A system for determining a position of a probe relative to an object such as a head of a body of a patient. The head includes a surface such as a forehead having a contour. The head is placed in a cradle equipped with an arc. During surgery, an optical scanner determines the position of the forehead relative to a base ring. An array for receiving radiation emitted from the probe and from the base ring generates signals indicating the position of the tip of the probe relative to the base ring.