Accurate Determination Of Plate Model Based On CCD Images With Different Orientations

During the reduction of CCD images for major Saturnian satellites (S1-S8), astrometrists are seldom concerned with the distortion of field of view (FOV). A 4-constant model (including the scale factor and orientation of a CCD image) is often supposed to be enough. In order to further improve the astrometric accuracy of planetary satellites from CCD images taken by a long focal length telescope, an experimental test was tried to study the distortion of a small CCD FOV.The first data set was obtained by a newly-fitted 2κ×2κCCD on the 1-m telescope at the Yunnan Observatory with different orientations by putting the CCD supporter in the normal position and its vertical one, respectively, with a Johnson I filter. In order to reduce the workload and artificial measurement error, this thesis designed a code which can search stars in a CCD image based on derivative search algorithm and use 2D Gaussian function to obtain more precise pixel position for a star image. Then, we matched each star in an image to its counterpart in the catalogue UCAC2. So as to investigate the effect of different constant models on the result of positional measurement of stars, we attempted to adopt 4-,6-,12-,20-and 30-constant models to reduce the measured coordinates of stars. The results reveal that the CCD's FOV has a plate constant model of up to the 4th power of the measured coordinates (x, y). After taking this model into account, the positional measurement accuracy of a bright UCAC2 star can be derived as well as-7 mas in each direction. This precision corresponds to a FOV of about7'×7'and the exposure time of 60s and to a level of the current best CCD observations of the like.In order to study the impact of filter to positional measurement, the second data set adopted in this thesis was also observed by the same telescope, but without filter. Imaging mode and reduction were the same as that for the first data set. We found that a low-order model could solve the distortion of CCD images. The main reason causing serious distortion of CCD images is using filter.