761 research outputs found
Optical Intensity Interferometry with the Cherenkov Telescope Array
With its unprecedented light-collecting area for night-sky observations, the
Cherenkov Telescope Array (CTA) holds great potential for also optical stellar
astronomy, in particular as a multi-element intensity interferometer for
realizing imaging with sub-milliarcsecond angular resolution. Such an
order-of-magnitude increase of the spatial resolution achieved in optical
astronomy will reveal the surfaces of rotationally flattened stars with
structures in their circumstellar disks and winds, or the gas flows between
close binaries. Image reconstruction is feasible from the second-order
coherence of light, measured as the temporal correlations of arrival times
between photons recorded in different telescopes. This technique (once
pioneered by Hanbury Brown and Twiss) connects telescopes only with electronic
signals and is practically insensitive to atmospheric turbulence and to
imperfections in telescope optics. Detector and telescope requirements are very
similar to those for imaging air Cherenkov observatories, the main difference
being the signal processing (calculating cross correlations between single
camera pixels in pairs of telescopes). Observations of brighter stars are not
limited by sky brightness, permitting efficient CTA use during also bright-Moon
periods. While other concepts have been proposed to realize kilometer-scale
optical interferometers of conventional amplitude (phase-) type, both in space
and on the ground, their complexity places them much further into the future
than CTA, which thus could become the first kilometer-scale optical imager in
astronomy.Comment: Astroparticle Physics, in press; 47 pages, 10 figures, 124 reference
Stellar Intensity Interferometry: Prospects for sub-milliarcsecond optical imaging
Using kilometric arrays of air Cherenkov telescopes, intensity interferometry
may increase the spatial resolution in optical astronomy by an order of
magnitude, enabling images of rapidly rotating stars with structures in their
circumstellar disks and winds, or mapping out patterns of nonradial pulsations
across stellar surfaces. Intensity interferometry (pioneered by Hanbury Brown
and Twiss) connects telescopes only electronically, and is practically
insensitive to atmospheric turbulence and optical imperfections, permitting
observations over long baselines and through large airmasses, also at short
optical wavelengths. The required large telescopes with very fast detectors are
becoming available as arrays of air Cherenkov telescopes, distributed over a
few square km. Digital signal handling enables very many baselines to be
synthesized, while stars are tracked with electronic time delays, thus
synthesizing an optical interferometer in software. Simulated observations
indicate limiting magnitudes around m(v)=8, reaching resolutions ~30
microarcsec in the violet. The signal-to-noise ratio favors high-temperature
sources and emission-line structures, and is independent of the optical
passband, be it a single spectral line or the broad spectral continuum.
Intensity interferometry provides the modulus (but not phase) of any spatial
frequency component of the source image; for this reason image reconstruction
requires phase retrieval techniques, feasible if sufficient coverage of the
interferometric (u,v)-plane is available. Experiments are in progress; test
telescopes have been erected, and trials in connecting large Cherenkov
telescopes have been carried out. This paper reviews this interferometric
method in view of the new possibilities offered by arrays of air Cherenkov
telescopes, and outlines observational programs that should become realistic
already in the rather near future.Comment: New Astronomy Reviews, in press; 101 pages, 11 figures, 185
reference
Resolving Horizon-Scale Dynamics of Sagittarius A*
Sagittarius A* (Sgr A*), the supermassive black hole at the heart of our
galaxy, provides unique opportunities to study black hole accretion, jet
formation, and gravitational physics. The rapid structural changes in Sgr A*'s
emission pose a significant challenge for traditional imaging techniques. We
present dynamic reconstructions of Sgr A* using Event Horizon Telescope (EHT)
data from April 6th and 7th, 2017, analyzed with a one-minute temporal
resolution with the Resolve framework. This Bayesian approach employs adaptive
Gaussian Processes and Variational Inference for data-driven
self-regularization. Our results not only fully confirm the initial findings by
the EHT Collaboration for a time-averaged source but also reveal intricate
details about the temporal dynamics within the black hole environment. We find
an intriguing dynamic feature on April 6th that propagates in a clock-wise
direction. Geometric modelling with ray-tracing, although not fully conclusive,
indicates compatibility with high-inclination configurations of about , as seen in other studies
Long-baseline optical intensity interferometry: Laboratory demonstration of diffraction-limited imaging
A long-held vision has been to realize diffraction-limited optical aperture
synthesis over kilometer baselines. This will enable imaging of stellar
surfaces and their environments, and reveal interacting gas flows in binary
systems. An opportunity is now opening up with the large telescope arrays
primarily erected for measuring Cherenkov light in air induced by gamma rays.
With suitable software, such telescopes could be electronically connected and
also used for intensity interferometry. Second-order spatial coherence of light
is obtained by cross correlating intensity fluctuations measured in different
pairs of telescopes. With no optical links between them, the error budget is
set by the electronic time resolution of a few nanoseconds. Corresponding
light-travel distances are approximately one meter, making the method
practically immune to atmospheric turbulence or optical imperfections,
permitting both very long baselines and observing at short optical wavelengths.
Previous theoretical modeling has shown that full images should be possible to
retrieve from observations with such telescope arrays. This project aims at
verifying diffraction-limited imaging experimentally with groups of detached
and independent optical telescopes. In a large optics laboratory, artificial
stars were observed by an array of small telescopes. Using high-speed
photon-counting solid-state detectors, intensity fluctuations were
cross-correlated over up to 180 baselines between pairs of telescopes,
producing coherence maps across the interferometric Fourier-transform plane.
These measurements were used to extract parameters about the simulated stars,
and to reconstruct their two-dimensional images. As far as we are aware, these
are the first diffraction-limited images obtained from an optical array only
linked by electronic software, with no optical connections between the
telescopes.Comment: 13 pages, 9 figures, Astronomy & Astrophysics, in press. arXiv admin
note: substantial text overlap with arXiv:1407.599
The Rapid Transient Surveyor
The Rapid Transient Surveyor (RTS) is a proposed rapid-response, high-cadence
adaptive optics (AO) facility for the UH 2.2-m telescope on Maunakea. RTS will
uniquely address the need for high-acuity and sensitive near-infrared spectral
follow-up observations of tens of thousands of objects in mere months by
combining an excellent observing site, unmatched robotic observational
efficiency, and an AO system that significantly increases both sensitivity and
spatial resolving power. We will initially use RTS to obtain the infrared
spectra of ~4,000 Type Ia supernovae identified by the Asteroid
Terrestrial-Impact Last Alert System over a two year period that will be
crucial to precisely measuring distances and mapping the distribution of dark
matter in the z < 0.1 universe. RTS will comprise an upgraded version of the
Robo-AO laser AO system and will respond quickly to target-of-opportunity
events, minimizing the time between discovery and characterization. RTS will
acquire simultaneous-multicolor images with an acuity of 0.07-0.10" across the
entire visible spectrum (20% i'-band Strehl in median conditions) and <0.16" in
the near infrared, and will detect companions at 0.5" at contrast ratio of
~500. The system will include a high-efficiency prism integral field unit
spectrograph: R = 70-140 over a total bandpass of 840-1830 nm with an 8.7" by
6.0" field of view (0.15" spaxels). The AO correction boosts the infrared
point-source sensitivity of the spectrograph against the sky background by a
factor of seven for faint targets, giving the UH 2.2-m the H-band sensitivity
of a 5.7-m telescope without AO.Comment: 15 pages, 10 figures, 3 table
Sparse Array Architectures for Wireless Communication and Radar Applications
This thesis focuses on sparse array architectures for the next generation of wireless communication, known as fifth-generation (5G), and automotive radar direction-of-arrival (DOA) estimation. For both applications, array spatial resolution plays a critical role to better distinguish multiple users/sources. Two novel base station antenna (BSA) configurations and a new sparse MIMO radar, which both outperform their conventional counterparts, are proposed.\ua0We first develop a multi-user (MU) multiple-input multiple-output (MIMO) simulation platform which incorporates both antenna and channel effects based on standard network theory. The combined transmitter-channel-receiver is modeled by cascading Z-matrices to interrelate the port voltages/currents to one another in the linear network model. The herein formulated channel matrix includes physical antenna and channel effects and thus enables us to compute the actual port powers. This is in contrast with the assumptions of isotropic radiators without mutual coupling effects which are commonly being used in the Wireless Community.\ua0Since it is observed in our model that the sum-rate of a MU-MIMO system can be adversely affected by antenna gain pattern variations, a novel BSA configuration is proposed by combining field-of-view (FOV) sectorization, array panelization and array sparsification. A multi-panel BSA, equipped with sparse arrays in each panel, is presented with the aim of reducing the implementation complexities and maintaining or even improving the sum-rate.\ua0We also propose a capacity-driven array synthesis in the presence of mutual coupling for a MU-MIMO system. We show that the appearance of\ua0grating lobes is degrading the system capacity and cannot be disregarded in a MU communication, where space division\ua0multiple access (SDMA) is applied. With the aid of sparsity and aperiodicity, the adverse effects of grating lobes and mutual coupling\ua0are suppressed and capacity is enhanced. This is performed by proposing a two-phase optimization. In Phase I, the problem\ua0is relaxed to a convex optimization by ignoring the mutual coupling and weakening the constraints. The solution of Phase I\ua0is used as the initial guess for the genetic algorithm (GA) in phase II, where the mutual coupling is taken into account. The\ua0proposed hybrid algorithm outperforms the conventional GA with random initialization.\ua0A novel sparse MIMO radar is presented for high-resolution single snapshot DOA estimation. Both transmit and receive arrays are divided into two uniform arrays with increased inter-element spacings to generate two uniform sparse virtual arrays. Since virtual arrays are uniform, conventional spatial smoothing can be applied for temporal correlation suppression among sources. Afterwards, the spatially smoothed virtual arrays satisfy the co-primality concept to avoid DOA ambiguities. Physical antenna effects are incorporated in the received signal model and their effects on the DOA estimation performance are investigated
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