| As an observational subject, astrometry develops rapidly with the improvements of observational methods and techniques. Especially after SOs of twenty centuries, many new techniques and methods appear, these improvements make the astrometry become more and more important to the whole astronomy. One of the main works of astrometry is to observe celestial bodies, reduce observational data and obtain the information of objects. To get reliable results, adopting suitable method to remove the systematic errors in observation is an important step. Long exposure times are needed to observe faint objects in astrometry. However, at the same time, tracking error can occur because of defects in the guiding device or mechanical system of the telescope. Tracking errors of any kind will degrade the image quality of a telescope. When the telescope works well (without any tracking error), the intensity distribution of star images is similar to a two-dimensional Gaussian distribution, assuming that the atmosphere is steady during the long exposure time. When the guiding device or the mechanical system of telescope does not work perfectly (i.e., when tracking error occurs), the intensity distribution of astrometric images will be not Gaussian but rather asymmetry. Although tracking error should have the same influence on the objects in the field of view, there is not any systematic analysis on the influence of it. Since the receivers were plates in old times, which have a limited linear response range, the influence of tracking error on star images differs from one area of the plate to another and for objects of different magnitudes. This effect makes it difficult to find a viable method for removing tracking error. Nowadays, CCD detector has become the main receiver of the telescope. When compared with photographic plates, CCD has three advantages: higher quantum efficiency, higher linearity and greater convenience. The linear response of a CCD makes it possible to observe many stars at the same time. When tracking error does exist with a CCD, its influence on all objects will be similar. This means that the tracking error has the characteristic of being spatially invariant. Part of the work of this thesis is in this field, image restoration is presented as a way to remove the influence of tracking error (in Chapter 2 and 3). Chapter 2 introduces the basicprinciples of image restoration. From the production of images to the degradation of images caused by various factors, and how to remove the influence of tracking error using Fourier Transformation method are all analyzed. Chapter 3 gives the detailed process of removing the influence of tracking error with image restoration. Constructing and maintaining a celestial reference system with high precision is one of the most important tasks of astrometry. At its 23rd General Assembly in 1997, the TAU adopted an international celestial reference frame (ICRF) that realizes the international celestial reference system (ICRS). As of 1 January 1998, ICRS become the IAU celestial reference system, and Hipparcos catalogue is the representative of ICRS in optical wavelength. In the 24th IAU GA, a new Working Group of ICRS was constructed based on the old WGRF. The main research fields of the new WG are as follows: maintenance and extension of the ICRS, densification at optical and IR, space astrometry and reference frames, link to the dynamical system, computation tools, astronomical standards, relation with IERS. Observing the extragalactic radio sources in both radio and optical wavebands is the imp...
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