10,320 research outputs found
Stellar intensity interferometry over kilometer baselines: Laboratory simulation of observations with the Cherenkov Telescope Array
A long-held astronomical vision is to realize diffraction-limited optical
aperture synthesis over kilometer baselines. This will enable imaging of
stellar surfaces and their environments, show their evolution over time, and
reveal interactions of stellar winds and gas flows in binary star 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 used also for
intensity interferometry. With no optical connection between the telescopes,
the error budget is set by the electronic time resolution of a few nanoseconds.
Corresponding light-travel distances are on the order of one meter, making the
method practically insensitive to atmospheric turbulence or optical
imperfections, permitting both very long baselines and observing at short
optical wavelengths. Theoretical modeling has shown how stellar surface images
can be retrieved from such observations and here we report on experimental
simulations. In an optical laboratory, artificial stars (single and double,
round and elliptic) are observed by an array of telescopes. Using high-speed
photon-counting solid-state detectors and real-time electronics, intensity
fluctuations are cross correlated between up to a hundred baselines between
pairs of telescopes, producing maps of the second-order spatial coherence
across the interferometric Fourier-transform plane. These experiments serve to
verify the concepts and to optimize the instrumentation and observing
procedures for future observations with (in particular) CTA, the Cherenkov
Telescope Array, aiming at order-of-magnitude improvements of the angular
resolution in optical astronomy.Comment: 18 pages, 11 figures; Presented at SPIE conference on Astronomical
Telescopes + Instrumentation in Montreal, Quebec, Canada, June 2014. To
appear in SPIE Proc.9146, Optical and Infrared Interferometry IV
(J.K.Rajagopal, M.J.Creech-Eakman, F.Malbet, eds.), 201
Quantum-inspired computational imaging
Computational imaging combines measurement and computational methods with the aim of forming images even when the measurement conditions are weak, few in number, or highly indirect. The recent surge in quantum-inspired imaging sensors, together with a new wave of algorithms allowing on-chip, scalable and robust data processing, has induced an increase of activity with notable results in the domain of low-light flux imaging and sensing. We provide an overview of the major challenges encountered in low-illumination (e.g., ultrafast) imaging and how these problems have recently been addressed for imaging applications in extreme conditions. These methods provide examples of the future imaging solutions to be developed, for which the best results are expected to arise from an efficient codesign of the sensors and data analysis tools.Y.A. acknowledges support from the UK Royal Academy of Engineering under the Research Fellowship Scheme (RF201617/16/31). S.McL. acknowledges financial support from the UK Engineering and Physical Sciences Research Council (grant EP/J015180/1). V.G. acknowledges support from the U.S. Defense Advanced Research Projects Agency (DARPA) InPho program through U.S. Army Research Office award W911NF-10-1-0404, the U.S. DARPA REVEAL program through contract HR0011-16-C-0030, and U.S. National Science Foundation through grants 1161413 and 1422034. A.H. acknowledges support from U.S. Army Research Office award W911NF-15-1-0479, U.S. Department of the Air Force grant FA8650-15-D-1845, and U.S. Department of Energy National Nuclear Security Administration grant DE-NA0002534. D.F. acknowledges financial support from the UK Engineering and Physical Sciences Research Council (grants EP/M006514/1 and EP/M01326X/1). (RF201617/16/31 - UK Royal Academy of Engineering; EP/J015180/1 - UK Engineering and Physical Sciences Research Council; EP/M006514/1 - UK Engineering and Physical Sciences Research Council; EP/M01326X/1 - UK Engineering and Physical Sciences Research Council; W911NF-10-1-0404 - U.S. Defense Advanced Research Projects Agency (DARPA) InPho program through U.S. Army Research Office; HR0011-16-C-0030 - U.S. DARPA REVEAL program; 1161413 - U.S. National Science Foundation; 1422034 - U.S. National Science Foundation; W911NF-15-1-0479 - U.S. Army Research Office; FA8650-15-D-1845 - U.S. Department of the Air Force; DE-NA0002534 - U.S. Department of Energy National Nuclear Security Administration)Accepted manuscrip
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
Photon-Efficient Computational 3D and Reflectivity Imaging with Single-Photon Detectors
Capturing depth and reflectivity images at low light levels from active
illumination of a scene has wide-ranging applications. Conventionally, even
with single-photon detectors, hundreds of photon detections are needed at each
pixel to mitigate Poisson noise. We develop a robust method for estimating
depth and reflectivity using on the order of 1 detected photon per pixel
averaged over the scene. Our computational imager combines physically accurate
single-photon counting statistics with exploitation of the spatial correlations
present in real-world reflectivity and 3D structure. Experiments conducted in
the presence of strong background light demonstrate that our computational
imager is able to accurately recover scene depth and reflectivity, while
traditional maximum-likelihood based imaging methods lead to estimates that are
highly noisy. Our framework increases photon efficiency 100-fold over
traditional processing and also improves, somewhat, upon first-photon imaging
under a total acquisition time constraint in raster-scanned operation. Thus our
new imager will be useful for rapid, low-power, and noise-tolerant active
optical imaging, and its fixed dwell time will facilitate parallelization
through use of a detector array.Comment: 11 pages, 8 figure
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
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