多目标光纤光谱大月夜减天光方法研究

Because the moonlit night sky is not uniform, it is very difficult to subtract the sky for multi-object fiber spectroscopic. To use the sky sampling fibers to obtain the sky background, two problems need to be solved: the uniform of the night sky and the difference of the throughput of optical fibers. The former can be done by build a model of the brightness of the moonlit night sky. For the latter, by observing the sky flat field we can obtain the relative efficiency of the fibers for flat field. By calculating the telescope vignetting, we can get the difference of the relative efficiency between flat field and target observation. Thus we can derive the difference of the fiber throughput in target observation. Using the results obtained, applying the flux of the sky sampling fibers, we can conclude the sky background in whole observing area. Then we can get the sky flux for any fiber to be deduced from the target spectrum. By applying this method for LAMOST observation and data processing, we can improve the ability of sky-subtraction for LAMOST in moonlit night, so that normal observations can be performed. In this way, we can improve not only the data quality, but also the observation efficiency.

由于大月夜的天光非常不均匀，大月夜的光纤光谱减天光是非常困难的。要使用天光采样光纤来得到观测天区的天光的话，需要解决天光的不均匀性和光纤之间的效率差异两大问题。对于前者，通过拟合月夜天光与不同参量的关系，建立月夜天光的亮度模型可以实现。对于后者，通过观测天光平场能够得到天光平场中光纤的相对效率，再通过计算天光平场与目标观测时望远镜渐晕的差异，可以得到目标观测中光纤的效率差异。利用得到的这些结果，可以通过天光采样来拟合得出整个观测天区的天光背景，进而得到任意一根光纤中的天光流量，从目标光谱中扣除。将这个方法用于LAMOST数据中，能够提高LAMOST在大月夜的减天光能力，使LAMOST在大月夜也能够正常观测，从而既提高了数据产品质量，又提高观测效率。

亮月夜减天光是光纤光谱观测中的难点问题。本课题以LAMOST为依托，来研究月夜天光的亮度模型，通过天光采样光纤得到的夜天光流量和不同光纤的相对效率差异来拟合出观测天区的夜天光分布，以此来提高LAMOST亮月夜的减天光精度。从测试观测的结果来看，本课题的方法能够有效提高亮夜的减天光精度，观测天区距离月亮越近，本课题的方法对减天光精度的提高效果越好，而在距离月亮大于30度之外，本课题的方法对减天光精度基本没有作用。通过对一个距离月亮约10度的测试观测的数据的分析，采用本课题方法之后的天光光谱减天光残差能够从原来的5.4%降低到3.3%。这对LAMOST能够进行在亮夜的科学观测，提高LAMOST的观测效率有重要意义。