An apparatus having: a system configured to measure an object; a base structure; an object support constructed to hold the object, the object support movably supported on the base structure; a balance mass configured to absorb a reaction force arising from movement of the object support; an actuator connected to the balance mass and the base structure, the actuator configured to apply a force to the balance mass and the base structure; a sensor configured to produce a signal for a measured characteristic of the base structure corresponding to a disturbance, or its effect, acting on the base structure; and a control system configured to determine, based on at least the signal for the measured characteristic of the base structure, a signal for the actuator to apply a force to the base structure and/or the balance mass.