Pointing accuracy is an important technical indicator of an astronomical telescope. This paper describes two ways to improve the pointing accuracy by using models: the basic parameter model and the spherical harmonic function model. After the technological update of the 1.2m telescope, in order to meet the operational needs of laser and adaptive optics, the author established an initial model to ensure that a star can enter the field view of the CCD detector. Subsequently, considering the structural characteristics of the 1.2m telescope, the author selected as the global pointing model a spherical harmonic function up to the fourth harmonic order and with the corresponding terms of each harmonic order being the first order tesseral harmonic terms. At the same time for the segmentation errors of the encoder measuring angles, a small-period model was used. After the corrections with the above several models, the pointing accuracy reaches within a few arcsecs, satisfying the requirements for observational tasks in recent years. Because of random factors, such as changes in temperature，humidity, wind speed or wind direction of the environment, also because of the non-uniform distribution of stars selected for setting the global pointing model, the result pointing accuracy from fitting being differ in some localized sky regions. In addition, to meet the needs of the lunar laser ranging experiment, measurement of orbits of high-altitude satellites, and differential photometry of stars, this paper proposes a unique and realized scheme to use a number of stars around a target as local calibrators of directions in the pointing model. With such a scheme the pointing accuracy of the telescope along the target orbit can be achieved with a precision as high as a few arcsecs. This study has not yet been reported. The factors that need to be considered in the modeling are especially presented. These include offset of the center of field view of CCD from optical axis, zero-point error of the encoder, the field rotation, size of field view of CCD and pixel equivalent, error of the algorithm to locate the center of image, error caused by tracking, etc. Through this study, the author has achieved a comprehensive understanding of the theory of pointing model for alt-az telescope. Based on many practices, the author further established various pointing models to accommodate to the structural characteristics of the telescope and special observational requirements. These make the 1.2m telescopes to reach a high pointing accuracy on the level of a few arcsecs, and in the mean time provide general references for achieving high performance of alt-az telescope.
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