平场和偏振本底改正是全日面单色像和偏振像数据处理的必要步骤.本文假设太阳自转一周过程中望远镜的平场和偏振本底不变,基于此从怀柔太阳观测基地第2144个卡林顿周的单色像和Stokes Q/I,U/I,V/I图像中利用中值算法提取出平场和Stokes I到Q,U,V的偏振本底.利用该方法提取的平场中包含一些小尺度的脏点、大尺度的光强不均匀性,提取的Stokes I到V的偏振本底成马鞍面分布,Stokes I到Q,U的偏振本底成倒钟形分布.平场改正后可以得到较理想的单色像.日面边缘偏振本底较大的宁静区扣除偏振本底后Stokes Q/I,U/I,V/I的平均值分别为-0.8×10~（-4）,0.9×10~（-4）,1.3×10~（-4）,远小于改正前的-6.8×10~（-4）,2×10~（-3）,1.7×10~（-3）.该方法无需定标观测,从常规观测数据中即可提取出平场和偏振本底,对天基、远程无人值守观测的数据标定具有重要的应用价值.
The correction of flat fields and polarization offset is a necessary step in the reduction of the full-disk solar observing data. The general used methods of the measurement of flat fields are summarized as follows. The first one is realized by randomly moving the solar telescope and making an average of thousands of observed images of the quiet region near the disk center, which is adopted by the large aperture solar telescopes with a small field-of-view. The second one is to shift the solar images with slightly different pointings, which is mostly used in the full-disk solar images. The third method is to use a diffuser to generate the uniform light source and it can be used in the full-disk solar images, too. Observing the nearby continuum of the selected working spectral line is generally used to obtain the polarization offset. It is worth to mention that, all these methods need special calibration observing mode and occupy the valuable scientific observation time. In the paper, we propose a new method to derive the flat fields and polarization offset from the routine observing data. Assuming that the flat fields and polarization offset do not change during a solar rotation, we firstly got the formula which can be used in the calculation of flat fields and polarization offset in the case of the ideal and real telescope guiding and pointing accuracy. The core algorithm in the formula is median process. Moreover, we obtained the flat fields and the crosstalk from Stokes I to Q, U and V using the above mentioned formula from series of the routine observed monochromatic image and Stokes Q/I, U/I and V/I images belonging to the Carrington Rotation 2144 observed by Huairou Solar Observing Station. The derived flat fields could not only reflect the small-scale dirty points but also the large-scale non-uniform pattern from the whole telescope system, which can be removed after the flat-fielding process. The distribution of the resulting Stokes V/I offset from Stokes I to V seems to be a Saddle surface, while that of
Chinese Science Bulletin
flat-fields, sun, image analysis, polarization, photometry measurement