Size Telescopes Research Articles

Unveiling the nature ofINTEGRALobjects through optical spectroscopy

(abridged) Hard X-ray surveys performed by the INTEGRAL satellite have discovered a conspicuous fraction (up to 30%) of unidentified objects among the detected sources. Here we continue our identification program by selecting probable optical candidates using positional cross-correlation with soft X-ray, radio, and/or optical archives, and performing optical spectroscopy on them. As a result, we identified or more accurately characterized 44 counterparts of INTEGRAL sources: 32 active galactic nuclei, with redshift 0.019 < z < 0.6058, 6 cataclysmic variables (CVs), 5 high-mass X-ray binaries (2 of which in the Small Magellanic Cloud), and 1 low-mass X-ray binary. This was achieved by using 7 telescopes of various sizes and archival data from two online spectroscopic surveys. The main physical parameters of these hard X-ray sources were also determined using the available multiwavelength information. AGNs are the most abundant population among hard X-ray objects, and our results confirm this tendency when optical spectroscopy is used as an identification tool. The deeper sensitivity of recent INTEGRAL surveys enables one to begin detecting hard X-ray emission above 20 keV from sources such as LINER-type AGNs and non-magnetic CVs.

ARC: an Achromatic Rotation-shearing Coronagraph

Context. Because of the larger size of future telescopes, the star leakage effect due to the finite diameter of the stars will become a major problem for coronagraphs with very small inner working angles (IWA). Aims. To reduce this star leakage, we propose a new instrumental concept, the Achromatic Rotation-shearing Coronagraph (ARC), that provides a variable IWA that is easily tunable to ensure the observation of Solar-like systems with an extremely large telescope. Methods. The ARC belongs to the nuller class of coronagraphs. It takes advantage of the achromatic dark output of a Mach-Zehnder interferometer (MZI). Field rotators set in the arms of the MZI rotate the π-phase shifted images of the MZI by an angle ψ. An on-axis point source (the star) is nulled as twin-images of an off-axis point source (the planet) are formed. For ψ = π, the ARC is equivalent to the achromatic interfero-coronagraph. For small ψ, the twin images of the off-axis source are formed close to each another, and eventually nulled if too close. In this way, the ARC can get rid of star leakage while optimizing the transmission of the planet. We describe the compensation of polarization effects induced by field rotators. An application to diluted apertures is shortly presented. Results. We perform the mathematical analysis of the technique using Cartesian and polar coordinates, and assuming that the system is optically perfect. Analytic expressions are given for the leakage in both the focal and aperture planes. Simplified expressions for the contrast and signal-to-noise ratio (SNR) in the focal plane are given for a circular aperture, explicitly showing that the level of star leakage decreases as sin 2 (ψ/2), while the SNR increases as the inverse of sin(ψ/2). Conclusions. The ARC is appropriate for the detection of exoplanets and is not intended to provide images of the environment of stars.

CTA—A project for a new generation of Cherenkov telescopes

Gamma-rays provide a powerful insight into the non-thermal universe and perhaps a unique probe for new physics beyond the standard model. Current experiments are already giving results in the physics of acceleration of cosmic rays in supernova remnants, pulsar and active galactic nuclei with a 100 sources detected at very-high-energies so far. Despite its relatively recent appearance, very high-energy gamma-ray astronomy has proven to have reached a mature technology with fast assembling, relatively cheap and reliable telescopes. The goal of future installation is to increase the sensitivity by a factor 10 compared to current installations, and enlarge the energy domain from few 10s of GeV to a 100 TeV. Gamma-ray spectra of astrophysical origin are rather soft thus hardly one single size telescope can cover more than 1.5 decades in energy, therefore an array of telescopes of 2–3 different sizes is required. Hereafter, we present design considerations for a Cherenkov Telescope Array (CTA), a project for a new generation of highly automated telescopes for gamma-ray astronomy. The status of the project, technical solutions and an insight in the involved physics will be presented.

An image of an exoplanet separated by two diffraction beamwidths from a star

Three exoplanets around the star HR 8799 have recently been discovered by means of differential imaging with large telescopes. Bright scattered starlight limits high-contrast imaging to large angular offsets, currently of the order of ten diffraction beamwidths, 10lambda/D, of the star (where lambda is the wavelength and D is the aperture diameter). Imaging faint planets at smaller angles calls for reducing the starlight and associated photon and speckle noise before detection, while efficiently transmitting nearby planet light. To carry out initial demonstrations of reduced-angle high-contrast coronagraphy, we installed a vortex coronagraph capable of reaching small angles behind a small, well-corrected telescope subaperture that provides low levels of scattered starlight. Here we report the detection of all three HR 8799 planets with the resultant small-aperture (1.5 m) system, for which only 2lambda/D separate the innermost planet from the star, with a final noise level within a factor of two of that given by photon statistics. Similar well-corrected small-angle coronagraphs should thus be able to detect exoplanets located even closer to their host stars with larger ground-based telescopes, and also allow a reduction in the size of potential space telescopes aimed at the imaging of very faint terrestrial planets.

How to simulate Galaxy Evolution: strengths and weaknesses of numerical techniques

Galaxy formation and evolution is one of the most challenging tasks in astrophysics. There is strongly increasing and substantial progress in this field not only due to the increase of telescope size and developments of new techniques providing observations allover the electromagnetic spectrum, but also the development of advanced computer architectures and of sophisticated numerical codes allow to tackle this problem from both sides and to link both research approaches to a fruitful progress in astrophysics. This review will not focus on the application of a single numerical method and the description of its result, but is instead aiming at providing an overview of different numerical tools on the basis of particle and grid schemes and to referring to important publications for the intersted reader.

The Antarctic Submillimetre Telescope

This report aims to provide a summary of the status of our Antarctic Submillimetre Telescope (AST) project up to date. It is a very new project for Antarctic astronomy. Necessary prerequisites for a future deployment of a large size telescope infrastructure have been tested in years 2007 and 2008. The knowledge of the transmission, frost formation and temperature gradient were fundamental parameters before starting a feasibility study. The telescope specifications and requirements are currently discussed with the industrial partnership.

A generation of astronomical telescopes, their users, and publications

Research astronomers and the telescopes they use each have typical life spans of about 40 years. Most of their journals live a good deal longer, though the second most important one today is only 40 years old. This paper looks at numbers for productivity and impact of specific astronomical facilities, changes in equality of opportunities and achievements in observational astronomy, and some aspects of national contributions. The focus is on optical astronomy, though something is also said about radio telescopes and astronomy from space. In summary, nothing stays “best of class” for very long; the fraction of the community with access to the most valuable facilities has increased with time (more equality of opportunity); but the fraction of citations earned by the few super-star papers has also increased (less equality of achievement); and the USA remains the host of the most-cited journals and the most productive telescopes, though Europe (meaning in this context the member nations of the European Southern Observatory, the European Space Agency, and the supporters of the journal Astronomy & Astrophysics) are fast closing the gap, with the UK retaining its own journal and some observing facilities not shared with either the USA or other European countries. Detailed examination of specific facilities indicates that size (of telescope, community, and budget) are all of great importance, but that the most significant “focal plane instrument” is still the astronomer at the virtual eyepiece. The changes have happened against a background of enormous increases in numbers of astronomers, sizes of available facilities (but not total number), numbers of papers (but not of journals), and numbers of citations per paper. A significant subset of the conclusions on turnover of people and facilities accompanying major growth: opportunity versus achievement; Europe versus the USA; and the trade-off between community size and the influence of individual scientists undoubtedly apply in many other fields.

TIDALLY INDUCED OUTBURSTS IN OJ 287 DURING 2005-2008

The blazar OJ 287 has produced two major optical outburst events during the years 2005-2008. These are the latest in a series of outbursts that have occurred repeatedly at 12 year intervals since early 1900s. It has been possible to explain the historical light fairly well by using a binary black hole model where the secondary black hole impacts the accretion disk of the primary twice during the 12 year orbital cycle. We will ask here how well does the latest light-curve fit with this model. We use a 10 million particle disk to model the accretion disk of the primary black hole. The rate of transfer of particles through the 10 Schwarzschild radius cylinder around the primary is followed. The secondary induces an inward flow through this surface. The inward flow rate is compared with the historical light as well as with the most recent observations reported in this paper. The observations have been carried out by using a number of small and medium size telescopes in different locations in order to ensure a dense light-curve coverage. The inflow light curve and the optical light of OJ 287 have a close resemblance to each other. It suggests that the tidally induced accretion flow is responsible for the main features of the optical light curve, with the exception of the quasi-periodic double peaks. It implies a close connection between the accretion disk and the jet where the optical synchrotron emission is presumably generated.

AKARI: space infrared cooled telescope

AKARI, formerly known as ASTRO-F, is the second Japanese space mission to perform infrared astronomical observations. AKARI was launched on 21 February 2006 (UT) and brought into a sun-synchronous polar orbit at an altitude of 700 km by a JAXA M-V rocket. AKARI has a telescope with a primary-mirror aperture size of 685 mm together with two focal-plane instruments on board: the Infrared Camera (IRC), which covers the spectral range 2–26 μm and the Far-Infrared Surveyor (FIS), which operates in the range 50–180 μm. The telescope mirrors are made of sandwich-type silicon carbide, specially developed for AKARI. The focal-plane instruments and the telescope are cooled by a unique cryogenic system that kept the telescope at 6K for 550 days with 180 l super-fluid liquid Helium (LHe) with the help of mechanical coolers on board. Despite the small telescope size, the cold environment and the state-of-the-art detectors enable very sensitive observations at infrared wavelengths. To take advantage of the characteristics of the sun-synchronous polar orbit, AKARI performed an all-sky survey during the LHe holding period in four far-infrared bands with FIS and two mid-infrared bands with IRC, which surpasses the IRAS survey made in 1983 in sensitivity, spatial resolution, and spectral coverage. AKARI also made over 5,000 pointing observations at given targets in the sky for approximately 10 min each, for deep imaging and spectroscopy from 2 to 180 μm during the LHe holding period. The LHe ran out on 26 August 2007, since which date the telescope and instrument are still kept around 40K by the mechanical cooler on board, and near-infrared imaging and spectroscopic observations with IRC are now being continued in pointing mode.

Modal Fourier Wavefront Reconstruction Using FPGA Technology

Atmospheric turbulence introduces optical aberration into wavefronts arriving at ground-based telescopes. The wavefront reconstruction using fast Fourier transforms (FFT) and spatial filtering is computationally tractable and sufficiently accurate in large Shack-Hartmann-based adaptive optics systems (up to 10,000 actuators). A first design has been developed using a Graphical Processing Units (GPU) platform. However, an increase in telescope size requires significant computational power. For this reason other hardware technologies must be taken into account during the development of a specific processor. In this paper, an improvement on the efficiency of the above methods is proposed based on Field Programmable Gate Array (FPGA) technology. To the best of our knowledge, there are not other wavefront reconstruction implementations based on FPGA technology. The basic advantages of this technology are its flexible architecture and extremely highperformance signal processing capability which are captured through parallelism. The implemented phase recoverer is, consequently, faster than the CPU or the GPU solution. Furthermore, the implemented design incorporating this technology meets current and future adaptive optics image processing frame rate requirements. Keywords: Adaptive optics, FPGA, Phase recovery, Fast fourier transform, DSP

Achieving resolution beyond the diffraction limit of telescopic optics with remote masking

The resolution of a telescope is traditionally limited by the size of its aperture and the associated diffraction limit. Increases in resolving power invariably require larger apertures. The dependence of resolution on a telescope size can be completely removed, however, by a technique referred to as remote masking. This technique involves the use of a small occulting object that is positioned to mask a portion of the target. The drop in total target intensity of the unresolved object corresponds directly to the brightness of the area being masked. Moving the mask in front of the target modulates the total observed intensity. The observer can thus create an image having a resolution comparable to the angular size of the mask. The concept was validated through a one-dimensional experiment, where two light sources were successfully resolved at an angular separation well below the diffraction limit of the telescope. The theoretical resolving power of a remotely masked telescope is limited only by the size of, and distance to, the occulting mask.

New Electronics for the Cherenkov Telescope Array

Very high energy gamma-ray astronomy is now bringing an invaluable contribution to the understanding of violent phenomena in the Universe, as well as the search for exotic physics such as indirect detection of dark matter or a test of Lorentz invariance violation. The current Imaging Arrays of Cherenkov Telescopes (IACT) show that this technique is mature. In Europe, the community is gathering around the Cherenkov Telescope Array consortium, to design and build the next generation ground-based array. It should reach an order of magnitude in sensitivity in a wide energy band, ranging from 10 GeV to more than 100 TeV. This goal can be achieved with an array of 50–100 telescopes of various sizes at various spacings. With about 2000 channels per camera, a specific effort has to be made to design front-end electronics with a lower cost and better performances. A gain in cost and performances can be obtained by maximising the integration of the front-end electronics in an ASIC. The amplifiers, analogue memories, digitization and first level buffering can be embedded in the same component. We present here the NECTAr project aiming at building a demonstrator element of a generic camera built around this component.

COMMISSION 29: STELLAR SPECTRA

The members of IAU Commission 29 Stellar Spectra are actively engaged in the quantitative analysis of spectra of various types of stars. With large and medium size telescopes equipped with high resolution spectrographs LTE and Non-LTE analysis of spectra of all types stars are being carried out. Spectra of stars in our Galaxy, in globular and open clusters, stars in LMC and SMC and in nearby galaxies are being studied. Accurate chemical composition analysis of various types of stars has been carried out during the past three years. Now the analysis of stellar spectra covers the wavelength range from X-ray region to IR and sub-millimeter range. Recently stellar spectra are being analysed using time-dependent, 3D, hydrodynamical model atmospheres to derive accurate stellar abundances.

A spectral line survey of Orion KL in the bands 486–492 and 541–577 GHz with the Odin satellite

Spectral line surveys are useful since they allow identification of new molecules and new lines in uniformly calibrated data sets. Nonetheless, large portions of the sub-millimetre spectral regime remain unexplored due to severe absorptions by H2O and O2 in the terrestrial atmosphere. The purpose of the measurements presented here is to cover wavelength regions at and around 0.55 mm -- regions largely unobservable from the ground. Using the Odin astronomy/aeronomy satellite, we performed the first spectral survey of the Orion KL molecular cloud core in the bands 486--492 and 541--576 GHz with rather uniform sensitivity (22--25 mK baseline noise). Odin's 1.1 m size telescope, equipped with four cryo-cooled tuneable mixers connected to broad band spectrometers, was used in a satellite position-switching mode. Two mixers simultaneously observed different 1.1 GHz bands using frequency steps of 0.5 GHz (25 hours each). An on-source integration time of 20 hours was achieved for most bands. The entire campaign consumed ~1100 orbits, each containing one hour of serviceable astro-observation. We identified 280 spectral lines from 38 known interstellar molecules (including isotopologues) having intensities in the range 80 to 0.05 K. An additional 64 weak lines remain unidentified. Apart from the ground state rotational 1(1,0)--1(0,1) transitions of ortho-H2O, H218O and H217O, the high energy 6(2,4)--7(1,7) line of para-H2O and the HDO(2,0,2--1,1,1) line have been observed, as well as the 1,0--0,1 lines from NH3 and its rare isotopologue 15NH3. We suggest assignments for some unidentified features, notably the new interstellar molecules ND and SH-. Severe blends have been detected in the line wings of the H218O, H217O and 13CO lines changing the true linewidths of the outflow emission.

An acre of glass: a history and forecast of the telescope

Ever since Galileo glimpsed the moons of Jupiter through his tiny telescope, astronomers have striven to understand how the universe is constructed and how it evolves. Every increase in the size and power of telescopes has led to new discoveries. Within the last century, telescopes have enabled us to learn about pulsars, quasars, gamma-ray bursters, black holes, and the acceleration of the universe's expansion. An Acre of Glass describes how recent innovations in telescope technology have led to the construction of giant, ground-based observatories and to an explosive development of astronomy. Today, telescopes with mirrors 30, 50, and even 100 meters in diameter are being built. Optical interferometers that cover an acre of ground are observing every night. J. B. Zirker shows us how telescopes past, present, and future are built and describes the exciting science they reveal-from planets beyond our solar system to supermassive black holes at the core of distant galaxies. For every striking image revealed through these enormous telescopes, technicians and scientists must overcome unique and incredible challenges. How many pack animals does it take to get a telescope to the top of a mountain? How do you make the shape of a 6-foot-wide mirror accurate to within a 1,000th the thickness of a human hair? In clear and accessible language, Zirker answers these questions and more, providing fascinating technical detail about how a telescope is made and what the next generation can hope to see.

Atmospheric turbulence compensation with laser phase shifting interferometry

Laser guide stars with adaptive optics allow astronomical image correction in the absence of a natural guide star. Single guide star systems with a star created in the earth's sodium layer can be used to correct the wavefront in the near infrared spectral regime for 8-m class telescopes. For possible future telescopes of larger sizes, or for correction at shorter wavelengths, the use of a single guide star is ultimately limited by focal anisoplanatism that arises from the finite height of the guide star. To overcome this limitation we propose to overlap coherently pulsed laser beams that are expanded over the full aperture of the telescope, traveling upwards along the same path which light from the astronomical object travels downwards. Imaging the scattered light from the resultant interference pattern with a camera gated to a certain height above the telescope, and using phase shifting interferometry we have found a method to retrieve the local wavefront gradients. By sensing the backscattered light from two different heights, one can fully remove the cone effect, which can otherwise be a serious handicap to the use of laser guide stars at shorter wavelengths or on larger telescopes. Using two laser beams multiconjugate correction is possible, resulting in larger corrected fields. With a proper choice of laser, wavefront correction could be expanded to the visible regime and, due to the lack of a cone effect, the method is applicable to any size of telescope. Finally the position of the laser spot could be imaged from the side of the main telescope against a bright background star to retrieve tip-tilt information, which would greatly improve the sky coverage of the system.

Instrumental stability requirements for exoplanet detection with a nulling interferometer: variability noise as a central issue

We revisit the nulling interferometer performances that are needed for direct detection and the spectroscopic analysis of exoplanets, e.g., with the DARWIN [European Space Agency-SCI 12 (2000)] or TPF-I [JPL Publ. 05-5, (2005)] missions. Two types of requirement are found, one concerning the mean value of the instrumental nulling function (nl(lambda)) and another regarding its stability. The stress is usually put on the former. It is stringent at short wavelengths but somewhat relaxed at longer wavelengths. The latter, which we call the variability noise condition, does not usually receive enough attention. It is required regardless of telescope size and stellar distance. The results from three nulling experiments performed in laboratories around the world are reported and compared with the requirements. All three exhibit 1/f noise that is incompatible with the performances required by the mission. As pointed out by Lay [Appl. Opt. 43, 6100-6123 (2004)], this stability problem is not fully solved by modulation techniques. Adequate solutions must be found that are likely to include servo systems using the stellar signal itself as a reference and internal metrology with high stability.

Wide-field spectroscopy and ELT science

Wide-field spectroscopy, in its various forms, has much to contribute to ELT science, so care is needed in trade-offs between telescope size and field of view. Integral field spectroscopy over large areas at high spatial resolution, and especially multiple integral fields, will be essential tools. For wide-field surveys, next-generation multi-object spectrographs (MOS) on 8m-class telescopes will likely out-perform similar instruments on ELTs, due to the smaller fields of view of the current ELT designs. However, there may be over wide fields. New technologies such as OH-suppression fibres offer revolutionary gains, so there is a difficult balance to be achieved in applying the latest technology and having instruments ready for ELT first light.

Astronomical quantum optics with Extremely Large Telescopes

Modern optics focuses on photonics and quantum optics, studying individual photons and statistics of photon streams. Those can be complex and carry information beyond that recorded by imaging, spectroscopy, polarimetry or interferometry. Since [almost] all astronomy is based upon the interpretation of subtleties in the light from astronomical sources, quantum optics has the potential of becoming another information channel from the Universe. The observability of quantum statistics increases rapidly with telescope size making photonic astronomy very timely in an era of very large telescopes.

Where are the X-ray quasi-periodic oscillations in active galaxies?

In this paper, we address the question of whether existing X-ray observations of Seyfert galaxies are sufficiently sensitive to detect quasi-periodic oscillations (QPOs) similar to those observed in the X-ray variations of Galactic black holes (GBHs). We use data from XMM‐Newton and simulated data based on the best Rossi X-ray Timing Explorer (RXTE) long-term monitoring light curves to show that if X-ray QPOs are present in Seyfert X-ray light curves ‐ with similar shapes and strengths to those observed in GBHs, but at lower frequencies commensurate with their larger black hole masses ‐ they would be exceedingly difficult to detect. Our results offer a simple explanation for the present lack of QPO detections in Seyferts. We discuss the improvements in telescope size and monitoring patterns needed to make QPO detections feasible. The most efficient type of future observatory for searching for X-ray QPOs in active Galactic nuclei (AGN) is an X-ray All-Sky Monitor (ASM). A sufficiently sensitive ASM would be ideally suited to detect low-frequency QPOs in nearby AGN. The detection of AGN QPOs would strengthen the AGN‐GBH connection, and could serve as powerful diagnostics of the black hole mass and the structure of the X-ray emitting region in AGN. Ke yw ords: galaxies: active ‐ galaxies: Seyfert ‐ X-ray: galaxies.