  During a tracking mode observation, every telescope with an alt-azimuthal
mount shows a rotation in the field of view (FoV) due to the diurnal motion of
the Earth. The angular extension of the rotation depends mainly on the time
length of the observation, but also on the telescope's latitude and pointing,
because it is determined by the evolution of the parallactic angle of the
target, which is a function of those two parameters. In many cases, the
rotation of the FoV can be exploited to assess some optomechanical properties
of the telescope, e.g. the alignment of the optical elements or the motors'
precision during the tracking. As a consequence, it could happen that a proper
simulation of the FoV rotation is crucial to program an observation aiming at
calibrating the whole system. We present a tool to simulate the apparent
rotation of the FoV, calculating the actual "star coverage" exploitable for
scientific goals. Given the FoV and the pointing direction, the software
calculates the angular extension of the rotation, considering only the stars
observable by the telescope below the magnitude limit. This tool will be
adopted to schedule the pointing calibration runs of the innovative ASTRI-Horn
Cherenkov telescope, developed by INAF for gamma-ray ground-based astronomy,
but with the potentiality to produce sky images as an ancillary output, using
the so-called Variance method. By exploiting the FoV rotation with the Variance
method, the critical assessment of the camera axis can be successfully
performed.
