aim for improved seeing over a field of view much larger than the isoplanatic patch. In this paper we present images of the central 2 arcmin region of the globular cluster M13 in the halo of the Milky Way, and an open cluster, where correction is almost uniform across the full field. The system has particular scientific application to extragalactic survey work, typically done in dark fields where guide stars are very faint, and where large samples of objects are required.

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RESOLVED OBSERVATIONS OF GEOSTATIONARY SATELLITES FROM THE 6.5 M MMT


Abstract: We report observations of a number of geostationary spacecraft recorded in the J, H, and Ks bands (centered around 1.2 µm, 1.6 µm, and 2.2 µm) at the 6.5 m MMT telescope in January 2015. With adaptive optics, the satellites were resolved at close to the diffraction limit in each of the wavebands. True color images may be recovered from the multiple wavebands, while the large aperture allows accurate photometric calibration with excellent time resolution of even small, faint objects in these distant orbits. Of note are our observations of solar panels, which can only be satisfactorily imaged in bands longer than their cut-off wavelengths. Since the cut-off is generally in the neighborhood of 1.5 – 2 µm, the panels will only be well resolved by telescopes larger than 4 m. In one case observed at the MMT, solar panels were seen to span approximately 24 m, twice the extent described in published data.

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IMAGE RESTORATION FROM SODIUM GUIDE STAR OBSERVATIONS IN DAYLIGHT


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Abstract: Current ground-based resolved imaging of resident space objects is mostly limited to dawn and dusk, thus severely restricting the timing of observations. The possibility of daylight imaging represents an advance that would increase the amount of sky accessible for space surveillance operations while enormously relaxing timing restrictions on data collections. Imaging in full daylight represents a challenge because of the level of atmospheric turbulence noise, high sky background, and the damaging effects of Rayleigh scattering on the signal-to-noise ratio. An important part of overcoming this challenge is the collection of wave-front sensor (WFS) measurements contemporaneous with the focal plane imagery. These measurements are used both to estimate frame-by-frame point-spread functions and to estimate the number and velocities of turbulent layers in a frozen flow model of the atmosphere. [...]

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HIGH-RESOLUTION SPACE SITUATIONAL AWARENESS IMAGING USING CARBON FIBER TELESCOPES


Abstract: High-resolution imagery of satellites from ground-based telescopes plays an important role in space situational awareness (SSA). Current approaches generally rely on large aperture telescopes equipped with adaptive optics. The cost and operational demands of such systems place severe limitations on worldwide surveillance capability. Coverage of the sky is restricted to the regions that happen to be above the small number of geographical locations where facilities exist. However, recent advances in carbon fiber reinforced polymer (CFRP) mirrors offer the potential for field-deployable telescopes with an aperture diameter of 1 m or larger for ground-based imaging of space objects. Concave CFRP mirrors are made from convex mandrels, which are challenging to fabricate. Therefore, a major cost driver of a CFRP telescope is the optical quality of the mandrel. Here we show, using both numerical simulations and real data, that a telescope with significant optical aberration can be used for high-resolution imaging if the telescope is equipped with a wavefront sensor (WFS) and the recorded images and WFS data are processed with an appropriate image restoration algorithm. We anticipate that the maturation of the technology to manufacture telescopes at this size from light-weight replicated components, at a cost considerably lower than conventional telescopes, will address a growing demand for SSA data.

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HIGH RESOLUTION IMAGING OF SATELLITES IN DAYLIGHT


Abstract: Ground-based imaging of satellites during the daytime represents a formidable challenge due the strong turbulence induced noise in the imagery and the high background noise. Two important approaches for overcoming the problem of imaging through this strong turbulence include aperture partitioning and the collection of wavefront measurements for use in image restoration post-processing. The aperture partitioning enables a reduction in the turbulence induced noise in the recorded imagery, while the wavefront measurements can be used to constrain a frozen flow estimate of the wave front. Together the WFS measurements and FFM enable the recovery of high spatial frequencies in the wave front, which leads to higher fidelity estimates of the PSFs necessary for estimating the recovered image of the satellite. Improvements in image restoration due to the aperture partitioning will be demonstrated by comparing image restoration algorithms that used WFS data with imagery acquired using a filled aperture versus imagery acquired using an aperture partitioning scheme. This comparison will be made using imagery acquired in daylight using a 3m-class telescope.

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A FAST WAVEFRONT RECONSTRUCTOR FOR THE NONLINEAR CURVATURE WAVEFRONT SENSOR


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Abstract: The Nonlinear Curvature Wavefront Sensor (nlCWFS), first proposed by Guyon,[1] determines wavefront shape from images of a reference beacon in a number of planes between the pupil and focal plane of a telescope. We describe a new algorithm that rapidly recovers the low-order aberrations accurately enough to allow practical use of the nlCWFS in an adaptive optics (AO) system. The algorithm was inspired by refractive strong scintillation in the interstellar me dium[2] , which behaves similarly to near-pupil linear curvature focusing, but over larger scales. The refractive component is extracted from the speckled images by binning with the lowest-order aberrations being additionally estimated through the use of first and second distribution moments. The linearity of the refractive scintillation process allows us to use a reconstructor matrix to compute an estimate of the pupil wavefront. [...]

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ADVANCES IN POLARIZED REMOTE ACOUSTIC IMAGING


Abstract: Small surface vibrations imparted from both internal and external driving acoustics, such as speech, machinery, or structural modes result in observable changes to the degree of linear polarization from light reflected off common materials such as windows, painted surfaces, and metals. We show that by passively sampling these oscillations, optical sensors measuring linear polarization can extract these signals and return them as both infrasound and audible sound. In this work we explore the physical signal characteristics and we additionally demonstrate a prototype instrument and its experimental results. From these results, we show simple sinusoids captured to provide a basis for SNR metrics. Additionally, we show more complex acoustic signals such as recorded human speech that were recovered from the vibrations of these polarizing surfaces.

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IMAGE REGISTRATION FOR DAYLIGHT ADAPTIVE OPTICS


Abstract: Daytime use of adaptive optics (AO) at large telescopes is hampered by shot noise from the bright sky background. Wave-front sensing may use a sodium laser guide star observed through a magneto-optical filter to suppress the background, but the laser beacon is not sensitive to overall image motion. To estimate that, laser-guided AO systems generally rely on light from the object itself, collected through the full aperture of the telescope. Daylight sets a lower limit to the brightness of an object that may be tracked at rates sufficient to overcome the image jitter. Below that limit, wave-front correction on the basis of the laser alone will yield an image that is approximately diffraction limited but that moves randomly. I describe an iterative registration algorithm that recovers high-resolution long-exposure images in this regime from a rapid series of short exposures with very low signal-to-noise ratio. The technique takes advantage of the fact that in the photon noise limit there is negligible penalty in taking short exposures, and also that once the images are recorded, it is not necessary, as in the case of an AO tracker loop, to estimate the image motion correctly and quickly on every cycle. The algorithm is likely to find application in space situational awareness, where high-resolution daytime imaging of artificial satellites is important.

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A GROUND-LAYER ADAPTIVE OPTICS SYSTEM WITH MULTIPLE LASER GUIDE STARS


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Abstract: To determine the influence of the environment on star formation, we need to study the process in the extreme conditions of massive young star clusters ( 104 solar masses) near the centre of our own Galaxy. Observations must be carried out in the near infrared because of very high extinction in visible light within the Galactic plane. We need high resolution to identify cluster members from their peculiar motions , and because most such clusters span more than 19, efficient observation demands a wide field of view. There is at present no space-based facility that meets all these criteria. Ground-based telescopes can in principle make such observations when fitted with ground-layer adaptive optics (GLAO), which removes the optical aberration caused by atmospheric turbulence up to an altitude of 500 m . A GLAO system that uses multiple laser guide stars has been developed at the 6.5-m MMT telescope, in Arizona. [...]

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REMOTE ACOUSTIC IMAGING OF GEOSYNCHRONOUS SATELLITES


Abstract: Identification and characterization of orbiting objects that are not spatially resolved are challenging problems for traditional remote sensing methods. Hyper temporal imaging, enabled by fast, low-noise electro-optical detectors is a new sensing modality which may allow the direct detection of acoustic resonances on satellites enabling a new regime of signature and state detection. Detectable signatures may be caused by the oscillations of solar panels, high-gain antennae, or other on-board subsystems driven by thermal gradients, fluctuations in solar radiation pressure, worn reaction wheels, or orbit maneuvers. Herein we present the first hyper-temporal observations of geosynchronous satellites. Data were collected at the Kuiper 1.54-meter telescope in Arizona using an experimental dual-channel imaging instrument that simultaneously measures light in two orthogonally polarized beams at sampling rates extending up to 1 kHz. In these observations, we see evidence of acoustic resonances in the polarization state of satellites. The technique is expected to support object identification and characterization of on-board components and to act as a discriminant between active satellites, debris, and passive bodies.

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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 costeffective 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.

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ATMOSPHERIC TOMOGRAPHY FOR ARTIFICIAL SATELLITE OBSERVATIONS WITH A SINGLE GUIDE STAR


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Abstract: 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.

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DAYLIGHT OPERATION OF A SODIUM LASER GUIDE STAR FOR ADAPTIVE OPTICS WAVEFRONT SENSING


Abstract: We report contrast measurements of a sodium resonance guide star against the daylight sky when observed through a tuned magneto-optical filter (MOF). The guide star was created by projection of a laser beam at 589.16 nm into the mesospheric sodium layer and the observations were made with a collocated 1.5-m telescope. While MOFs are used with sodium light detecting and ranging systems during the day to improve the signalto-noise ratio of the measurements, they have not so far been employed with laser guide stars to drive adaptive optics (AO) systems to correct atmospherically induced image blur. We interpret our results in terms of the performance of AO systems for astronomy, with particular emphasis on thermal infrared observations at the next generation of extremely large telescopes now being built.

Read Article

A COMPREHENSIVE APPROACH TO HIGH-RESOLUTION DAYLIGHT IMAGING FOR SSA


Abstract:High resolution daytime imaging of resident space objects (RSO) from the ground is presently severely challenging. At visible wavelengths, where diffraction-limited resolution is the highest before the atmosphere becomes opaque in the UV, shot noise from the bright background degrades the information that may be recovered from RSO imagery. Total exposure times must be limited in order to avoid motion blur induced either by the object’s intrinsic rotation or simply by its orbital motion over the site. Fundamentally, then, one cannot collect enough light from the object to achieve adequate signal-to-noise ratio (SNR) in the presence of very high noise before the apparent shape of the object has changed. To overcome this limitation, we propose in this paper a suite of techniques which we believe will collectively enable high-resolution imaging during daylight [...]

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HIGH-RESOLUTION SPECKLE IMAGING THROUGH STRONG ATMOSPHERIC TURBULENCE


Image

Abstract: We demonstrate that high-resolution imaging through strong atmospheric turbulence can be achieved by acquiring data with a system that captures short exposure (“speckle”) images using a range of aperture sizes and then using a bootstrap multi-frame blind deconvolution restoration process that starts with the smallest aperture data. Our results suggest a potential paradigm shift in how we image through atmospheric turbulence. No longer should image acquisition and post processing be treated as two independent processes: they should be considered as intimately related.

Read Article