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.     

Stellar intensity interferometry: Prospects for sub-milliarcsecond optical imaging

Using 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, enabling images of rapidly rotating hot stars with structures in their circumstellar disks and winds, or mapping out patterns of nonradial pulsations across stellar surfaces. Intensity interferometry (once 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 (～10 m) with very fast detectors (～ns) are becoming available as the arrays primarily erected to measure Cherenkov light emitted in air by particle cascades initiated by energetic gamma rays. Planned facilities (e.g., CTA, Cherenkov Telescope Array) envision many tens of telescopes distributed over a few square km. Digital signal handling enables very many baselines (from tens of meters to over a kilometer) to be simultaneously synthesized between many pairs of telescopes, while stars may be tracked across the sky with electronic time delays, in effect synthesizing an optical interferometer in software. Simulated observations indicate limiting magnitudes around m_V = 8, reaching angular resolutions ～30 μarcsec 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 directly provides the modulus (but not phase) of any spatial frequency component of the source image; for this reason a full image reconstruction requires phase retrieval techniques. This is feasible if sufficient coverage of the interferometric (u, v)-plane is available, as was verified through numerical simulations. Laboratory and field experiments are in progress; test telescopes have been erected, intensity interferometry has been achieved in the laboratory, and first full-scale tests of 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.     

Optical long baseline intensity interferometry: prospects for stellar physics

More than sixty years after the first intensity correlation experiments by Hanbury Brown and Twiss, there is renewed interest for intensity interferometry techniques for high angular resolution studies of celestial sources. We report on a successful attempt to measure the bunching peak in the intensity correlation function for bright stellar sources with 1 meter telescopes (I2C project). We propose further improvements of our preliminary experiments of spatial interferometry between two 1 m telescopes, and discuss the possibility to export our method to existing large arrays of telescopes.     

Determination of linear polarization and faraday rotation of pulsar signals from spectral intensity modulation

Most of the known pulsars are sources of highly linearly polarized radiation. Faraday rotation in the intervening medium rotates the plane of the linear polarization as the signals propagate through the medium. The Rotation Measure (RM), which quantifies the amount of such rotation as a function of wavelength, is useful in studying the properties of the medium and in recovering the intrinsic polarization characteristics of the pulsar signal. Conventional methods for polarization measurements use telescopes equipped with dual orthogonally polarized feeds that allow estimation of all 4 Stokes parameters. Some telescopes (such as the Ooty Radio Telescope) that offer high sensitivity for pulsar observations may however be receptive to only a single linear polarization. In such a case, the apparent spectral intensity modulation, resulting from differential Faraday rotation of the linearly polarized signal component within the observing bandwidth, can be exploited to estimate the RM as well as to study the linear polarization properties of the source. In this paper, we present two improved procedures by which these observables can be estimated reliably from the intensity modulation over large bandwidths, particularly at low radio frequencies. We also highlight some other applications where such measurements and procedures would be useful.     

Ground-based interferometry

This paper summarizes the limits of ground-based interferometry for differential astrometry as well as ground-based interferometry for direct detection of exo-planets and exo-zodi dust levels. For direct detection, ground-based interferometry at near IR wavelengths using large telescopes with adaptive optics offers a significant advantage over single telescopes with adaptive optics. Ground-based differential astrometry for exo-planet detection is extremely accurate with sufficient accuracy to detect Neptune mass planets around 400–600 nearby stars. Ground-based interferometry using large (>6m) telescopes is also capable of detecting the 10 m emission of the zodiacal light around nearby stars with zodi levels similar to our solar system     

Submillimeter interferometry

The technical feasibility of submillimeter interferometry at Mauna Kea, Hawaii, by connecting existing and planned optical/IR telescopes as well as submillimeter telescopes is discussed.Paper presented at the Symposium on the JNLT and Related Engineering Developments, Tokyo, November 19–December 2, 1988.Nobeyama Radio Observatory is a branch of the National Astronomical Observatory, the Ministry of Education, Science and Culture of Japan.     

Asteroseismology and interferometry

Asteroseismology provides us with a unique opportunity to improve our understanding of stellar structure and evolution. Recent developments, including the first systematic studies of solar-like pulsators, have boosted the impact of this field of research within astrophysics and have led to a significant increase in the size of the research community. In the present paper we start by reviewing the basic observational and theoretical properties of classical and solar-like pulsators and present results from some of the most recent and outstanding studies of these stars. We centre our review on those classes of pulsators for which interferometric studies are expected to provide a significant input. We discuss current limitations to asteroseismic studies, including difficulties in mode identification and in the accurate determination of global parameters of pulsating stars, and, after a brief review of those aspects of interferometry that are most relevant in this context, anticipate how interferometric observations may contribute to overcome these limitations. Moreover, we present results of recent pilot studies of pulsating stars involving both asteroseismic and interferometric constraints and look into the future, summarizing ongoing efforts concerning the development of future instruments and satellite missions which are expected to have an impact in this field of research.     

Spectral speckle interferometry

A method of obtaining an objective prism spectrum of a stellar object with diffraction-limited spatial resolution is described.Paper presented at the Symposium on the JNLT and Related Engineering Developments, Tokyo, November 29–December 2, 1988.     

The development of astronomical interferometry

Astronomical interferometry was pioneered by Fizeau and Michelson in the 19^th century. In the 1920s, the first stellar diameters were measured. The development of radio interferometry began in the 1950s, and led to the construction of powerful synthesis arrays operating at cm, mm, and sub-mm wavelengths. Modern computer and control technology has enabled the interferometric combination of light from separate telescopes also in the visible and infrared regimes. Imaging with milliarcsecond resolution and astrometry with microarcsecond precision have thus become reality.     

Microwave radiometry and interferometry of Uranus

We present high-resolution interferometry of Uranus at 6 cm wavelength and single-dish observations of the disk-averaged brightness temperature, T_B, at 2.8 and 4.8 cm wavelength. The 1978 measurements of T_B of 228 ± 2,243 ± 9, and 259 ± 4 K at 2.8, 4.8, and 6 cm, respectively, support the finding of M. J. Klein and J. A. Turegano (1978, Astrophy. J.224, L31–L34) that the brightness temperature of Uranus has been rising. There is no evidence for radio emission from outside the visible disk at 6 cm. Radiation from a synchrotron radiation belt or from the Uranian rings is certainly less than 10% of the total radio flux. The interferometry shows a possible 55 ± 20 K difference in brightness temperature between the equator and the currently exposed pole. The pole appears to be ～275 K while the equator is ～220 K. However, a permanent gradient of this magnitude is insufficient to account for the rise in disk-averaged brightness by simple reorientation of Uranus' globe relative to our line of sight. The changing insolation probably triggers a redistribution of the trace constituent NH₃ which is responsible for the radio opacity. The NH₃ may be interacting strongly with H₂S on Uranus.     

Infrared interferometry with single telescopes

We examine the merits of three basic interferometric techniques using two-dimensional infrared arrays: speckle interferopmetry, pupil plane interferometry, and speckle holography.Paper presented at the Symposium on the JNLT and Related Engineering Developments, Tokyo, November 29–December 2, 1988.     

Combining optical spectroscopy and interferometry

Modern optical spectrographs and optical interferometers push the limits in the spectral and spatial regime, providing important new tools for the exploration of the Universe. In this contribution I outline the complementary nature of spectroscopic and interferometric observations and discuss different strategies for combining such data. Most remarkable, the latest generation of “spectro‐interferometric” instruments combine the milliarcsecond angular resolution achievable with interferometry with spectral capabilities, enabling direct constraints on the distribution, density, kinematics, and ionization structure of the gas component in protoplanetary disks. I will present some selected studies from the field of star‐ and planet formation and hot star research in order to illustrate these fundamentally new observational opportunities. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Compression of ephemerides

Approximations in the normL ₁ by Chebyshev polynomials are generated to represent astronomical ephemerides over large intervals of time.     

Optical interferometry – A practice primer

The existing and planned interferometers differ largely in the size of telescope area, baseline length and wavelength at which they operate. In this article we focus on the common principle between all of them and discuss the three main ingredients of an interferometer: the light collectors, the delay lines and the beam combiner. We take a look into the different approaches taken, e.g. beam travel in vacuum or air, the use of spatial filters, interferometer layouts etc. Furthermore, we describe the subsystems which are helping in the formation of fringes, namely adaptive optics, fringe trackers and phase reference systems. This revised version was published online in July 2006 with corrections to the Cover Date.     

Observations of circumstellar disks with infrared interferometry

The study of circumstellar disks around young stellar objects is arguably the area of astrophysics on which the technique of infrared interferometry has had the biggest impact. Here I will review the existing set of observations in this field, concentrating on disks but also including jets/winds and stellar properties. At the end, there is a brief discussion of how ongoing technical developments and observational improvements will expand the impact of infrared interferometry on the study of star formation.     

Spatial interferometry in the mid-infrared region

The potential of high-resolution spatial interferometry for detailed mapping and precision astrometry in the mid-infrared region, somewhat analogous to interferometry now done in the microwave region, is discussed from an instrumental point of view. Some results from a prototype system and from tests of atmospheric properties are given. The design of a more advanced two-telescope system now under construction is outlined. This involves movable telescopes of 1.65 m aperture and of high precision, using heterodyne detection of infrared in the 10 Μm atmospheric window.     

The angular diameter of vesta from speckle interferometry

It is shown that the autocorrelation function of a telescope diffraction-limited image is closely approximated by a subtraction of the mean cross correlation of pairs of speckle photographs from the mean autocorrelation of the same set of data. This fact is used to derive the angular diameter of the asteroid Vesta from a series of speckle interferometry data. The resultant apparent angular diameter of 0″.40 ± 0″.04 corresponds to an absolute diameter of 513 ± 51 km.