A sterile master input handle assembly comprising: - a non-sterile manipulandum; - a sterile surgical drape draping said non-sterile manipulandum; - said sterile master input handle assembly is mechanically ungrounded and comprises features designed to be tracked by a tracking system to determine position and/or orientation of the master input handle assembly within a predefined work volume for controlling at least one paired slave surgical instrument; characterized in that - said sterile master input handle assembly further comprises at least one sterile accessory, suitable to form a sterile interface of the sterile master input handle assembly for the surgeon's fingers; - said at least one sterile accessory mechanically engages with said non-sterile manipulandum across said sterile surgical drape; - said at least one sterile accessory covers at least one portion of the outer surface of the surgical drape; - said at least one sterile accessory includes at least one interface surface forming said sterile interface for the surgeon's fingers to manipulate the non-sterile manipulandum across said sterile surgical drape; - said at least one sterile accessory forming a sterile handgrip; - said sterile master input handle assembly comprises a further, second sterile accessory so as to have at least two sterile accessories, said sterile accessory and said further, second sterile accessory provide two opposite interface surfaces (307, 307') for the surgeon's fingers to grasp said non-sterile manipulandum across said sterile surgical drape; - said sterile master input handle assembly comprises a degree of freedom between said two opposite interface surfaces (307, 307'). - a non-sterile manipulandum; - a sterile surgical drape draping said non-sterile manipulandum; - said sterile master input handle assembly is mechanically ungrounded and comprises features designed to be tracked by a tracking system to determine position and/or orientation of the master input handle assembly within a predefined work volume for controlling at least one paired slave surgical instrument; - said sterile master input handle assembly further comprises at least one sterile accessory, suitable to form a sterile interface of the sterile master input handle assembly for the surgeon's fingers; - said at least one sterile accessory mechanically engages with said non-sterile manipulandum across said sterile surgical drape; - said at least one sterile accessory covers at least one portion of the outer surface of the surgical drape; - said at least one sterile accessory includes at least one interface surface forming said sterile interface for the surgeon's fingers to manipulate the non-sterile manipulandum across said sterile surgical drape; - said at least one sterile accessory forming a sterile handgrip; - said sterile master input handle assembly comprises a further, second sterile accessory so as to have at least two sterile accessories, said sterile accessory and said further, second sterile accessory provide two opposite interface surfaces (307, 307') for the surgeon's fingers to grasp said non-sterile manipulandum across said sterile surgical drape; - said sterile master input handle assembly comprises a degree of freedom between said two opposite interface surfaces (307, 307').