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In this paper, we present the results of Monte Carlo simulations of$\gamma$-ray and cosmic ray proton induced extensive air showers as detected bythe TACTIC atmospheric Cherenkov imaging telescope for optimizing its triggerfield of view and topological trigger generation scheme. The simulation studyhas been carried out at several zenith angles. The topological triggergeneration uses a coincidence of 2 or 3 nearest neighbour pixels for producingan event trigger. The results of this study suggest that a trigger field of11$\times$11 pixels ($\sim$ $3.4^o$ $\times$ $3.4^o$) is quite optimum forachieving maximum effective collection area for $\gamma$-rays from a pointsource. With regard to optimization of topological trigger generation, it isfound that both 2 or 3 nearest neighbour pixels yield nearly similar results upto a zenith angle of ~$25^o$ with a threshold energy of $\sim$ 1.5 TeV for$\gamma$-rays. Beyond zenith angle of ~$25^o$, the results suggest that a2-pixel nearest neighbour trigger should be preferred. Comparison of thesimulated integral rates has also been made with corresponding measured valuesfor validating the predictions of the Monte Carlo simulations, especially theeffective collection area, so that energy spectra of sources (or flux upperlimits in case of no detection) can be determined reliably. Reasonably goodmatching of the measured trigger rates (on the basis of $\sim$ 207 hours ofdata collected with the telescope in NN-2 and NN-3 trigger configurations) withthat obtained from simulations reassures that the procedure followed by us inestimating the threshold energy and detection rates is quite reliable.

Simulation studies for optimizing the trigger generation criteria for the TACTIC telescope