Ground-Based Optical Astrometry in the Time of Space Astrometry

The main task of fundamental astrometry is described. It includes to determine the positions and motions of some celestial objects, to construct a quasi-inertial celestial reference frame, to determine some astronomical constants so that a uniform astronomical constant system can be set up, and to provide useful information for astrometry, celestial mechanics, astrophysics and geophysics. In spite of the effects of the atmosphere and the instrumental deformation from the gravity, traditional optical astrometry on the ground has played for a long time an important role for the task.The observations obtained by the VLBI and the Hipparcos satellite have very high precision, and the extra galactic radio reference frame and the Hipparcos star reference frame have been constructed, respectively.If the two frames are connected, the shortage of the VLBI reference frame for the optical observation can be avoided.Even so, the Hipparcos refercence frame has only about 120 000 stars with magnitude brighter than 9 mag, which does not meet the needs of big quantities and faint magnitude of reference stars. Furthermore, since the observation time was short, the precision of the proper motions from the Hipparcos Catalogue is not high.The greatest difficulty in these observations is that the celestial objects in the solar system can not be observed effectively to determine the zero-point in the right ascention system.All these shortages can be avoided compensationally by the optical observation on the ground.One of the advantages of optical astrometry on the ground is its protracted nature.It can observe stars in a long time so that the precision of their proper motions can be raised, its long time observations for the celestial objects in our solar system can be used to determine the orientation of a reference frame, the flexibility of the ground observation enables us to observe some special region of the sky or some special objects repeatedly so that the demands of some researches can be satisfied.When a CCD camera is used to a traditional meridian circle, a star as faint as 17.5 mag can be observed.This kind of observational manner can extend the number and the magnitude of the observed stars in a great degree, and provide the information on the rotation and space motions of our earth and on the geophysics. This observational manner is not traditional one on the ground. Some new methods for the observational manner and the determination of instrumental errors should be studied, and effects of the gravity and heat deformations on the instrument should be avoided and systematic errors from the atmospheric factors should be eliminated.At last, a precision comparable to space observation can be reached, and astrometry from space and on the ground can be coordinated and developed.