A cell transistor includes sourcedrain regions formed at a lower level than part of its channel region. The light source provides an illumination light. If the lateral arm causes an imbalance, a strain occurs on the lever to which a strain sensor is attached. In this way, the guidance control is more responsive to actual variance of the true heading error and is less responsive to random error in sensing the target heading, which random error would otherwise increase in its contribution to corrective guidance movements approaching the point of intersection. The cannula comprises a sterilizable polymer which functions as a waveguide.