Microarcsecond instability of the celestial reference frame

The fluctuation of the angular positions of reference extragalactic radio and optical sources under the influence of the irregular gravitational field of visible Galactic stars is considered. It is shown that these angular fluctuations range from a few up to hundreds of microarcseconds. This leads to a small rotation of the celestial reference frame. The non-diagonal coefficients of the rotation matrix are of the order of a microarcsecond. The temporal variation of these coefficients due to the proper motion of the foreground stars is of the order of one microsecond per 20 years. The celestial reference frame can therefore be considered inertial and homogeneous only to microarcsecond accuracy. Astrometric catalogues with microarcsecond accuracy will be unstable, and must be re-established every 20 years.     

Relativistic astrometry

Concepts of relativistic astrometry — such as Weyl's stellar compass or the concept of flat-space plus forces — are discussed. To visualize effects from light deflection pictures showing the stellar sky as seen from the vicinity of a strongly gravitating source are presented.Communication presented at the International Conference on Astrometric Binaries, held on 13–15 June, 1984, at the Remeis-Sternwarte Bamberg, Germany, to commemorate the 200th anniversary of the birth of Friedrich Wilhelm Bessel (1784–1846).     

Monitoring the Microarcsecond Quasar J1819+3845

We present the results of a year-long monitoring campaign on J1819+3845.We interpret the results of this WSRT campaign to infer critical source parameters such as source lifetime and structure on tens of microarcseconds. The long lifetime of the source at such high brightness temperatures requires continuous energy injection or exotic emission processes. We have previously interpreted the extreme scintillation ofJ1819+3845 as due to a relatively nearby (∼ 20 pc) scattering screen. We show this screen has a velocity w.r.t. the LSR of ∼ 25 kms^-1, as measured by the changing scintillation properties throughout the year: the `velocity parallax'. This revised version was published online in July 2006 with corrections to the Cover Date.     

Australian research effort for the SKA

The next generation radio telescope being planned is the Square Kilometer Array (SKA): an international project which is currently in the research and development phase. Australia is one of the partner countries in the SKA consortium; here I describe some of the SKA research being undertaken in Australia. This revised version was published online in July 2006 with corrections to the Cover Date.     

Solar astrometry by photolithography

Observed temporal variations of shape and size of the solar disk as viewed from Earth may act as constraints for theories of the interior of the Sun. In addition to existing programs of solar diameter measurements we investigate a ground-based photographic method.The solar limb profile is recorded on a photoresist-coated substrate over a 20 radial length simultaneously all along the circonference as a three-dimensional 21 mm diameter, 0.0015mm thick permanent object available for inspection by interferometric methods. The exposure time is long enough for filtering much of the atmospheric turbulence, whereas the slope of the observed solar limb should help to locate a standard solar limb. The first results of February 1989 at large zenith distance and low altitude are a set of differential measurements of the position of a solar limb around a circle with, after taking into account the 3.7 atmospheric differential refraction, a 0.34r.m.s. dispersion of the residuals for a fit to a circular solar disk.We estimate that this method of accuracy comparable to other ground-based methods, with potentially more than 600 independent simultaneous measurements along the circonference, could help to discriminate between terrestrial and solar causes for variations of shape and size of the solar disk.We note that operation outside the Earth's atmosphere would provide access not only to undisturbed images but also to UV wavelengths, i.e., to a better definition of the solar limb.     

Precision astrometry with SuperCOSMOS

We describe the new, fast, high-precision microdensitometer SuperCOSMOS. Some aspects of hardware and software design that enable high-precision astrometry from photographic plates are explained. We show that the positioning repeatability of the measuring machine is less than 0.1 μ μ m standard error in either coordinate, and the absolute positional accuracy is about 0.15 μ m standard error. Furthermore, measurements of the same plate in different orientations show that the sampling errors are small (e.g. ∼ 0.2 μ m, rising to ∼ 1.0 μ m at the plate limit, for stellar images in a IIIaJ emulsion), thus allowing the extraction of relative positional information from Schmidt plates at accuracies less than 1 μ m. We demonstrate that SuperCOSMOS is capable of measuring the positions of bright stars (i.e. those more than ∼ 4 mag above the plate limit) to a precision ∼ 0.5 μ m with survey–grade photographic plates employing fine–grained emulsions.     

Geodynamic implications of astrometry

The transition from local horizon and terrestrial BIH-systems to celestial reference frames is well known to be affected by various geodetic parameters such as polar motion (x_p(t), y_p(t)), UT1-TUC (where UT1 is basically dependent on variations in UT0 and t=time), plumb line deflections (, ) of observation stations, global and local tidal deformations etc. Variations of such quantities with (relative) resolution of the order of 0.001 and better, such as VLBI, demand the application of continuous high-precision (world-wide) geodynamic surveys whenever global theories and sufficient models are not available and the introduction of improved local and global models (geophysical and relativistic) is needed in order to match astrometric observations related to different reference frames. Prediction of parameters for immediate transformation from one system of reference into the other is sometimes of interest.The paperreviews recent results of different observations,points out a number of still open and unresolved problems in observations and modeling, anddiscusses related consequences. Conclusions for geodynamics drawn from comparison of observed data with models based on astronomical and geophysical observations give way to new understanding of basic phenomena of relevance for various disciplines.     

Strong gravitational lensing with SKA

The advent of new observational facilities in the last two decades has allowed the rapid discovery and high-resolution optical imaging of many strong lens systems from galaxy to cluster scales, as well as their spectroscopic follow-up. Radio telescopes have played the dominant role in the systematic detection of dozens of new arcsec-scale lens systems. For the future, we expect nothing less! The next major ground- and space-based facilities, especially the Square Kilometer Array can discover tens of thousands of new lens systems in large sky surveys. For optical imaging and spectroscopic follow-up a strong synergy with planned optical facilities is needed. Here, we discuss the field where strong gravitational lensing is expected to play the dominant role and where SKA can have a major impact: The study of the internal mass structure and evolution of galaxies and clusters to z 1. In addition, studies of more exotic phenomena are contemplated. For example, milli- and micro-lensing can provide a way to measure the mass-functions of stars and CDM substructure at cosmological distances. All-sky radio monitoring will also rapidly develop the field of time-domain lensing.     

Tracking Galaxy Evolution Through Low-Frequency Radio Continuum Observations using SKA and Citizen-Science Research using Multi-Wavelength Data

We present a brief review of progress in the understanding of general spiral and elliptical galaxies, through merger, star formation and AGN activities. With reference to case studies performed with the GMRT, we highlight the unique aspects of studying galaxies in the radio wavelengths where powerful quasars and bright radio galaxies are traditionally the dominating subjects. Though AGN or quasar activity is extremely energetic, it is extremely short-lived. This justify focussing on transitional galaxies to find relic-evidences of the immediate past AGN-feedback which decide the future course of evolution of a galaxy. Relic radio lobes can be best detected in low frequency observations with the GMRT, LOFAR and in future SKA. The age of these relic radio plasma can be as old as a few hundred Myr. There is a huge gap between this and what is found in optical bands. The very first relic-evidences of a past quasar activity (Hanny’s Voorwerp) was discovered in 2007 by a Galaxy Zoo citizen-scientist, a school teacher, in the optical bands. This relic is around a few tens of thousand years old. More discoveries needed to match these time-scales with star formation time-scales in AGN host galaxies to better understand black hole galaxy co-evolution process via feedback-driven quenching of star formation. It is now well-accepted that discovery and characterization of such faint fuzzy relic features can be more efficiently done by human eye than a machine. Radio interferometry images are more complicated than optical and need the citizen-scientists to be trained. RAD@home, the only Indian citizen-science research project in astronomy, analysing TIFR GMRT Sky Survey (TGSS) 150 MHz data and observing from the Giant Meterwave Radio Telescope (GMRT), was launched in April 2013. Unique, zero-infrastructure zero-funded design of RAD@home as a collaboratory of 69 trained e-astronomers is briefly described. Some of the new-found objects like episodic radio galaxies, radio-jet and companion galaxy interaction, radio galaxy bent by motion of the intra-filament medium in a Mpc-scale galaxy filament etc. are briefly presented as demonstration of its potential. Citizen-science has not only opened up a new way for astronomy research but also possibly the only promising way to extract maximum science out of the Big Data in the SKA-era. This possibly can convert the Big Data problem into a prospect. Citizen-science can contribute to the knowledge creation in never-seen-before speed and in approach. As it is based on internet, it can provide an equal opportunity of academic-growth to people even in the under-developed regions where we always need to put our optical and radio telescopes. This can liberate the research-activity of city-based research-institutes out of the four brick walls and alleviate various socio-economic and geo-political constraints on growth of citizens educated in undergraduate-level science but located in remote areas.     

Modern methods in the astrometry of double stars

The techniques for studying double stars continue to evolve in a predictable way. The most recent major breakthroughs have been the area scanner and speckle interferometry. On the horizon looms the application of large format CCDs which will replace the photographic plate for many astrometric problems.Communication presented at the International Conference on Astrometric Binaries, held on 13–15 June, 1984, at the Remeis-Sternwarte Bamberg, Germany, to commemorate the 200th anniversary of the birth of Friedrich Wilhelm Bessel (1784–1846).     

Generalisation of the dependences method in photographic astrometry

A general scheme of the astrometric plate reduction techniques is considered. It is shown that each classical plate-constants method may be formulated as a generalised Schlesinger's dependences method with N parameters, where N is the number of reference stars. The parameters depend upon the model of reduction adopted and common configuration of reference stars and the object.     

The accretion history of the universe with the SKA

In this paper, we investigate how the Square Kilometre Array (SKA) can aid in determining the evolutionary history of active galactic nuclei (AGN) from redshifts z = 0 → 6. Given the vast collecting area of the SKA, it will be sensitive to both ‘radio-loud’ AGN and the much more abundant ‘radio-quiet’ AGN, namely the radio-quiet quasars and their ‘Type-II’ counterparts, out to the highest redshifts. Not only will the SKA detect these sources but it will also often be able to measure their redshifts via the Hydrogen 21-cm line in emission and/or absorption. We construct a complete radio luminosity function (RLF) for AGN, combining the most recent determinations for powerful radio sources with an estimate of the RLF for radio-quiet objects using the hard X-ray luminosity function of [ApJ 598 (2003) 886], including both Type-I and Type-II AGN. We use this complete RLF to determine the optimal design of the SKA for investigating the accretion history of the Universe for which it is likely to be a uniquely powerful instrument.     

Video techniques applied to astrometry

In this work we report on the application of video techniques to the astrometry of fast moving objects in the Solar System. Various phenomena may be observed for astrometric purposes, and two-dimensional photometry is well adapted for this. In this paper we describe in particular the method used to acquire and analyse such observations and we also give the first results obtained concerning two observations of mutual phenomena of the Galilean satellites of Jupiter which occurred during the last (1985) campaign of observations.     

Global astrometry by space techniques

The European Space Agency project of an astrometric satellite-HIPPARCOS-is shortly described. It will measure the angles between stars situated in fields separated by about 70°. The precision of the elementary measurements is expected to be of the order of 0.005. A similar accuracy is found to apply to the basic reduction giving the abcissae of stars referred to great circles on the sky. The final overall reduction should yield accuracies better than 0.002 in position and parallaxes and 0.002 per year in proper motions.The main features of the final catalogue are described and some possible consequences for fundamental astronomy are given.Presented at the Symposium Star Catalogues, Positional Astronomy and Celestial Mechanics, held in honor of Paul Herget at the U.S. Naval Observatory, Washington, November 30, 1978.     

Solar system science with SKA

Radio wavelength observations of solar system bodies reveal unique information about them, as they probe to regions inaccessible by nearly all other remote sensing techniques and wavelengths. As such, the SKA will be an important telescope for planetary science studies. With its sensitivity, spatial resolution, and spectral flexibility and resolution, it will be used extensively in planetary studies. It will make significant advances possible in studies of the deep atmospheres, magnetospheres and rings of the giant planets, atmospheres, surfaces, and subsurfaces of the terrestrial planets, and properties of small bodies, including comets, asteroids, and KBOs. Further, it will allow unique studies of the Sun. Finally, it will allow for both indirect and direct observations of extrasolar giant planets.     

Prospects for astrometry with the Hubble Space Telescope

The Hubble Space Telescope (HST), a large optical telescope having an aperture of 2.4 meters and a length of 8.8 meters, is being developed by the National Aeronautics and Space Administration. This telescope will be placed into earth orbit by the space shuttle. Astrometric observations with the HST are made using a Fine Guidance Sensor which is capable of measuring the position of one object relative to another with an accuracy of ±0.002 arcseconds. The astrometric user of HST will be provided with an Astrometric Data Reduction Software package (ADRS). The variety of astrometric problems to be investigated with HST is discussed.     

Objectives of ground-based astrometry after HIPPARCOS

After a short summary of the results expected from the HIPPARCOS mission, the author gives several examples illustrating the needs that will arise afterwards in order to maintain the level of precision achieved and to extend it to many more objects. Three domains are particularly dealt with in this paper:

1. The maintenance of the HIPPARCOS reference system that will degrade at the rate of 2 mas per year. The instruments intended to contribute to this task should concentrate on observing a few stars as accurately as possible. The best candidates are astrolabes and optical interferometers.
2. The extension of the HIPPARCOS catalogue to more stars using astrographs, Schmidt telescopes, and photoelectric meridian circles. A particularly important objective would be to reduce the GSE catalogue to this system and determine its proper motions with second epoch plates.
3. The determination of new parallaxes and double star parameters in particular with CCD astrometry, astrometric photometry and various interferometric techniques.

     

CCD suitable for double star astrometry

Summary Astrometric measurements of 18 visual double stars carried out with a CCD are given. The present observations are a test of the propriety of the CCD technique for astrometric observations of visual double stars. The duration of the observations was 75 minutes. The precision of the CCD measures corresponds the precision of the photographic method, so the advantages of CCD's are the great loss of reduction work and the corresponding time saving, inclusive shortering the observing time. Based on observations made at E.S.O., La Silla, Chile.