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

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

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

Optical aperture synthesis with electronically connected telescopes

Highest resolution imaging in astronomy is achieved by interferometry, connecting telescopes over increasingly longer distances, and at successively shorter wavelengths. Here, we present the first diffraction-limited images in visual light, produced by an array of independent optical telescopes, connected electronically only, with no optical links between them. With an array of small telescopes, second-order optical coherence of the sources is measured through intensity interferometry over 180 baselines between pairs of telescopes, and two-dimensional images reconstructed. The technique aims at diffraction-limited optical aperture synthesis over kilometre-long baselines to reach resolutions showing details on stellar surfaces and perhaps even the silhouettes of transiting exoplanets. Intensity interferometry circumvents problems of atmospheric turbulence that constrain ordinary interferometry. Since the electronic signal can be copied, many baselines can be built up between dispersed telescopes, and over long distances. Using arrays of air Cherenkov telescopes, this should enable the optical equivalent of interferometric arrays currently operating at radio wavelengths.Comment: 9 pages, 2 figures; published under open access in Nature Communications, http://www.nature.com/ncomms

Blur resolved OCT: full-range interferometric synthetic aperture microscopy through dispersion encoding

We present a computational method for full-range interferometric synthetic aperture microscopy (ISAM) under dispersion encoding. With this, one can effectively double the depth range of optical coherence tomography (OCT), whilst dramatically enhancing the spatial resolution away from the focal plane. To this end, we propose a model-based iterative reconstruction (MBIR) method, where ISAM is directly considered in an optimization approach, and we make the discovery that sparsity promoting regularization effectively recovers the full-range signal. Within this work, we adopt an optimal nonuniform discrete fast Fourier transform (NUFFT) implementation of ISAM, which is both fast and numerically stable throughout iterations. We validate our method with several complex samples, scanned with a commercial SD-OCT system with no hardware modification. With this, we both demonstrate full-range ISAM imaging, and significantly outperform combinations of existing methods.Comment: 17 pages, 7 figures. The images have been compressed for arxiv - please follow DOI for full resolutio

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

Imaging stellar surfaces with intensity interferometry

Context. Intensity interferometry was invented and used by R.Hanbury Brown and R.Q.Twiss in the 1960's to measure stellar angular diameters. Its main advantage over conventional interferometry is that it enables very long baselines and is insensitive to poor seeing. However, because it requires very large light collectors, it was never pursued further. The Cherenkov Telescope Array (CTA) is a new upcoming facility that will detect rapid flashes of optical Cherenkov light induced by extraterrestrial gamma-rays. Its large telescopes could very well be used part-time for intensity interferometry. With its 2 km maximum baseline, it could image surfaces of hot stars at an unprecedented sub-milliarcsecond resolution. Aim. To experimentally simulate intensity interferometry in the laboratory with an array analogous to the planned CTA. Methods. Small pinhole apertures were illuminated by experimentally produced light with appropriate quantum statistics to simulate stars. High-speed single-photon counting avalanche diode detectors mounted on laboratory telescopes made up the array, enabling more than 100 baselines. A digital data processor was used to calculate the spatial coherence of the stars. Results. Intensity interferometry was successfully performed for stars of different sizes and shapes. With all the baselines available, it was possible to reconstruct two-dimensional maps of the spatial coherence required for image restoration. Conclusions. The results experimentally demonstrated the validity and potential of a multi-telescope array similar to the CTA for stellar surface imaging.Stjärnorna på himlen syns vara små eftersom de är avlägsna objekt, solar på enorma avstånd. Den närmaste stjärnan är Alfa Centauri på ett avstånd av 4,4 ljusår, cirka 41 miljon miljoner kilometer. Solen är den enda stjärna vars yta vi kan se i detalj medan andra stjärnor är så avlägsna att de inte ens i de största teleskopen syns som mer än små ljusa prickar. De skarpaste bilder som i dag erhålls av himmelsobjekt fås med så kallade interferometrar. Dessa är anläggningar där flera teleskop kopplas ihop för att bilda ett gemensamt större instrument. Kraftfullast bland dessa är Europeiska Sydobservatoriets interferometer i Chile och dess amerikanska motsvarighet i Kalifornien. Med dessa har man lyckats avbilda ett fåtal stora stjärnor. Någon visade sig inte vara rund utan kraftigt avplattad eftersom den snurrar jättesnabbt kring sin axel. Andra stjärnor kan tänkas ha andra former eller kan bestå av flera stjärnor i omloppsbanor tätt kring varandra. Att se stjärnor som utsträckta objekt kan lära oss mycket om dem men också om vår egen stjärna, solen. De stjärnor som hittills kunnat avbildas är jättestjärnor, mycket större än solen, och det finns tusentals ljusa stjärnor som fortfarande bara kan ses som prickar. Bildskärpan i en interferometer bestäms av avståndet mellan de teleskop som ingår i anläggningen: större avstånd ger bättre skärpa. Fastän man sedan länge drömt om att länka teleskop över många kilometrar, är det ännu inte möjligt över mer än ett par hundra meter. Begränsningarna sätts av kraven på extrem precision i hur ljuset mellan teleskopen måste kombineras, samt av luftoron i jordens atmosfär. En annan teknik, så kallad intensitets-interferometri, tillåter längre avstånd mellan teleskopen och därmed en högre bildskärpa. Metoden innebär att det synliga ljuset i teleskopet omvandlas till elektroniska signaler som överförs i kablar utan att störas av luftens turbulens. Nackdelen är att viss information går förlorad, vilket gör det svårare att återskapa bilder av himmelsobjekten. Dessutom kräver denna teknik mycket ljus och därför också stora teleskop. Genom en historisk tillfällighet uppförs nu en anläggning med sådana stora teleskop, CTA, ”Cherenkov Telescope Array”, för ett helt annat huvudändamål, att observera gammastrålning från världsrymden. När energirika gammastrålar tränger in i jordens atmosfär, skapas partiklar som utsänder blixtar av blåaktigt ljus, så kallad Tjerenkovstrålning. Eftersom denna är mycket ljussvag, måste teleskopen vara både stora och många. Teleskopens prestanda råkar motsvara vad som krävs för intensitets-interferometri och möjligheten till denna tillämpning har uppmärksammats inom projektet. Teleskopen kommer att ligga på avstånd upp till ett par kilometrar vilket möjliggör en bildskärpa som är storleksordningen bättre än med dagens anläggningar. Detta kommer att möjliggöra avbildning av främst stjärnor som är hetare än solen (tekniken fungerar bäst för varmare stjärnor). Möjligen kommer man till och med att kunna se silhuetter av planeter när de syns passera över stjärnskivan! Eftersom tekniken aldrig använts med modern digital elektronik, måste metoderna utvecklas och testas innan observationer i full skala kan påbörjas. Detta är vad som gjorts i detta examensarbete. Många små teleskop sattes upp i ett laboratorium i ett mönster motsvarande det kommande CTA. Med denna installation mättes olika konstgjorda stjärnor. Efter analys av mätningarna, kunde storlek och form på de olika ”stjärnorna” bestämmas och det kunde experimentellt visas att teorin fungerade. Detta är första gången som avbildande intensitets-interferometri genomförts för astronomiskt relevanta objekt. Med denna teknik torde det bli möjligt att erhålla bilder av stjärnytor när CTA kommer i drift någon gång kring år 2020

Intensity interferometry: Optical imaging with kilometer baselines

Optical imaging with microarcsecond resolution will reveal details across and outside stellar surfaces but requires kilometer-scale interferometers, challenging to realize either on the ground or in space. Intensity interferometry, electronically connecting independent telescopes, has a noise budget that relates to the electronic time resolution, circumventing issues of atmospheric turbulence. Extents up to a few km are becoming realistic with arrays of optical air Cherenkov telescopes (primarily erected for gamma-ray studies), enabling an optical equivalent of radio interferometer arrays. Pioneered by Hanbury Brown and Twiss, digital versions of the technique have now been demonstrated, reconstructing diffraction-limited images from laboratory measurements over hundreds of optical baselines. This review outlines the method from its beginnings, describes current experiments, and sketches prospects for future observations.Comment: 12 pages, 3 figures, 92 references. Invited keynote talk presented at the conference 'SPIE Astronomical Telescopes + Instrumentation', Edinburgh, Scotland (2016); to be published in SPIE Proc. 9907, 'Optical and Infrared Interferometry and Imaging V

Interferometric synthetic aperture sonar system supported by satellite

Tese de doutoramento. Engenharia Electrotécnica e de Computadores. Faculdade de Engenharia. Universidade do Porto. 200