Tsinghua-NAOC (National Astronomical Observatories of China) Telescope (hereafter, TNT) is an 80-cm Cassegrain reflecting telescope located at Xinglong Observatory of NAOC, with 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 in this paper a systematic test and analysis of the photometric performance of this telescope. Based on the calibration observations on twelve photometric nights, spanning the period from year 2004 to year 2012, we derived an accurate transformation relationship between the instrumental ubvri magnitudes and standard Johnson U BV and Cousins RI magnitudes. In particular, the color terms and the extinction coefficients of different passbands are well determined. With these data, we also obtained the limiting magnitudes and the photometric precision of TNT. It is worthwhile to point out that the sky background at Xinglong Observatory may become gradually worse over the period from year 2005 to year 2012 (e.g., ∼21.4 mag vs. ∼20.1 mag in the V band).

We present optical observations of BL Lacertae (BL Lac) in the standard BVRI bands with the 80 cm Tsinghua Centre for Astrophysics-National Astronomical Observatories of China telescope. In 2004, BL Lac was monitored for 25 nights, between September and November, to study its long-term optical variability behaviour. The source initially faded by R ∼ 1 mag in ∼30 d and then brightened by R ∼ 0.8 mag in ∼20 d. It was further monitored for six consecutive nights on 2005 September 3-8, focusing on its intranight variability characteristics. The source continuously brightened by R ∼ 0.7 mag during the six nights, with some visible fluctuations superimposed on to the brightness-increasing trend. The amplitudes of intranight variations tend to increase from I to B wavelengths. BL Lac exhibited strong bluer-whenbrighter behaviour for the internight variations, which is less pronounced for some of the intranight variations. The intranight variations between different bands are correlated without measurable time lags in most time, but the flare on 2005 September 6 showed that the R-band variations lagged the B band ones by 1052 +859 −759 s. Our results indicate that the optical variability properties of BL Lac (the prototype of low-energy-peaked BL Lac objects, LBLs) remarkably resemble the X-ray variability properties of high-energy-peaked BL Lac objects (HBLs). The similarities imply common origin of the variations, plausibly the most energetic tails of the synchrotron emission produced by the relativistic electrons in the jets, for both the optical emission of LBLs and the X-ray emission of HBLs.

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