南京大学65 cm天文望远镜指向精度的修正研究

天望远镜的准确指向是正常观测的重要保障，也是望远镜系统本身的一项重要测试指标，南京大学天系65cm望远镜的成像OCD的视场较小，实际观测时，指向精度不高，待测天体的像往往不在视场内，为了修正南京大学天系65cm望远镜的指向精度，2004年7月-8月，对该望远镜的指向精度进行了测量并通过指向误差函数进行了指向精度的修正，首先建立指向误差修正模型，通过对不同天区恒星的观测可得到该望远镜在相应方位的观测指向误差值，将观测值和理论模型拟合，从而得到指向误差修正函数．建立南京大学65cm望远镜的指向误差修正模型，然后对其观测的指向误差值进行拟合，最后用拟合函数对观测进行指向修正，结果表明，通过指向误差函数对指向误差进行修正，提高了该望远镜系统的指向精度，目前，不仅可以使观测的星象能在CCD视场内，而且改正后的指向精度略优于1′，在修正前望远镜系统的指向精度RMS(均方差值)是117″，修正后望远镜系统的指向精度RMS(均方差值)提高到38″。

Correction of Pointing Accuracy of NJU-65 cm Telescope

For an astronomical telescope, high pointing accuracy is a key technical specification not only fundamental to routine observation, but also important in evaluating its performance. The pointing error of the telescope is caused by machine work and assembling, atmospheric refraction and so on. The pointing error makes the telescope difficult to point to a observing target. So a correction to the pointing error is very important in order to improve the pointing accuracy. When a higher pointing accuracy is required, a way of adjusting only the telescope structure to improve the pointing accuracy is usually not feasible. After the telescope has been assembled, the pointing error have to be corrected by a real observation so as to improve the pointing accuracy. Before the work was done in this paper, the pointing accuracy of NJU-65 cm telescope had been as poor as 117 arcseconds RMS so that the stars could not always be imaged within the chip of CCD camera of the telescope. In order to improve the pointing accuracy of the telescope, considering the diversity of the observed pointing errors and the complexity of the pointing error rule, two pointing error correction models were compared and one of them was then adopted. By observing the positions of stars in different sky area, pointing errors could be recorded for each corresponding star and be fitted with the theoretical model, resulting in a final error correcting function, by which the telescope pointing is to be compensated for during observation. In July and August of 2004, we implemented this method to the NJU-65 cm telescope and corrected its pointing error successfully. Currently, its corrected pointing accuracy can reach 38 arcseconds RMS, 4 times better than before. The present pointing accuracy is good enough to always point the star image within the visible field of the CCD chip.