CCD联合平差方法及其初步实例检验

在常规照相天体测量工作中 ,对暗天体的定位是采用逐级定标星过渡的方法实现的。由于星表系统差和局部差的存在 ,最终定位结果可能包含难以把握的误差积累。在CCD小视场观测情况下时常难以找到足够数量的定标星。鉴于此 ,推导了CCD观测联合平差方法的严格矢量表达式 ,以期通过共同星的过渡 ,实现大天区统一平差。原理上此方法可实现相对于河外天体的定位 ,以削弱星表系统差和局部差对定位结果的影响。讨论了CCD联合平差方法实现中的问题 ,包括底片常数模型选择、切点位置改正、共同星的较差改正和法方程解算方法等 ,并分析了具体的处理措施。用实例初步检验了CCD联合平差方法的效果 ,表明通过对多张CCD观测的联合平差 ,可以削弱单张CCD观测参考星数目过少、单张CCD观测参考星分布不均、个别参考星存在位置偏差和局部区域参考星存在系统性位置偏差等不利因素对归算结果的影响 ,进而达到扩大视场和提高归算精度的目的

THE SYNTHESIZED REDUCTION METHOD OF CCD OBSERVATIONS AND ITS PRELIMINARY EXAMINATION

In the routine photographic astrometry, the position determination of faint celestial body is realized by using multi-step transition of reference stars. Because of the systematic and regional errors of star catalogues, in the ultimate position determination there may exist serious accumulation of various errors. Also, it is difficult to find sufficient reference stars in the small viewing field of a single CCD observation. In this paper, the rigorous vectorial expression of the block adjustment of CCD observations is deduced in order to realize a synthesized reduction of CCD observations within large sky coverage and connected by common stars of overlapping observations. Theoretically, the position determination relative to extragalactic bodies can be realized by using this method and so the influence of the systematic and regional errors of catalogues on the final positioning results could be eliminated. Some key points in the realization of the block adjustment of CCD observations are discussed. The pertinent issues are the selection of plate constant models, the position correction of the tangential point, the differential corrections of common stars and the special treatment of the normal observation equation. Their corresponding measures are recommended. The effects of the block adjustment of CCD observations are also preliminarily examined with real case data. It is shown that the block adjustment method can improve the final reduction results compared with the independent reduction of CCD observations in several cases including that (1) no sufficient reference stars in a single CCD observation, (2) uneven distribution of reference stars in a single CCD observation, (3) a reference star with position bias and (4) regional reference stars with systematic position biases. Therefore, the block adjustment method of CCD observations can effectively extend the viewing field of CCD observation and improve the precision of the reduction results.