  In this paper, we present the results of Monte Carlo simulations of
$\gamma$-ray and cosmic ray proton induced extensive air showers as detected by
the TACTIC atmospheric Cherenkov imaging telescope for optimizing its trigger
field of view and topological trigger generation scheme. The simulation study
has been carried out at several zenith angles. The topological trigger
generation uses a coincidence of 2 or 3 nearest neighbour pixels for producing
an event trigger. The results of this study suggest that a trigger field of
11$\times$11 pixels ($\sim$ $3.4^o$ $\times$ $3.4^o$) is quite optimum for
achieving maximum effective collection area for $\gamma$-rays from a point
source. With regard to optimization of topological trigger generation, it is
found that both 2 or 3 nearest neighbour pixels yield nearly similar results up
to 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 a
2-pixel nearest neighbour trigger should be preferred. Comparison of the
simulated integral rates has also been made with corresponding measured values
for validating the predictions of the Monte Carlo simulations, especially the
effective collection area, so that energy spectra of sources (or flux upper
limits in case of no detection) can be determined reliably. Reasonably good
matching of the measured trigger rates (on the basis of $\sim$ 207 hours of
data collected with the telescope in NN-2 and NN-3 trigger configurations) with
that obtained from simulations reassures that the procedure followed by us in
estimating the threshold energy and detection rates is quite reliable.
