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

Robustness analysis and station-keeping control of an interferometer formation flying mission in low Earth orbit

The impact of formation flying on interferometry is growing over the years for the potential performance it could offer. However, it is still an open field, and many studies are still required. This article presents the basic principles behind interferometry focusing first on a single array and secondly on a formation of satellites. A sensitivity analysis is carried out to evaluate how the performance of the interferometry is affected by an error in the relative position in the formation geometry. This is estimated by computing the loss of the performance in terms of percentage deviation due to a non-nominal relative trajectory, including two-dimensional errors and defining a payload index. The main goal of this study is to estimate whether some errors in the relative state are more impacting than others. The final objective is to compute the link between a position error and a specific loss of performance, to foresee the origin of the the error. Furthermore, a dynamical model is developed to describe the relative motion in the Low Earth Orbit environment, considering both the unperturbed and the J2 and drag contributions. A Proportional, Integral and Derivative controller is implemented for the position control of a multiple satellite formation flying, considering a low thrust control profile. The Formation Flying L-band Aperture Synthesis study is taken as the case scenario, analysing both nominal and non-nominal configurations. This study serves as a starting point for the development of a combined tool to assess the performance of the interferometry and the control on the relative state for future remote sensing studies involving relative motion

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

TriHex: combining formation flying, general circular orbits and alias-free imaging, for high resolution L-band aperture synthesis

The Soil Moisture and Ocean Salinity (SMOS) mission of the European Space Agency (ESA), together with NASA’s Soil Moisture Active Passive (SMAP) mission, is providing a wealth of information to the user community for a wide range of applications. Although both missions are still operational, they have significantly exceeded their design life time. For this reason, ESA is looking at future mission concepts, which would adequately address the requirements of the passive L-band community beyond SMOS and SMAP. This article proposes one mission concept, TriHex, which has been found capable of achieving high spatial resolution, radiometric resolution, and accuracy, approaching the user needs. This is possible by the combination of aperture synthesis, formation flying, the use of general circular orbits, and alias-free imaging.Peer ReviewedPostprint (author's final draft

Sparse aperture imaging satellite

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2002.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references (p. 213-217).The quest for higher angular resolution in astronomy will inevitably lead to larger and larger apertures. Unfortunately, the diameter size of primary mirrors for space telescopes is limited by the volume and mass constraints of current launch vehicles as well as the scaling laws of manufacturing costs. Efforts are ongoing to break this trend by employing exotic technologies such as deployed segmented mirror telescopes, and sparse aperture optics using interferometry. In order to better understand the technological difficulties involved in designing and building a sparse aperture array, the challenge of building a white light Golay-3 telescope was undertaken. The MIT Adaptive Reconnaissance Golay- 3 Optical Satellite (ARGOS) project exploits wide-angle Fizeau interferometer technology with an emphasis on modularity in the optics and spacecraft subsystems. Unique design procedures encompassing the nature of coherent wavefront sensing, control and combining as well as various systems engineering aspects to achieve cost effectiveness, are developed. To demonstrate a complete spacecraft in a 1-g environment, the ARGOS system is mounted on a frictionless air-bearing, and has the ability to track fast orbiting satellites like the ISS or the planets. Wavefront sensing techniques are explored to mitigate initial misalignment and to feed back real-time aberrations into the optical control loop. This paper presents the results and the lessons learned from the conceive, design and implementation phases of ARGOS. A preliminary assessment shows that the beam combining problem is the most challenging aspect of sparse optical arrays. The need for optical control is paramount due to tight beam combining tolerances. The wavefront sensing/control requirements appear to be a major technology and cost driver.by Soon-Jo Chung.S.M

A geostationary orbit microwave multi-channel radiometer

The geostationary orbit microwave multi-channel radiometer has the advantages of high real-time performance and large coverage, which plays an important role in typhoon, strong precipitation detection, and medium-to-short-term meteorological/oceanic forecasting. However, due to the difficulty in engineering development of the payload, its application on-orbit has not yet been achieved at present. To satisfy the requirements of fine and quantitative application of satellite observation data, a geostationary orbit microwave multi-channel radiometer with a 10-m-caliber is developed, in which the spatial resolution at horizontal polarization is better than 24 km at 54 GHz. In geostationary orbit microwave multi-channel radiometer, a quasi-optical feed network covering nearly 28 frequency octave bands and ranging from 23.8 to 664 GHz is proposed to solve the technical problem of multi-frequency sharing in the system. Meanwhile, a high-precision reflector preparation method and a high-precision unfolding scheme are proposed, which are considered as a solution for the large-diameter reflector with a high maintaining surface accuracy. A high-precision antenna prototype with 0.54-m is developed, and the tests are performed to verify the key technologies, such as the preparation of high-precision grating reflectors at the micron level, high surface accuracy detection, and sub-millimeter wave antenna electrical performance testing. The results indicate that measured main beam efficiency of the 664 GHz antenna is better than 95.5%. In addition, the system sensitivity is greater than 1.5 K, and the calibration accuracy is better than 1.8 K, according to the results of an analysis of the multi-channel radiometer’s essential parameters and calibration errors

透過アレーアンテナの高性能化と多機能化に関する研究

Tohoku University陳強課

Interferometric synthetic aperture sonar system supported by satellite

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