Introduction

It should be noted that the narrow angle astrometry baseline is different from the wide angle astrometry baseline. The narrow angle astrometry baseline is determined by the end point of the metrology system (in the StS). The metrology system can be viewed as measuring the distance from the FSU input fibre in an image plane in the laboratory to an image plane in the StS, situated close to the roof mirror which separates the beams from the two stars in the StS. In this way the metrology system is creating a virtual reference surface within the ATs against which the narrow angle astrometry is performed. See Section 21.1 for further details.

The metrology reference surface viewed through the AT entrance pupils defines the narrow angle astrometry baseline. If any of the powered optics between the entrance pupil and the reference surface are moved then the baseline will be changed. If, for example, M2 is moved one $\mu$m off axis, the baseline changes by a few tens of $\mu$m, generating an error term which contributes significantly to the overall error budget.

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. 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.