Abstract: An Auto Guiding Unit (AGU) of an Altazimuth telescope is used to guide the primary field of view, but the field of view itself is only about 1 arc minute. The two-dimensional motion of an AGU needs to be controlled through certain software to ensure a selected star appearing in its field of view. The controlling software also needs to give the bias error when there is a drift of the pointing of the AGU so as to correct the telescope pointing and make tracking observation of the star. In this paper we use homogeneous equations based on the celestial projection coordinate system to build two models to correct the pointing and calculate the orientation accuracy of the AGU. Using these models an AGU can be used to point to a star (represented in the equatorial coordinates) and give the bias error of pointing.