Scientific and technical results from VINCI using coherent estimation of fringe visibility

Although primarily intended as a test and alignment instrument in order to commission the VLTI, VINCI has taken useful scientific data in its first year and a half of operation. Our results employ coherent integration of fringe visibility in which the actual amplitudes of the raw scans are combined linearly after correcting for the position of the fringe within each scan. In addition to reducing the effect of noise compared to incoherent integration, the result contains a broader range of information, including an estimate of the complex visibility spectrum. Such an estimator is thus sensitive to instrumental phase and spectral characteristics, including the variable component of dispersion introduced by the excess air paths in the delay lines. Calibration of such instrumental effects demonstrates the ability to detect source phase at a fine level as will be required for direct interferometric detection of extra solar planets. We present diameters for five stars obtained by observing the visibility null in their correlated spectra. Using coherent integration we have also observed the peculiar correlated spectra seen in many Mira variables, possibly due to changes in the apparent diameter with wavelength. Calibration of the zero-baseline power from o Ceti is used with other interferometric observations of this star over a period of 90 days to plot diameter variations associated with its pulsation cycle. This revised version was published online in July 2006 with corrections to the Cover Date.     

Extragalactic Astronomy with the VLTI: a new window on the Universe

Interferometry in the optical and near infrared has so far played a marginal role in Extragalactic Astronomy. Active Galactic Nuclei are the brightest and most compact extragalactic sources, nonetheless only a very limited number could be studied with speckle interferometry and none with long baseline interferometry. The VLTI will allow the study of moderately faint extragalactic objects with very high spatial resolution thus opening a new window on the universe. With this paper we focus on three scientific cases to show how AMBER and MIDI can be used to tackle open issues in extragalactic astronomy. This revised version was published online in July 2006 with corrections to the Cover Date.     

Interferometry: The tool to study giant, supergiant and Mira stars

Late-type giant stars have been traditional targets for infrared interferometers. They are bright and big and are therefore easy targets to resolve and to detect. Considerable progress has been and is being made on the spatial structure of these objects thanks to existing interferometers. Beyond the classical measurement of their diameters, pulsations have been directly detected, spatial intensity distributions are more and more understood and more important, consistent scenarios for both spectroscopic and interferometric measurements are on the verge to be validated. All this has been possible with prototype instruments having a small number of baselines and very limited spectral capabilities. AMBER and MIDI will surely open a new era with high spectral resolution, high efficiency and imaging capabilities. This is not an exhaustive review of all the work done in the field but rather a presentation of the context. A recent review of Mira star observations with interferometers was recently written (Scholz, 2003).A brief science case is first introduced in this paper. Achievements with high angular resolution single-telescope techniques are presented. The contributions of optical and infrared interferometers are then explained. Eventually, some hints about the possible progress with VLTI are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Long Range Science Perspectives for the VLTI

VLTI interferometry will allow imaging of galactic and extragalactic sources with milliarcsecond angular resolution. For moderately bright sources the spectral resolution will be of the order of 10000. These capabilities will allow detailed studies of solar system objects, stars, proto-planetary systems and the detection of hot extra-solar planets. The observations of galactic nuclei will allow unprecedented measurements of physical parameters in these systems. VLTI will be a prime instrument to study the immediate environment of the massive black hole at the center of the Milky Way. With the exception of a few `self-referencing' sources the observations of extragalactic nuclei will benefit from an extended capability for simultaneous measurements of nearby reference sources for fringe tracking. With beam combination instruments like AMBER, MIDI, PRIMA, and GENIE the VLTI will reach full maturity at a time when other interferometric instruments at different wavelengths will be fully operational. Most important are ALMA (in the mm- and sub-mm-domain), LOFAR and SKA (in the radio meter to centimeter domain) and of course VLB-networks in the radio, and other – at that time –well developed interferometers in the optical. A major scientific potential of future scientific VLTI programs will lie in an efficient combination of these high angular resolution capabilities. This revised version was published online in July 2006 with corrections to the Cover Date.     

MIDI – the 10 μm instrument on the VLTI

After more than five years of preparation, the mid-infrared interferometric instrument MIDI has been transported to Paranal where it will undergo testing and commissioning on theVery Large Telescope Interferometer VLTI from the end of 2002through large part of this year 2003. Thereafter it will be available as a user instrument to perform interferometric observations over the8 μm–13 μm wavelength range, with a spatial resolution of typically 20 milliarcsec, a spectral resolution of up to 250, and an anticipated point source sensitivity of N = 3–4 mag or 1–2.5 Jy for self –fringe tracking, which will be the only observing mode during the first months of operation. We describe the layout of the instrument, laboratory tests, and expected performance, both for broadband and spectrally resolved observing modes. We also briefly outline the planned guaranteed time observations. This revised version was published online in July 2006 with corrections to the Cover Date.     

Unveiling the Launching Region of YSO Jets with AMBER

The interplay between accretion and ejection in the environment of young stellar objects (YSOs) is believed to be a crucial element in the star formation process. Since most of the properties of the models are set up in the first few AUs from the source (below the so-called Alfvèn surface), to validate and constrain the models observationally we need very high angular resolution. With HST (resolution ∼ 0.'1, i.e. about 14 AU in Taurus)we have been able to access the external border of the acceleration region, for jets in the Taurus-Auriga cloud. Here we see an onion-like kinematic structure in the first 200 AU of the flow, and indications for rotation around the symmetry axis for the resolved low/moderate velocity component. We have now planned observations with AMBER on the VLTI to investigate at 1 mas resolution (in J) the core of the central engine, down to 0.1 AU from the source. Here we describe a joint project by several Institutes in the AMBER consortium dedicated to the study of the morphology and detailed kinematics of a few selected targets. On one hand we will use the large UV coverage of the ATs to explore at medium spectral resolution the structure of the flow. On the other hand the large collection area of the UTs combined with the high resolution mode (R=10000) of AMBER will allow us to search for interesting kinematic features, among which signatures of rotation around the axis, that would constitute an important validation of the proposed models for the jet launching. This revised version was published online in July 2006 with corrections to the Cover Date.     

Measuring the sizes of stars

The Chatterton Astronomy Department aims to apply interferometers with very high resolving power to optical astronomy. The programme of the stellar intensity interferometer at Narrabri Observatory was completed in 1972 and since then the work has been directed towards building a more sensitive instrument with higher resolving power. As a first step a much larger intensity interferometer was designed but was not built because it was large, expensive and not as sensitive as desired. Efforts are now being made to design a more sensitive and cheaper instrument. A version of Michelson’s stellar interferometer is being built using modern techniques. It is hoped that it will reach stars of magnitude +8 and will work reliably in the presence of atmospheric scintillation. It is expected to cost considerably less than an intensity interferometer of comparable performance. The pilot model of this new instrument is almost complete and should be ready for test in 1984. Text of an Academy Lecture delivered at the Raman Research Institute, Bangalore on January 27, 1984.     

Preparation of the AMBER integrations

The Astronomical Multi-Beam Recombiner (AMBER) is a near infrared/red focal interferometric instrument. Its integration takes place in Grenoble where each sub-system is tested, aligned and the AMBER requirements validated. In a preliminary phase the environment of the AMBER integration room was characterized. Several tests were made in order to determine, and when required to reduce, environmental constraints (temperature, turbulence and vibrations of the optical table). This revised version was published online in July 2006 with corrections to the Cover Date.     

African Meridian B-Field Education and Research (AMBER) Array

The AMBER array contains four magnetometers and spans across the geomagnetic equator from L of 1 to an L of 1.4. In addition to filling the largest land-based gap in global magnetometer coverage, the AMBER array will address two fundamental areas of space physics: (1) the processes governing electrodynamics of the equatorial ionosphere as a function of latitude (or L-shell), local time, longitude, magnetic activity, and season, and (2) ULF pulsation strength and its connection with equatorial electrojet strength at low/mid-latitude regions. Satellite observations show unique equatorial ionospheric structures in the African sector, though these have not been confirmed by observation from the ground due to lack of ground-based instruments in the region. In order to have a complete global understanding of equatorial ionosphere motions, deployment of ground-based magnetometers in Africa is essential. One focus of IHY is the deployment of networks of small instruments, including the development of research infrastructure in developing nations through the United Nations Basic Space Science (UNBSS) Small Instrument Array. Therefore, AMBER magnetometer array in partnership with parallel US funded GPS receivers in Africa will allow us to understand the electrodynamics that governs equatorial ionosphere motions. While AMBER routinely observes the F region plasma drift mechanism (E × B drift), the GPS stations will monitor the structure of plasma at low/mid-latitudes in the African sectors. In addition to new scientific discoveries and advancing the space science research into Africa by establishing scientific collaborations between scientists in the developing and developed nations, the AMBER project also contributes to developing the basic science of heliophysics through cross-disciplinary studies of universal process. This includes the creation of sustainable research/training infrastructure within the developing nations (Africa).     

Interferometry with the Large Binocular Telescope

The Large Binocular Telescope (LBT) will be the largest single telescope in the world when it is completed in 2005. The unique structure of the telescope incorporates two, 8.4 meter diameter primary mirrors on a 14.4 meter center-to-center mounting. This configuration provides the equivalent collecting area of a 12 meter telescope, and when combined coherently, the two optical paths offer very interesting possibilities for interferometry. Two initial interferometric instruments are planned for the LBT. A group based at the University of Arizona is constructing LBTI, a pupil-plane, nulling beam combiner operating in the thermal infrared N band. This instrument will search for and measure zodiacal light in candidate stellar systems for the Terrestrial Planet Finder (TPF) and Darwin missions. Expansion ports can accomodate additional instruments. A second group, based in Heidelberg, Arcetri, and Köln, is building LINC-NIRVANA, a near-infrared Fizeau-mode beam combiner. This type of observation preserves phase information and allows true imagery over a wide field of view. Using state-of-the-art detector arrays, coupled with advanced adaptive optics, LINC-NIRVANA will deliver the sensitivity of a 12 m telescope and the spatial resolution of a 23 m telescope, over a field of view up to 2 arc minutes square.     

The Subaru Telescope High Dispersion Spectrograph (HDS)

The High Dispersion Spectrograph (HDS) is the échelle spectrograph for an open‐use instrument of the Subaru Telescope. The current status of the instrument is reviewed. The new image slicers that significantly improve the efficiency of observations with very high resolving power have been installed in the past three years. Brief overview of recent science results is given on studies of early generations of stars and extra‐solar planets. An upgrade plan and future prospects of this instrument are discussed. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Solar dynamics imaging system a back-end instrument for the proposed NLST

The Solar Dynamics Imaging System (SDIS) will be one of the focal plane instruments operated at the National Large Solar Telescope (NLST). The prime objective of the instrument is to obtain high spatial and temporal resolution images of the region of interest on the Sun in the wavelength range from 390 nm to 900 nm. The SDIS provides filtergrams using broad-band filters while preserving the Strehl ratio provided by the telescope. Furthermore, the SDIS is expected to provide observations that allow image reconstruction to extract wave front information and achieve a homogenous image quality over the entire FOV.     

Scientific rationale for the D-CIXS X-ray spectrometer on board ESA's SMART-1 mission to the Moon

The D-CIXS X-ray spectrometer on ESA's SMART-1 mission will provide the first global coverage of the lunar surface in X-rays, providing absolute measurements of elemental abundances. The instrument will be able to detect elemental Fe, Mg, Al and Si under normal solar conditions and several other elements during solar flare events. These data will allow for advances in several areas of lunar science, including an improved estimate of the bulk composition of the Moon, detailed observations of the lateral and vertical nature of the crust, chemical observations of the maria, investigations into the lunar regolith, and mapping of potential lunar resources. In combination with information to be obtained by the other instruments on SMART-1 and the data already provided by the Clementine and Lunar Prospector missions, this information will allow for a more detailed look at some of the fundamental questions that remain regarding the origin and evolution of the Moon.     

Interferometric Observation at Mid-Infrared Wave-lengths with MIDI

MIDI, the MID-Infrared Interferometricnterferometric Instrument for ESO's Very Large Telescope Interferometer (VLTI), will be the first instrument for combining mid-infrared light directly in order to obtain angular resolution up to 10 mas (assuming a 200 m baseline) in a wavelength range from 8 to 13 μm. Currently in the phase of commissioning at Paranal, the start of its scientific operation is expected for summer 2003. Direct interferometry at thermal infrared wavelengths demands special requirements on the instrument and also on the procedures of preparation of data reduction. Hereafter MIDI's different observing modes are described and an example for an interferometric observation is given. This revised version was published online in July 2006 with corrections to the Cover Date.     

Study of X-ray transients with Scanning Sky Monitor (SSM) onboard AstroSat

Scanning Sky Monitor (SSM) onboard AstroSat is an X-ray sky monitor in the energy range 2.5–10 keV. SSM scans the sky for X-ray transient sources in this energy range of interest. If an X-ray transient source is detected in outburst by SSM, the information will be provided to the astronomical community for follow-up observations to do a detailed study of the source in various other bands. SSM instrument, since its power-ON in orbit, has observed a number of X-ray sources. This paper discusses observations of few X-ray transients by SSM. The flux reported by SSM for few sources during its Performance Verification phase (PV phase) is studied and the results are discussed.     

Experiments on the dish verification antenna china for the SKA

The Square Kilometre Array (SKA) is expected to become the world’s most powerful radio telescope at meter and centimeter wavelength in the coming decades. The construction of SKA will be divided into two phases. The first phase (SKA1), scheduled for completion in 2023, will construct 10 % of the whole collecting area. The second phase (SKA2) will build the rest 90 % collecting area. The SKA1 consists of several types of arrays including SKA1-low and SKA1-mid. The latter is a dish array consisting of ~200 medium-size antennas. The integrated dish array in SKA2 will expand to 2500 dishes, spreading 3000 kilometers across the southern part of Africa. The demanding specifications and enormous number of the SKA dish raise challenges in the dish development such as mass production with high performance at low cost, quick installation and high reliability. Dish Verification Antenna China (DVA-C) was built as one of three initial prototypes. A novel single-piece panel reflector made of carbon fiber reinforced polymer (CFRP) was adopted. In this study, an L-band receiver is installed to make DVA-C a complete system for experiments on antenna performance test and preliminary observations. The performance of DVA-C including the system noise temperature, pointing accuracy, antenna pattern, and aperture efficiency has been tested. Preliminary observations such as pulsars and HI are then conducted, which indicates that the DVA-C can not only serve as an educational instrument and key technology test bed, but also be applied for scientific work such as pulsar timing, all-sky HI survey, multi-frequency monitoring of variable sources etc.     

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.     

Prospects of high angular resolution measurements of binary stars properties with VLTI

We present a program for observations of spectroscopic binaries with VLTI. The aim of such observations is to obtain high accuracy measurements of stellar parameters, in particular masses, combining the visual orbit with spectroscopic orbital elements. We selected a sample of spectroscopic binaries, including pre-main sequence stars, using the estimated angular nodal separation as a criterion for feasibility. A cross-check with the 4th catalog of measurements of binary stars (Hartkopf, 2001) was carried out and we discuss the statistical properties of the sample. This revised version was published online in July 2006 with corrections to the Cover Date.     

Solar System Capabilities of the Thirty Meter Telescope

The TMT Project is completing the design of a telescope with a primary mirror diameter of 30 m, yielding ten times more light gathering power than the largest current telescopes. It is being designed from the outset as a system that will deliver diffraction-limited resolution (8, 15 and 70 milliarcsec at 1.2, 2.2 and 10 microns, respectively) and high Strehl ratios over a 30 arcsecond science field with good performance over a 2 arcmin field. Studies of a representative suite of instruments that span a very large discovery space in wavelength (0.3–30 microns), spatial resolution, spectral resolution and field-of-view demonstrate their feasibility and their tremendous scientific potential. Of particular interest for solar system research, one of these will be IRIS (Infrared Imaging Spectrometer), a NIR instrument consisting of a diffraction-limited imager and an integral-field spectrometer. IRIS will be able to investigate structures with dimensions of only a few tens of kilometers at the distance of Jupiter. Two other instruments, NIRES and MIRES (Near- and Mid IR Echelle Spectrographs) will enable high angular, high spectral resolution observations of solar system objects from the ground with sensitivities comparable to space-based missions. The TMT system is being designed for extremely efficient operation including the ability to rapidly switch to observations with different instruments to take advantage of “targets-of-opportunity” or changing conditions. Thus TMT will provide capabilities that will enable very significant solar system science and be highly synergistic with JWST, ALMA and other planned astronomy missions.