2,007 research outputs found
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
Stellar intensity interferometry: Optimizing air Cherenkov telescope array layouts
Kilometric-scale optical imagers seem feasible to realize by intensity
interferometry, using telescopes primarily erected for measuring Cherenkov
light induced by gamma rays. Planned arrays envision 50--100 telescopes,
distributed over some 1--4 km. Although array layouts and telescope sizes
will primarily be chosen for gamma-ray observations, also their interferometric
performance may be optimized. Observations of stellar objects were numerically
simulated for different array geometries, yielding signal-to-noise ratios for
different Fourier components of the source images in the interferometric
-plane. Simulations were made for layouts actually proposed for future
Cherenkov telescope arrays, and for subsets with only a fraction of the
telescopes. All large arrays provide dense sampling of the -plane due to
the sheer number of telescopes, irrespective of their geographic orientation or
stellar coordinates. However, for improved coverage of the -plane and a
wider variety of baselines (enabling better image reconstruction), an exact
east-west grid should be avoided for the numerous smaller telescopes, and
repetitive geometric patterns avoided for the few large ones. Sparse arrays
become severely limited by a lack of short baselines, and to cover
astrophysically relevant dimensions between 0.1--3 milliarcseconds in visible
wavelengths, baselines between pairs of telescopes should cover the whole
interval 30--2000 m.Comment: 12 pages, 10 figures; presented at the SPIE conference "Optical and
Infrared Interferometry II", San Diego, CA, USA (June 2010
Stellar Intensity Interferometry: Astrophysical targets for sub-milliarcsecond imaging
Intensity interferometry permits very long optical baselines and the
observation of sub-milliarcsecond structures. Using planned kilometric arrays
of air Cherenkov telescopes at short wavelengths, intensity interferometry may
increase the spatial resolution achieved in optical astronomy by an order of
magnitude, inviting detailed studies of the shapes of rapidly rotating hot
stars with structures in their circumstellar disks and winds, or mapping out
patterns of nonradial pulsations across stellar surfaces. Signal-to-noise in
intensity interferometry 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. Prime candidate sources have been
identified among classes of bright and hot stars. Observations are simulated
for telescope configurations envisioned for large Cherenkov facilities,
synthesizing numerous optical baselines in software, confirming that
resolutions of tens of microarcseconds are feasible for numerous astrophysical
targets.Comment: 12 pages, 4 figures; presented at the SPIE conference "Optical and
Infrared Interferometry II", San Diego, CA, USA (June 2010
On the Bohnenblust-Hille inequality and a variant of Littlewood's 4/3 inequality
The search for sharp constants for inequalities of the type Littlewood's 4/3
and Bohnenblust-Hille, besides its pure mathematical interest, has shown
unexpected applications in many different fields, such as Analytic Number
Theory, Quantum Information Theory, or (for instance) in deep results on the
-dimensional Bohr radius. The recent estimates obtained for the multilinear
Bohnenblust-Hille inequality (in the case of real scalars) have been recently
used, as a crucial step, by A. Montanaro in order to solve problems in the
theory of quantum XOR games. Here, among other results, we obtain new upper
bounds for the Bohnenblust-Hille constants in the case of complex scalars. For
bilinear forms, we obtain the optimal constants of variants of Littlewood's 4/3
inequality (in the case of real scalars) when the exponent 4/3 is replaced by
any As a consequence of our estimates we show that the optimal
constants for the real case are always strictly greater than the constants for
the complex case
- …