Astrophysics The photometric system of the Tsinghua-NAOC 80-cm telescope at NAOC Xinglong Observatory ∗

Telescope (hereafter, TNT) is an 80-cm Cassegrain reflecting telescope located at the Xinglong Observatory of NAOC, with the main scientific goals of monitoring various transients in the universe such as supernovae, gamma-ray bursts, novae, variable stars, and active galactic nuclei. We present a systematic test and analysis of the photometric performance of this telescope. Based on the calibration observations taken on 12 pho-tometric nights, spanning the period from 2004 to 2012, we derived an accurate trans-formation equation between the instrumental ubvri magnitudes and standard Johnson UBV and Cousins RI magnitudes. In particular, the color terms and the extinction coefficients of different passbands are well determined. With these data, we also ob-tained the limiting magnitudes and the photometric precision of TNT. It is worthwhile to point out that the sky background at the Xinglong Observatory became gradually worse over the period from 2005 to 2012 (e.g., ∼21.4 mag vs. ∼20.1 mag in the V band). Key words: instrumentation: detectors — site testing — telescopes