Siegfried Roeser

THE PPMXL CATALOG OF POSITIONS AND PROPER MOTIONS ON THE ICRS. COMBINING USNO-B1.0 AND THE TWO MICRON ALL SKY SURVEY (2MASS)

USNO-B1.0 and the Two Micron All Sky Survey (2MASS) are the most widely used all-sky surveys. However, 2MASS has no proper motions at all, and USNO-B1.0 published only relative, not absolute (i.e., on the International Celestial Reference Frame (ICRS), proper motions. We performed a new determination of mean positions and proper motions on the ICRS system by combining USNO-B1.0 and 2MASS astrometry. This catalog is called PPMXL (VO access to the catalog is possible via http://vo.uni-hd.de/ppmxl), and it aims to be completed from the brightest stars down to about V ≈ 20 all sky. PPMXL contains about 900 million objects, some 410 million with 2MASS photometry, and is the largest collection of ICRS proper motions at present. As representative for the ICRS, we chose PPMX. The recently released UCAC3 could not be used because we found plate-dependent distortions in its proper motion system north of –20° declination. UCAC3 served as an intermediate system for δ ≤ –20°. The resulting typical individual mean errors of the proper motions range from 4 mas yr–1 to more than 10 mas yr–1 depending on observational history. The mean errors of positions at epoch 2000.0 are 80-120 mas, if 2MASS astrometry could be used, 150-300 mas else. We also give correction tables to convert USNO-B1.0 observations of, e.g., minor planets to the ICRS system.

GAIA: origin and evolution of the Milky Way

GAIA is a short-listed candidate for the ESA Cornerstone mission C5, meeting the ESA Survey Committee requirement for an observatory mission, dedicated to astrometry, providing 10 micro-arcsecond accuracy at 15th magnitude. The GAIA mission concept follows the dramatic success of the ESA HIPPARCOS mission, utilizing a continuously scanning spacecraft, accurately measuring 1D coordinates along great circles, in two simultaneous fields of view, separated by a known angle. These 1D relative coordinates are later converted to the five astrometric parameters of position and motions in a global analysis. GAIA will provide precise astrometry and multi-color photometry for all the one billion stars, quasars, and compact galaxies to I equals 20 on the sky. GAIA will additionally provide the sixth phase- space parameter, radial velocity, from a slitless spectroscopic survey of most stars brighter than about magnitude 17. The technical challenges are considerable, but achievable. The scientific returns are than about magnitude 17. The technical challenges are considerable, but achievable. The scientific returns are spectacular, with greatest impact in the study of stellar populations and dynamical structure of the galaxies of our local group, and in providing the first complete census of the stars and massive planets in the solar neighborhood. GAIA will revolutionize our knowledge of the origin and evolution of our Milky Way Galaxy, and of the distribution of planetary system around other stars.

DIVA optical telescope

The German Instrument for Multi-channel Photometry and Astrometry (DIVA), dedicated to the German (DLR) small extraterrestrial satellite program, is intended as a kind of technology precursor mission to GAIA. DIVA is scheduled for launch in 2004 and shall perform a sky survey to measure within 2 years life time the positions, parallaxes, magnitudes, etc. of about 35 million stars. The main instrument, covering the spectral range of 400-1000nm, observes 2 fields of view (0.6° x 0.77°) by a single Focal Plane Assembly (FPA). The focal length is 11200mm. The DIVA Optomechanics is based on a high precision Three Mirror Anastigmat (TMA) concept with 8 mirrors, 5 of them flat. An extremely high short term stability (torsion tolerance) of 0.3 mas over 10h only has to be realized only by passive means to achieve the astrometrical performance requirements. The paper describes the phase B2 design activities wrt. the optomechanical and thermal design of the main instrument. Special emphasis is given to an exhausting, but very pragmatic thermomechanical and optical performance trade off between a cost effective athermal design concept, applying mirrors and an optical bench made from a specially treated isotropic aluminum alloy, and a thermally stable hybrid material concept based on a Carbon Fiber Reinforced Plastics (CFRP) sandwich structure and Zerodur mirrors. The selection of the final baseline design solution shall be reported. According to the very high long and short scale surface properties of the candidate aluminum mirrors a sophisticated manufacturing procedure was established based on conventional and ion beam polishing techniques. The representative breadboard mirror test results will be given.

K giants as astrometric reference stars for the Space Interferometry Mission

Wide angle astrometry with the Space Interferometry Mission needs a set of several thousand grid stars distributed uniformly over the sky. The requirements for candidate grid stars are quite stringent: the photocenters of these stars have to be astrometrically stable to within a few microarcseconds, which makes binary stars unacceptable as grid stars. We search the most precise and comprehensive astrometric catalogs available today--the Hipparcos, Tycho-1 and Tycho-2 Catalogues--for possible grid stars, and discuss the properties of samples of K giants derived from these catalogs. Furthermore, we present results of a precise radial velocity study of a small proxy sample of Hipparcos K giants. We demonstrate that it is possible to find K giants with radial velocity variations smaller than a few tens of meters per second on timescales of several months. It is thus possible to detect stellar companions in samples of candidate grid stars by means of a radial velocity survey. We discuss the results of Monte Carlo simulations that address the consequences of the measurement accuracy of a radial velocity survey on the portion of undetected binary systems.

Optical design for DIVA

DIVA (deutsches interferometer fuer vielkanalphotometrie und astrometrie) is a project of a small satellite, aiming to measure positions, proper motions, parallaxes and spectra of several million stars. DIVA will carry two Fizea interferometers with a baseline of 100 mm using a novel telescope design. It consists of a Gregory configuration with high secondary magnification and a four-component field lens system at the intermediate focus. We present the optical layout which allows diffraction-limited imaging over a 0.5 degree of view in the wavelength range 400-1000 nm. Critical aspects of the design are discussed. The present status of the project is briefly outlined.