Norbert Zacharias

The large quasar reference frame (LQRF) An optical representation of the ICRS

Context. The large number and all-sky distribution of quasars from different surveys, along with their presence in large, deep astrometric catalogs, enables us to build of an optical materialization of the International Celestial Reference System (ICRS) following its defining principles. Namely: that it is kinematically non-rotating with respect to the ensemble of distant extragalactic objects; aligned with the mean equator and dynamical equinox of J2000; and realized by a list of adopted coordinates of extragalatic sources. Aims. The Large Quasar Reference Frame (LQRF) was built with the care of avoiding incorrect matches of its constituents quasars, homogenizing the astrometry from the different catalogs and lists in which the constituent quasars are gathered, and attaining a milli-arcsec global alignment with the International Celestial Reference Frame (ICRF), as well as typical individual source position accuracies higher than 100 milli-arcsec � . Methods. Starting from the updated and presumably complete Large Quasar Astrometric Catalog (LQAC) list of QSOs, the initial optical positions of those quasars are found in the USNO B1.0 and GSC2.3 catalogs, and from the SDSS Data Release 5. The initial positions are next placed onto UCAC2-based reference frames, This is followed by an alignment with the ICRF, to which were added the most precise sources from the VLBA calibrator list and the VLA calibrator list – when reliable optical counterparts exist. Finally, the LQRF axes are inspected through spherical harmonics, to define right ascension, declination and magnitude terms. Results. The LQRF contains 100,165 quasars, well represented accross the sky, from −83.5 to +88.5 ◦ in declination, being 10 arcmin the average distance between adjacent elements. The global alignment with the ICRF is 1.5 mas, and the individual position accuracies are represented by a Poisson distribution that peaks at 139 mas in right ascension and 130 mas in declination. As a by-product, significant equatorial corrections are found for all the catalogs used (apart from the SDSS DR5), an empirical magnitude correction can be discussed for the GSC2.3 intermediate and faint regimes, both the 2MASS and the preliminary northernmost UCAC2 positions are shown of astrometry consistent with the UCAC2 main catalog, and the harmonic terms are found to be always small. Conclusions. The LQRF contains J2000 referred equatorial coordinates, and is complemented by redshift and photometry information from the LQAC. It is designed to be an astrometric frame, but it is also the basis for the GAIA mission initial quasars’ list, and can be used as a test bench for quasars’ space distribution and luminosity function studies. The LQRF is meant to be updated when new quasar identifications and newer versions of the astrometric frames used are realized. In the later case, it can itself be used to examine the relations between those frames.

VERY LARGE ARRAY PLUS PIE TOWN ASTROMETRY OF 46 RADIO STARS

We have used the Very Large Array (VLA), linked with the Pie Town Very Long Baseline Array antenna, to determine astrometric positions of 46 radio stars in the International Celestial Reference Frame (ICRF). Positions were obtained in the ICRF directly through phase referencing of the stars to nearby ICRF quasars whose positions are accurate at the 0.25 mas level. Radio star positions are estimated to be accurate at the 10 mas level, with position errors approaching a few milli-arcseconds for some of the stars observed. Our measured positions were combined with previous measurements taken from as early as 1978 to obtain proper motion estimates for all 46 stars with average uncertainties of ~1.7 mas/yr. We compared our radio star positions and proper motions with the Hipparcos Catalogue data, and find consistency in the reference frames produced by each data set on the 1-sigma level, with errors of ~2.7 mas per axis for the reference frame orientation angles at our mean epoch of 2003.78. No significant spin is found between our radio data frame and the Hipparcos Celestial Reference Frame (HCRF) with largest rotation rates of +0.55 and -0.41 mas/yr around the x and z axes, respectively, with 1-sigma errors of 0.36 mas/yr. Thus, our results are consistent with a non-rotating Hipparcos frame with respect to the ICRF.We have used the Very Large Array, linked with the Pie Town Very Long Baseline Array antenna, to determine astrometric positions of 46 radio stars in the International Celestial Reference Frame (ICRF). Positions were obtained in the ICRF directly through phase referencing of the stars to nearby ICRF quasars whose positions are accurate at the 0.25 mas level. Radio star positions are estimated to be accurate at the 10 mas level, with position errors approaching a few milliarcseconds for some of the stars observed. Our measured positions were combined with previous measurements taken from as early as 1978 to obtain proper-motion estimates for all 46 stars with average uncertainties of ≈1.7 mas yr-1. We compared our radio star positions and proper motions with the Hipparcos Catalogue data and found consistency in the reference frames produced by each data set on the 1 σ level, with errors of ~2.7 mas per axis for the reference frame orientation angles at our mean epoch of 2003.78. No significant spin is found between our radio data frame and the Hipparcos Celestial Reference Frame, with the largest rotation rates of +0.55 and -0.41 mas yr-1 around the x- and z-axes, respectively, with 1 σ errors of 0.36 mas yr-1. Thus, our results are consistent with a nonrotating Hipparcos frame with respect to the ICRF.

Optical Positions of ICRF Sources Using UCAC Reference Stars

New optical positions on the 30 mas precision level have been obtained for 172 extragalactic International Celestial Reference Frame (ICRF) sources mainly in the range -30° ≤ δ ≤ +25°. Results are presented from a pilot investigation including four Cerro Tololo (CTIO) 0.9 m runs (1999–2001). Reference stars in the R ≈ 10–16.5 mag range from a preliminary US Naval Observatory CCD Astrograph Catalog (UCAC) are used. Systematic errors have been investigated, and a field distortion pattern based on the residuals has been removed. The errors of the fainter stars in the CTIO data were assessed by evaluating an auxiliary set of CCD observations of common ICRF sources, taken at the 1.60 m Cassegrain telescope of the Laboratorio Nacional de Astrofisica, Brazil. A significant improvement in the optical positions was achieved over a previous determination of source positions. The mean optical positions are compared with the ICRF radio positions. The overall optical minus radio offsets are -6 and -15 mas for right ascension and declination, respectively. The formal internal error of these mean offsets is ≈2.3 mas. This indicates a possible systematic error in the UCAC declinations of ≈10 to 15 mas. Both the optical counterpart observations and the optical reference stars are observed about 9 yr after the Hipparcos mean epoch, and our results set an upper limit for a possible Hipparcos system rotation with respect to the International Celestial Reference System for the z-axis of about 0.7 mas yr-1.

Deep Astrometric Standards and Galactic Structure

The advent of next-generation imaging telescopes, such as the Large Synoptic Survey Telescope (LSST) and the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS), has revitalized the need for deep and precise reference frames. The proposed weak-lensing observations with these facilities put the highest demands on image quality over wide angles on the sky. It is particularly difficult to achieve a subarcsecond point-spread function on stacked images, where precise astrometry plays a key role. Current astrometric standards are insufficient to achieve the science goals of these facilities. We thus propose the establishment of a few selected deep ( ) astrometric standards (DAS). These will enable a reliable geometric calibration of solid-state V p 25 mosaic detectors in the focal plane of large ground-based telescopes, and will make a substantial contribution to our understanding of stellar populations in the Milky Way. In this paper we examine the need for such standards and discuss the strategy for selecting them and their acquisition and reduction techniques. The feasibility of DAS is demonstrated by a pilot study around the open cluster NGC 188, using the Kitt Peak National Observatory 4 m CCD Mosaic camera, and by Subaru Suprime-Cam observations. The goal of reaching an accuracy of 5-10 mas in positions and obtaining absolute proper motions good to 2 mas yr 1 over a several square-degree area is challenging, but reachable with the NOAO 4 m telescopes and CCD mosaic imagers, or a similar setup. Our proposed DAS aims to establish four fields near the Galactic plane, at widely separated coordinates. In addition to their utilitarian purpose for DAS, the data we will obtain in these fields will enable fundamental Galactic science in their own right. The positions, proper motions, and VI photometry of faint stars will address outstanding questions of Galactic disk formation and evolution, stellar buildup, and mass assembly via merger events.

CTIO 0.9m observations of ICRF optical counterparts

Abstract : We present astrometric results from 7 observing runs at the Cerro Tololo Interamerican Observatory (CTIO) 0.9m telescope of 197 extragalactic reference frame sources, selected from the original International Celestial Reference Frame (ICRF) catalog. This is part of the U.S. Naval Observatory (USNO) reference frame link program. Contemporaneous to the CTIO deep imaging, wide-field CCD data were taken with the USNO Twin Astrogragh to provide accurate secondary reference stars in the 13 to 16 mag range. The optical positions are on the Hipparcos system (via Tycho-2 stars). The unweighted, mean RMS position difference optical-radio for a single source is 28 and 25 mas for RA and Dec, respectively.

Optical Positions of Extragalactic Radio Sources Using the UCAC1

Abstract : Extra-galactic radio source positions referred to the first USNO CCD Astrograph Catalog (UCAC1) are presented. They were derived from CCD observations taken with the 1.6-meter Cassegrain telescope at the Laboratorio Nacional de Astrofisica, Brazil (LNA). The observations started in 1997 and were carried out in a joint program between USNO, Observatorio do Valongo/UFRJ, Brazil, and Observatorio Nacional/CNPq, Brazil. They were planned to provide, together with CTIO observations, fiducial points to link the UCAC to the ICRF. The results presented here refer to 24 radio source targets distributed mainly between -30 degrees and -70 degrees declinations. Reduction techniques follow standard procedures. The results are compared with VLBI and with similar precise optical positions, in order to contribute to the evaluation of the UCAC astrometry.

DIVISION I / WORKING GROUP ASTROMETRY BY SMALL GROUND-BASED TELESCOPES

At the IAU XXVI General Assembly in 2006, the Division I decided to create the Working Group on Astrometry by Small Ground-Based Telescopes (WG-ASGBT). Its scientic goals are to foster the follow-up of small bodies detected by the large surveys including the NEOs; to set-up a dedicated observation network for the follow-up of objects which will be detected by Gaia ; to contribute to the observation campaigns of the mutual events of natural satellites, stellar occultations, and binary asteroids; and to encourage teaching astrometry for the next generation. The present report gives the main activities carried out in these areas with small telescopes (diameter less than 2m).