Image Enhancement with Blind Deconvolution in Millimeter-Wave 3-D FLoSAR

As having the imaging ability of area in front of flight direction, forward-looking synthetic aperture radar (SAR) has become a hot topic in areas of SAR research. Nevertheless, existing imaging algorithms of forward-looking SAR are all based on the assumption that the platform is moving in low velocity. When this assumption is not satisfied, the imaging result of existing algorithms will show focus out effect and deviation of imaging position from real position. Aiming at this problem and taking airborne forward-looking SAR system using single transmitting and multiple receiving antennas as the research object, this paper analyzes the feasibility of forward-looking SAR imaging of high velocity platform, and proposes a forward-looking SAR imaging algorithm suitable for high velocity platform according to its spatial geometry and the model of echo signal. Finally, the raw echo data of an array of point targets is simulated, and the focused images are obtained using the algorithm proposed in this paper. Imaging results demonstrate the validity of the algorithm. (6 pages)

Side-looking SAR cannot image targets in flight direction. Introduced here is the bistatic missile-borne forward-looking SAR system, as an efficient approach to forward-looking SAR imaging. According to the spatial geometry and the model of echo signal, the signal spectrum derived by the method of series reversion, the phase compensation factors and the realized step of the proposed algorithm are given. Several simulation and processing results are presented to demonstrate the validity of the proposed algorithm.

The past ten years has seen very significant advances in high resolution imaging arising from the construction and routine operation of Very Long Baseline Interferometer arrays in the US, Europe and Australia. Coupled with the launch of the first VLBI antennas into earth orbit, angular resolution below a milliarcsecond has been achieved on a large number of compact objects. Nevertheless, while the application of phase referencing to VLBI observations has increased the number of sources that can be imaged by VLBI, the astrophysical impact of interferometry on baselines greater than a few tens of km, has been severely limited by sensitivity. The next decade promises to bring about very significant increases in continuum sensitivity at high angular resolution through new developments in VLBI recording devices to achieve recording bandwidths of 4 to 8 Gbits per second, and by direct optical fibre-linked long baseline arrays. In the following decade, the Square Kilometre Array will herald a new era of high resolution radio astronomy with a factor of 100 sensitivity increase. In addition to vastly increasing the sample of non-thermal sources accessible to VLBI, the SKA will open up high angular resolution imaging to a new regime of astrophysics, by enabling milliarcsecond resolution imaging of thermal radio emission. At wavelengths of a few centimetres the SKA, in combination with next generation space VLBI missions, will allow direct imaging of the x-ray emitting accretion disks in galactic nuclei.

Aims. We aim to investigate the origin of the HI Brγ emission in young stars by using GRAVITY to image the innermost region of circumstellar disks, where important physical processes such as accretion and winds occur. With high spectral and angular resolution, we focus on studying the continuum and the HI Brγ-emitting area of the Herbig star HD 58647. Methods. Using VLTI-GRAVITY, we conducted observations of HD 58647 with both high spectral and high angular resolution. Thanks to the extensive uv coverage, we were able to obtain detailed images of the circumstellar environment at a sub-au scale, specifically capturing the continuum and the Brγ-emitting region. Through the analysis of velocity-dispersed images and photocentre shifts, we were able to investigate the kinematics of the HI Brγ-emitting region. Results. The recovered continuum images show extended emission where the disk major axis is oriented along a position angle of 14°. The size of the continuum emission at 5-σ levels is ~1.5 times more extended than the sizes reported from geometrical fitting (3.69 mas ± 0.02 mas). This result supports the existence of dust particles close to the stellar surface, screened from the stellar radiation by an optically thick gaseous disk. Moreover, for the first time with GRAVITY, the hot gas component of HD 58647 traced by the Brγ has been imaged. This allowed us to constrain the size of the Brγ-emitting region and study the kinematics of the hot gas; we find its velocity field to be roughly consistent with gas that obeys Keplerian motion. The velocity-dispersed images show that the size of the hot gas emission is from a more compact region than the continuum (2.3 mas ± 0.2 mas). Finally, the line phases show that the emission is not entirely consistent with Keplerian rotation, hinting at a more complex structure in the hot gaseous disk.

High angular resolution millimeter observations of circumstellar disks

Large surveys at high angular resolution have a lot of interest for astrophysical studies. Their achievements with space missions imply a significant data transmission if a resolution close to 0.1 arcsec is wished. A satisfying telemetry rate is conceivable thanks to a selection of the significant information on board. An image composed of detected stars is first subtracted. A thresholding is then applied in order to keep significant wavelet coefficients. Coding these bright stars as a catalogue with a position and a magnitude estimated on board is less expensive for the telemetry than the coding of their images on the focal plane. Tests were carried out with the technical features of the European astrometric Gaia mission. The consequence of such a lossly compression on the restored images are illustrated. At the end of the space mission, thanks to a combination of fields with different orientations, an improvement of 2 to 3 magnitudes for the detection and a higher resolution are obtained. Even though this approach showed us some difficulties and limits for the Gaia mission, it allowed us to conceive a specific mission dedicated to a full-sky imaging at high angular resolution.

High angular resolution (HAR) observations of evolved giants and super-giants in the infrared can answer some of the most interesting questions related to grain condensation, mass loss and hence stellar evolution. Lunar occultation is a powerful and productive technique at present for achieving HAR particularly in the near infrared region, because of the reduced background of scattered moon light compared to the optical region. The technique consists of recording the straight edge diffraction pattern of the star light produced by the sharp edge of the moon and obtaining HAR by mathematical modelling. The most attractive aspect of the method is its ability to achieve high one dimensional angular resolution down to 1–2 mas with an accuracy which can be as high as ∽ 0.1 mas. However careful consideration of various factors like optical bandwidth of the filter used, telescope aperture and time constant of recording system that influence the occultation light curve is necessary before such HAR can be achieved in practice.

Conventional techniques for target discrimination, by high resolution, show severe disadvantages when faced with ballistic missiles. High range resolution (HRR), for example, usually applies to a single, isolated target that moves with almost constant velocity. Ballistic missile targets are often multiple and tend not to move uniformly, so, for these targets, the usual HRR techniques must be adapted. A ballistic missile target can extend for 1 km or more while requiring a range resolution of less than 1 m to resolve it. Often the number of frequencies, available for HRR is restricted by the radar platform. For example, the case here discussed is restricted to 256 frequencies. This paper considers new methodologies for stepped frequency waveforms that have been designed to cope with ballistic missile characteristics. A 1 km profile could be generated, to a desired resolution, by a long sequence of stepped frequencies. Alternatively, it could be generated by concatenating (stitching together) a number of smaller profiles derived from shorter sequences sampled in different range bins. We have investigated two super-resolution techniques for long sequences and two for stitched sequences. These show that a long profile can be achieved. The technique of stitching short profiles has its problems. If the profiles are simply bolted together (hard stitching) then the inaccuracies of alignment can be severe. For this reason we have investigated so called soft stitching. This aligns the profiles and creates an image in which they overlap. The result is a 1 km long profile without severe inaccuracy due to motion.

A light field print (LFP) displays three-dimensional (3D) information to the naked-eye observer under ambient white light illumination. Changing perspectives of a 3D image are seen by the observer from varying angles. However, LFPs appear pixelated due to limited resolution and misalignment between their lenses and colour pixels. A promising solution to create high-resolution LFPs is through the use of advanced nanofabrication techniques. Here, we use two-photon polymerization lithography as a one-step nanoscale 3D printer to directly fabricate LFPs out of transparent resin. This approach produces simultaneously high spatial resolution (29–45 µm) and high angular resolution (~1.6°) images with smooth motion parallax across 15 × 15 views. Notably, the smallest colour pixel consists of only a single nanopillar (~300 nm diameter). Our LFP signifies a step towards hyper-realistic 3D images that can be applied in print media and security tags for high-value goods.

Compared to traditional digital cameras, light field (LF) cameras measure not only the intensity of rays, but also their light field information. As LF cameras trade a good deal of spatial resolution for extra angular information, they provide lower spatial resolution than traditional digital cameras. In this paper, we show a hybrid imaging system consisting of a LF camera and a high-resolution traditional digital camera, achieving both high spatial resolution and high angular resolution. We build an example prototype using a Lytro camera and a DSLR camera to generate a LF image with 10 megapixel spatial resolution and get high-resolution digital refocused images, multi-view images and all-focused images.

We have used the Australia Telescope Compact Array (ATCA) to make the first high resolution observations of a large sample of class~I methanol masers in the 95-GHz ($8_0$--$7_1$A$^+$) transition. The target sources consist of a statistically complete sample of 6.7-GHz class~II methanol masers with an associated 95-GHz class~I methanol maser, enabling a detailed study of the relationship between the two methanol maser classes at arcsecond angular resolution. These sources have been previously observed at high resolution in the 36- and 44-GHz transitions, allowing comparison between all three class~I maser transitions. In total, 172 95-GHz maser components were detected across the 32 target sources. We find that at high resolution, when considering matched maser components, a 3:1 flux density ratio is observed between the 95- and 44-GHz components, consistent with a number of previous lower angular resolution studies. The 95-GHz maser components appear to be preferentially located closer to the driving sources and this may indicate that this transition is more strongly inverted nearby to background continuum sources. We do not observe an elevated association rate between 95-GHz maser emission and more evolved sources, as indicated by the presence of 12.2-GHz class~II masers. We find that in the majority of cases where both class~I and class~II methanol emission is observed, some component of the class~I emission is associated with a likely outflow candidate.

Context. Photonic lanterns (PLs) are waveguide devices enabling high-throughput single-mode spectroscopy and high angular resolution. Aims. We aim to present the first on-sky demonstration of a PL operating in visible light, to measure its throughput and assess its potential for high-resolution spectroscopy of compact objects. Methods. We used the SCExAO instrument (a double-stage extreme adaptive optics system installed at the Subaru Telescope) and FIRST mid-resolution spectrograph (R 3000) to test the visible capabilities of the PL on internal source and on-sky observations. Results. The best averaged coupling efficiency over the PL field of view was measured at 51% ± 10%, with a peak at 80%. We also investigated the relationship between coupling efficiency and the Strehl ratio for a PL, comparing them with those of a single-mode fiber (SMF). Findings show that in the adaptive optics regime a PL offers a better coupling efficiency performance than an SMF, especially in the presence of low-spatial-frequency aberrations. We observed Ikiiki (α Leo – mR = 1.37) and ‘Aua (α Ori – mR = −1.17) at a frame rate of 200 Hz. Under median seeing conditions (about 1 arcsec measured in the H band) and large tip or tilt residuals (over 20 mas), we estimated an average light coupling efficiency of 14.5% ± 7.4%, with a maximum of 42.8% at 680 nm. We were able to reconstruct both star’s spectra, containing various absorption lines. Conclusions. The successful demonstration of this device opens new possibilities in terms of high-throughput single-mode fiber-fed spectroscopy in the visible. The demonstrated on-sky coupling efficiency performance would not have been achievable with a single SMF injection setup under similar conditions, partly because the residual tip or tilt alone exceeded the field of view of a visible SMF (18 mas at 700 nm). This emphasizes the enhanced resilience of PL technology to such atmospheric disturbances. The additional capabilities in high angular resolution are also promising but still have to be demonstrated in a forthcoming investigation.

High angular resolution techniques at infrared and centimeter to millimeter wavelengths have become of ever increasing importance for astrophysical research in the past decade. They have led to important breakthroughs, like the direct imaging of protoplanetary discs and of the first exoplanets, the measurement of stellar orbits around the black hole at the center of the Milky Way, or the detection of sub-parsec-scale jets in low luminosity AGN. With adaptive optics in a mature state, infrared/optical astronomy is pushing toward extreme adaptive optics, extremely large telescopes, and infrared/optical interferometry with large aperture telescopes. At longer wavelengths, large arrays start to conquer the sub-millimeter window, with the mid-term goal of global VLBI at sub-millimeter wavelengths. These new techniques will have enormous impact on the field because they will enable us to address issues such as directly measuring the properties of exoplanets, imaging the surfaces of stars, examining stellar dynamics in extremely dense cluster cores, disentangling the processes at the bottom of black hole accretion flows in the jet launching region, or testing general relativity in the strong gravity regime near the event horizon of supermassive black holes. The conference The Universe under the Microscope: Astrophysics at High Angular Resolution, held at the Physikzentrum of the Deutsche Physikalische Gesellschaft in Bad Honnef, Germany, on 12–25 April 2008, aimed at an interdisciplinary approach by bringing together astrophysicists from the three great branches of the field, instrumentation, observation, and theory, to discuss the current state of research and the possibilities offered by the next-generation instruments. Editors of the proceedings Rainer Schödel Instituto de Astrofísica de Andalucía -CSIC, Granada, Spain Andreas Eckart I. Physikalisches Institut der Universität zu Köln, Köln, Germany Susanne Pfalzner I. Physikalisches Institut der Universität zu Köln, Köln, Germany Eduardo Ros Max-Planck-Institut für extraterrestrische Physik, Bonn, Germany Scientific organizing committee Dennis Downes Institut de Radio Astronomie Millimétrique, France Wolfgang Duschl University of Kiel, Germany Andrea Ghez University of California, Los Angeles, USA Vladimir Karas Academy of Sciences, Czech Republic Andreas Eckart University of Cologne, Germany Sera Marko University of Amsterdam, The Netherlands Susanne Pfalzner University of Cologne, Germany Sebastian Rabien Max-Planck-Institute for Extraterrestrial Physics, Germany Daniel Rouan Observatoire de Paris-Meudon, France Eduardo Ros Max-Planck-Institute for Radio Astronomy, Germany Rainer Schödel Instituto de Astrofísica de Andalucía -CSIC, Spain Zhiqiang Shen Shanghai Astronomical Observatory, China Anton Zensus Max-Planck-Institute for Radio Astronomy, Germany Local organizing committee Sebastian Fischer University of Cologne, Germany Devaky Kunneriath University of Cologne, Germany Leo Meyer University of Cologne, Germany Koraljka Muzic University of Cologne, Germany Rainer Schödel University of Cologne, Germany/IAA -CSIC, Spain Christian Straubmeier University of Cologne, Germany Mohammad Zamaninasab University of Cologne, Germany

To accelerate the acquisition of simultaneously high spatial and angular resolution diffusion imaging. Accelerated imaging is achieved by recovering the diffusion signal at all voxels simultaneously from under-sampled k-q space data using a compressed sensing algorithm. The diffusion signal at each voxel is modeled as a sparse complex Gaussian mixture model. The joint recovery scheme enables incoherent under-sampling of the 5-D k-q space, obtained by randomly skipping interleaves of a multishot variable density spiral trajectory. This sampling and reconstruction strategy is observed to provide considerably improved reconstructions than classical k-q under-sampling and reconstruction schemes. The complex model enables to account for the noise statistics without compromising the computational efficiency and theoretical convergence guarantees. The reconstruction framework also incorporates compensation of motion induced phase errors that result from the multishot acquisition. Reconstructions of the diffusion signal from under-sampled data using the proposed method yields accurate results with errors less that 5% for different accelerations and b-values. The proposed method is also shown to perform better than standard k-q acceleration schemes. The proposed scheme can significantly accelerate the acquisition of high spatial and angular resolution diffusion imaging by accurately reconstructing crossing fiber architectures from under-sampled data.

Super-multiview content with high angular resolution: 3D quality assessment on horizontal-parallax lightfield display