This WG was created following the decision of the Division 1 at the XXIVth IAU GA in Manchester. During the three years it was chaired by M. Stavinschi (Romania) and Jean Kovalevsky (France). His members are Dafydd Wyn Evans (UK), Carlos Lopez (Argentina), Dan Pascu (USA), Antonio Pugliano (Italy), Manuel Sanchez (Spain), Ramachrisna Teixeira (Brazil), and Arthur Upgren (USA).

The first action taken by the Working Group was to identify the existing small astrometric instruments. A questionnaire was distributed among observatories, and we received 30 descriptions of instruments in 12 different countries. Many of them are working in some programs, but others are now closed. The conclusion was that there are many instruments that could be used efficiently, if adequate programs were to be proposed. They are described in the Working Group web site: http://www.astro.ro/wg.html

The following step was to gather interested people in order to discuss possible programs to be undertaken with those or analogous instruments. The first meeting took place in 2001 in Munich during JENAM, as a joint discussion on European Astronomy with Small Telescopes. A report was published in the EAS Newsletter, issue 22, December 2001.

Then, during the "Journées 2002: Systèmes de référence spatio-temporels" in Bucharest, a second half-day meeting was organized. The report will be published in the proceedings of the meeting by Observatoire de Paris.

In 2002, a call was made to the community in contact with us to propose cooperative programs using small or medium-sized telescopes for some defined scientific objective. The proposals received are presented further down.

Clearly, the traditional astrometric instruments, such as meridian circles, astrolabes or astrographs, performing as they did twenty years ago, cannot compete the results obtained with space astrometry for the position of celestial objects, or with VLBI or GPS for the rotation of the Earth. They must be upgraded so as they can still play a role in astronomy. The Working Group has identified several possibilities.

1) CCD detectors . Systems with 4096 x 4096 pixels are now available and the read-out capabilities have considerably improved. In addition, the scan mode allows increasing the field of view, at least in one direction.

2) The CMOS development seems the most promising for the future.

3) It is now possible, with small and medium-sized telescopes to reach the diffraction limit in resolution. Speckle interferometry is currently being installed at the focus of such telescopes. It involves recording speckle patterns on a CCD and combining several hundreds images such as shown on the figure and to reconstruct a perfect image by a computer. One needs a fast computer to compute the autocorrelation function. Speckle interferometry is particularly suited to observe close binary stars.

4) Photometers are easy to install at the focus of a telescope and can be used to many programs. Small telescopes are sufficient and it would be a good use of it to dedicate it to single or, better, multichannel photometry. In addition to astrometric applications (eclipses, occultations, eclipsing binaries), there are plenty of possible programs of monitoring variable stars.

5) One can also mention the solar astrolabe used to observe the variations of the solar diameter. But it is rather expensive and difficult to build.
 
 

The principal condition is that, whenever space astrometry can potentially achieve results that ground-based astrometry cannot compete, such programs should not be undertaken. But, space astrometry missions have their own limitations.

•They are not flexible: observations are either constrained by a scanning law (GAIA) or by overall programming (SIM or HST)
•They are not designed for monitoring: it is not possible to get long sequences of observation of a single body
•They have a limited lifetime: many astronomical features must be observed either indefinitely or at least a longer time
•They often need preliminary data: for instance, ephemerides or prediction of magnitudes of irregular variables

In response to the call for cooperative astrometric programs using ground-based small or medium-size instruments, the Working Group received five proposals. Some of them are new, some were already operational, but sought support in order to enlarge participation.

1- Mutual phenomena of Jupiter and Saturn satellites (PHEMU - PHESAT)

Jean-Eudes Arlot; IMCCE, Paris Observatory

Using the opportunity of the transit of the Sun and the Earth through the equatorial plane of Jupiter and Saturn, observations of mutual occultations or eclipses with CCDs provide relative positions of these satellites with a high astrometric accuracy. They are used to improve the models describing their motion.

2- Ground-based monitoring of astrometric binaries (GMAB)

George Gontcharov, Pulkovo Observatory

About hundred astrometric binary candidates have been discovered by Hipparcos. They should have periods of 10-50 years. Additional observation from the ground must be made in order to determine periods and other orbital elements and reveal photocentric orbits that will allow to determine the masses of the components.

3- Dedicated astrometric network for the follow-up of GAIA

William Thuillot, IMCCE, Paris Observatory

The objective of the network is to observe on alerts to get CCD images of targets in order to confirm their detection and follow its evolution. The network should be planned now with tests of acquisition of images of test targets and the development of the processing of the images. Preliminary programs of observations may be organized, for instance on minor planets or comets.

4- Radiosources optical counterparts

Gennady Pinigin, Nikolaev Observatory, Ukraine

The link between the International Celestial Reference Frame (ITRF) and the optical realizations like the Hipparcos catalogue is a major problem to which ground-based astrometry can bring valuable contributions using CCD observations.

5- Variation of the solar diameter

Alexandro Humberto Andrei, National Observatory, Rio de Janeiro

The solar diameter has periodic and irregular variations that are not well understood. In particular, they may be partly originated by atmospheric effects depending on the geographical position of the observatory. It is therefore necessary to have several well distributed solar astrolabes. A close technical cooperation among stations has proven to be highly desirable.

Other possible objectives for ground-based astrometry

There are many other scientifically sound and prospective objectives that can be assigned to ground-based instruments. Some need coordinated observations; some may be made as a single observatory program. Examples:

Solar system:

• Observation of minor planet positions for the determination of the masses of perturbing bodies
• Observations of Earth-cruisers, for which one needs to update frequently the orbits
• Bulk observations of newly discovered objects in order to get good orbits for recognition purposes
• Observation of small planetary satellites using mask technique to dim the luminosity of the planet

Stars and stellar systems:

• Observation of close binaries by speckle interferometry. The large number of these objects and the potential of obtained data for determining stellar masses call for more instruments devoted to speckle interferometry.
• Determination of stellar diameters by speckle interferometry or using occultations by the Moon.
• Photometry of variable stars, in particular eclipsing binaries, which permits to determine stellar masses.
• Recognition of stars and determination of their proper motions in highly crowded fields badly conditioned for space astrometry.