Mahmood,

Search on the topic of inertial navigation. You'll want to look at using rate gyros for platform stabilization on which accelerometers are mounted. This'll be old technology you are searching for - probably between 1945 and 1975. A decent enough place to start is the Journal of Aeronautics and Astronautics. An outstanding author to look for would be Charles Stark Draper. Singer-Kearfott published a tech note between 1960 and 1965 describing the used of synchros and resolvers as platform position feedback mechanisms. Ignore anything you find regarding strapdown inertial nav systems - it's not what you want for stabilization of a physical platform.

Seems to me the amount of torque or force needed is dependent not only on the mass of the platform and associated equipment, but also the focal length of the object viewed by the camera.

Servos are fun projects so enjoy yourself,

Bruce