XVIIth IAU General Assembly, Rio de Janeiro
Chair: W. Thuillot
In October 2007 IAU Symposium 248, "A Giant Step: From Milli- to Micro-arcsecond Astrometry", was held in Shanghai, China. The Symposium exceeded all expectations, with close to 200 astronomers attending and presenting an array of outstanding talks. I was asked by the Scientific Organizing Committee to give an invited talk on the educational needs of students who might wish to study Astrometry in the golden age of Micro-arcsecond Astrometry and to organize a round-table discussion on the topic. In the process of preparing my talk and organizing the session, I realized that I had a unique opportunity to bring together the experts on virtually all topics that might be covered in an introductory text on Astrometry and incorporate relativistic formulations that look forward to the solution of cutting-edge problems in astrophysics. The result, which I have edited, consists of 29 chapters written by 28 authors from 13 countries with publication expected in early 2010.
The text opens with an exploration of the opportunities and challenges
facing astrometry in the 21st century from space and the ground. Ground-based
astrometry will continue to play a vital role in modern astrophysics, since
large apertures and long-term investigations will not be possible in space for
the foreseeable future. We then lay out the new formalisms of relativity required
to take advantage of micro-arcsecond astrometry, celestial mechanics, coordinate
systems and review the problems of observing through the Earth’s atmosphere.
The topics of diffraction-limited imaging, interferometry in the optical and
radio, geometrical optics, imaging detectors are explored as they apply to astrometry.
We then develop the tools required to analyze our astrometric data, such as
basic algorithms, statistics, image deconvolution, dealing with poorly sampled
images, transforming our observations from detected photons to the celestial
sphere, determining parallaxes and creating astrometric catalogs. The text closes
with a survey of numerous applications of astrometry to topics of current interest
to astronomy and astrophysics, including galactic kinematics, binary stars,
star clusters, the solar system, extra-solar planets and cosmology.
Second-epoch observations for the Yale/San-Juan Southern Proper Motion (SPM) program were officially completed in June 2008. There now exist either CCD or plate material in two epochs for the sky south of -20 degrees declination. All CCD data and both first- and second-epoch plate data have been reduced to celestial coordinates in anticipation of the construction of the SPM4 Catalog of absolute proper motions. The first step in its construction is building a first- epoch positional catalog utilizing the significant overlap of the 6-degree plates taken on 5-degree-maximum field centers, in combination with an external reference catalog. Tycho-2 was chosen as the external reference for this first- epoch positional catalog we have named the YSJ1. (Yale/San Juan 1st-epoch.)
The YSJ1 contains over 166 million stars and galaxies brighter than roughly V=18 and includes contributions from all first-epoch plates that, in addition to the near-complete sky coverage south of -20 degrees, also extend to a number of more northern fields between -20 and -5 degrees. The YSJ1 is based on PMM scans of the SPM plates made by the US Naval Observatory Flagstaff Station and a new determination by USNO Washington of the detections and centers using the scan pixel data. (The YSJ1 will also be used in the calculation of proper motions to be included in present/future releases of the UCAC.)
The SPM4 Catalog should be completed in August 2009, and made available shortly thereafter. It is expected to contain absolute proper motions of over 100 million stars and galaxies south of -20 degrees declination. A proposal to continue CCD observations to complete coverage of the SPM fields north of -20 degrees that have first-epoch material, as well as to replace some scientifically interesting second-epoch "plate" fields with superior, longer-baseline CCD observations, has been approved by the NSF. This project will also provide funds to allow Galactic-kinematics investigations employing the SPM4, as originally envisioned when the SPM program was conceived.
The SPM has been supported in part by a series of grants from
the US NSF, an initial grant from the Ford Foundation to build the telescope
and facilities, continuing infrastructure support from the Univ. San Juan,
Arg., and occasional grants from the USNO, NASA, the Argentine CONICET and
Yale University. We are indebted to the contributions of many individuals
over nearly 50 years of operations that have resulted in the SPM4. In particular,
we would like to acknowledge the observers who took the photographic plates
during the first epoch and the beginning of the second epoch, the Directors
of the Observatório Astronómico Felix Aguilar who provided the
infrastructure support, especially C. U. Cesco, J. A. Lopez and E. Actis,
Rector of the Univ. San Juan, Tulio A. Del Bono and Deputy Provost for the
Sciences at Yale Univ., B. F. Carmichael, both of whom provided critical support
on a number of occasions. Finally, we wish to thank the many graduate students
and postdoctoral associates at Yale University who have supported the SPM
throughout the years.
Astrometry of natural satellites has been performed at the 1.6
m and 0.6 m (B&C and Zeiss) telescopes of the Laboratorio Nacional de
Astrofisica, Itajuba, Brazil (IAU code 874). Classical astrometry has been
attempted for Jupiter satellites (Elara, Himalia, Pasiphae, Carme), for Saturn
ones (Titan, Phoebe, Iapetus, Hiperion), for Uranus (all main five ones) and
Neptune (Triton, Nereid). This group has also given important attention to
the international campaigns of observation of the mutual events of the satellites
lead by IMCCE, recently for the Uranian satellites, and currently for Jupiter
and Saturn mutual events. It is also strongly involved in the IMCCE campaign
of observation of the mutual events of the binary asteroids (collaboration
with P. Descamps, F. Marchis et al.). Through a collaboration with B. Sicardy
(Paris Observatory), predictions of stellar occultations by Pluto, TNOs or
natural satellites are periodically made on the basis of the Brazilian astrometic
observations. The improvement of these predictions thanks to last minute astrometry
helps to choose the location of the mobile telescopes. Occultations themselves
have also been recorded by the group using 0.6m – 1.6m LNA telescopes,
like the ones by Pluto in May and June 2007. Collaboration with astronomers
from the Bucharest Astronomical Institute, Romania, is under way, for the
observation of ICRF sources using the 0.6 m Zeiss telescope at Belogradchik
Observatory, Bulgaria.
This program has benn undertaken jointly by researchers from SYRTE (J. Souchay, S. Bouquillon, F. Taris) and from IMCCE (F.Colas) The astrometric requirements needed on the position and velocity of Gaia on its orbit are very stringent. It has been shown (Perryman 2005, Mignard 2005) that the uncertainty on position and velocity must be, at most, 150m (20mas) and 2.5mm/s (1mas/h) respectively. The classic Doppler and ranging techniques can only deliver 6km and 8mm/s. To achieve those high level requirements Bastian (2002) has proposed to use small telescopes in the frame of the GBOT (Ground Based Optical Tracking). Gaia's location roundabout the L2 Lagrange point is approximately 1.5 million km from the Earth, facing roughly opposite of the sun. It's visual magnitude would be approximately 18 (but this value can be off by a huge amounts, +/-2mag.). In order to prepare the GBOT of Gaia, observations of WMAP (Wilkinson Microwave Anisotropy Probe) has been performed. That probe is located around the L2 Lagrange point and its magnitude (roughly 19) is very near from the expected magnitude of Gaia. WMAP is then a reasonable model for the brightness and observability of Gaia. The precise astrometric position of WMAP from ephemerides is provided by Dale Fink, Navigator of WMAP Spacecraft Control Team at NASA.
First results of O-C have been obtained with the ESO 2.2m+WFI telescope in April 2008. Astrometric reductions are at the level of 50/70mas, mostly due to the imprecision of the reference catalog used (UCAC2); Another test campaign (July 2008) has been initiated to test the capability of a 1m class telescope to be used during the GBOT. The T105 of the Pic du Midi was chosen to achieve that goal. As previously, astrometric reduction are limited by the astrometric reference catalog (UCAC2) at the level of 50/60mas, slightly better than with the ESO telescope probably due to the greatest number of reference stars in the field of WMAP. Photometric determination has been done that show a magnitude around 18 even if the UCAC2 stars used for the calibration are not photometric standards.
The campaign under progress (February to August 2009) uses the
T120 of the Observatoire de haute Provence (OHP). It is a long term program
with several aims like finding the S/N ratio of a Gaia-like object against
the phase of the Moon, finding the best moment on the orbit to observe the
object and compute its position among the reference stars etc…. The
speed of WMAP/Gaia on its orbit varies in the range 0-20mas/s. Tests with
asteroids have also been undertaken together with different tracking methods
(stars/object). Observations with different filters have been scheduled to
get information about the reflectivity of the kapton used for thermal protection
of the probes.
This special action, a cornerstone project, is organized on 23 and 24 October 2009 in the frame of the International Year of Astronomy and is devoted to the observation of Jupiter and the Galilean moons by a large public. This event is also the opportunity to draw the attention of the amateur and professional astronomers to the mutual events of the Galilean satellites. These rare mutual eclipses and occultations arise every six years and are observable in the Jovian system in 2009-2010. Exceptionally, due to the closeness of the Jovian and Saturnian equinoxes, this kind of events which also arise in the Saturnian system but every 15 years, are also observable in 2009-2010. They provide high accurate astrometric data through photometric measurements and are well adapted to the observation by small telescopes. A network of observers is organized by J.-E. Arlot, Paris Observatory (Arlot et al., 2009), who chaired a special meeting on the PHEMU09 campaign during this GA on August 10.
The European Gaia mission will be launched in 2012 for a five
years mission. Astrometry down to a few microarcseconds level, but also photometry
and spectroscopy of one billion of objects, will be performed. The observing
process is founded on a scanning law, no pointing will be possible. Among
the large number of celestial objects observed, moving objects will be detected
and identified when possible. Therefore many Solar System objects down to
the limiting magnitude 20 will be observed. We are awaiting the discovery
of new objects such as Near-Earth asteroids or comets. In some cases - faint
objects, high excentricity, low solar elongation - Gaia will not be able to
re-observe these objects or to identify them in a second scan done several
days or months later. In this context, astrometry by small ground-based telescopes
is a very useful method in order to perform supplementary observations on
alert. It will avoid the loss of these objects; it will allow their identification
and their dynamical and physical characterization. For these objectives, a
dedicated network is set up in the frame of the preparation of the Gaia mission.