Introduction

The metrology system retro-reflector (RR2 and RR3 in the StS) will only be free of differential path fluctuations if the metrology beams are kept at a stable position in the image plane at M10. If the beams wander in the image plane, they will reflect off different points on RR3 giving OPD changes. It is absolutely essential that all the optics from M10 to the FSU are designed to minimise the movement of the beams in the image plane. The metrology system will cancel out path length errors after M10 to first order if the stellar beams are correctly superimposed on the metrology beams at the M10 image plane. The optical components in the stellar beam before M9 must be used to accurately steer the stellar beams onto the locations of the metrology spots.