a system for controlling deflections of an elongate body transverse to a longitudinal axis of the body , the system including : a sensor mounted on the body for producing an output signal when the body experiences a deflection in at least one direction transverse to its axis ; heating and / or cooling elements thermally coupled to the body at a location for heating or cooling the body in a manner to thermally deflect the body in a direction transverse to the longitudinal axis ; and a control unit , connected between the sensor and the heating or cooling elements , responsive to the output signal produced by the sensor .