Abstract: A controller for moving a first part of a surgical robot arm, the surgical robot arm comprising a plurality of arm segments separated by a plurality of driven joints, in response to an external force being imparted on a second part of the surgical robot arm, the controller being configured to: determine a torque at each of the plurality of joints which results from the force imparted on the second part of the surgical robot arm; calculate from the determined torques a resultant force which acts on the first part of the surgical robot arm as a result of the external force being imparted on the second part of the surgical robot arm; calculate a desired velocity of the first part of the surgical robot arm for a time subsequent to a current time by evaluating an equation of motion modelling Coulomb and viscous frictional forces using a backward Euler approximation, the equation of motion having inputs of the calculated resultant force which acts on the first part of the surgical robot arm; and the current velocit

Abstract: A rotatable mirror assembly comprises a rotatably mounted support (1); and a mirror (7) mounted to the support for rotation therewith. The mirror (7) is mounted at a position (5) laterally offset from the axis (6) of rotation of the support, the mirror being spaced from the remainder of the support to allow the mirror to distort when it is rotated.