A workstation for calibrating a robotic instrument (42) has a distal tip (46) within an X-ray image space (35) of an X-ray modality (32). The workstation employs a calibration controller (50) for calibrating a remote center of motion (RCM) length of the robotic instrument (42) responsive to X-ray images (36) of different poses of the distal tip (46) of the robotic instrument (42) within the X-ray image space (35), and further employs a robotic instrument controller (40) for controlling a guidance of the robotic instrument (42) within the X-ray image space (35) from the RCM calibration. The robotic instrument (42) may include an endoscope whereby the calibration controller (50) is further employed to calibrate a focal length of the robotic instrument (42) responsive to the X-ray images (36) and one or more endoscope image(s) (48) of the X-ray image space (35) for controlling the guidance of the robotic instrument (42) within the X-ray image space (35) from the RCM/focal length calibrations.