关于云南天文台2.4米望远镜YFOSC的CCD的平场

Notes on the CCD Flatfields of the YFOSC on the 2.4m Reflector of the Yunnan Observatory

与国家天文台兴隆站2.16 m安装的北京天文台暗天体分光成像仪(BAO FaintObject Spectrograph and Camera,BFOSC)系统相似,云南天文台2.4 m望远镜安装了云南暗弱天体光谱及成像仪(Yunnan Faint-Object Spectrograph and Camera,YFOSC),对它在直接照相时的CCD平场性能做了测定。使用不同旋转角度拍摄平均而得的平场,可以使不同位置拍摄的像场,所受最大误差≤0.015 mag。但与卡焦的VersArray CCD照相机不同,即使在视场中间直径6.'5的圆内,误差约在0.010～0.013 mag之间,达不到≤0.005 mag的精度。

Similar to the BFOSC mounted on the 2.16m reflector of the National Observatories at the Xinglong station, the YFOSC instrument is mounted on the 2.4m reflector of the Yunnan Observatory. We measured the CCD flatfields of the YFOSC in the direct imaging mode in the V band. We constructed an average flatfield for the flatfields observed at different CCD orientation angles. It has the maximum error of 0. 015mag. Unlike the direct-imaging VersArray CCD camera at the Cassegrain focus, the maximum error for the YFOSC cannot be less than 0. 005mag even within the 6.＇5-diameter central circular region of its field of view. The causes for the difference are as follows. The errors for the VersArray CCD are only due to the uncorrected scattered light from the baffles of the 2. 4m reflector, but those for the YFOSC are further contributed by the uncorrected scattered light inside the YFOSC itself. We expect that errors for the B-, R-, and I-band flatfields are similar since the same error sources are present. Fringes will appear in R-and I-band images obtained with the YFOSC. This is due to the interference of the night-sky emission lines within the silicon of the CCD. A V-band image does not suffer from this effect because shorter-wavelength emission lines are absorbed more by the silicon, and are weaker and fewer. The fringing is an additive effect. The fringing patterns must be subtracted from CCD images.