Title:The TACTIC atmospheric Cherenkov Imaging telescope

Abstract: The TACTIC $\gamma$-ray telescope, equipped with a light collector of area $\sim$9.5m$^2$ and a medium resolution imaging camera of 349-pixels, has been in operation at this http URL, India since 2001. This paper describes the main features of its various subsystems and its overall performance with regard to (a) tracking accuracy of its 2-axes drive system, (b) spot size of the light collector, (c) back-end signal processing electronics and topological trigger generation scheme, (d) data acquisition and control system and (e) relative and absolute gain calibration methodology. Using a trigger field of view of 11$\times$11 pixels ($\sim$ 3.4$^\circ$$\times3.4^\circ$), the telescope records a cosmic ray event rate of $\sim$2.5 Hz at a typical zenith angle of 15$^\circ$. Monte Carlo simulation results are also presented in the paper for comparing the expected performance of the telescope with actual observational results. The consistent detection of a steady signal from the Crab Nebula above $\sim$1.2 TeV energy, at a sensitivity level of $\sim5.0\sigma$ in $\sim$25 h, alongwith excellent matching of its energy spectrum with that obtained by other groups, reassures that the performance of the TACTIC telescope is quite stable and reliable. Furthermore, encouraged by the detection of strong $\gamma$-ray signals from Mrk 501 (during 1997 and 2006 observations) and Mrk 421 (during 2001 and 2005-2006 observations), we believe that there is considerable scope for the TACTIC telescope to monitor similar TeV $\gamma$-ray emission activity from other active galactic nuclei on a long term basis.