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 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 . - 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 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 . <EOS>