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

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.