Introduction

The image plane(s) in the star separator positioned at the same location as the StS roof mirror (the mirror which separates the beams from the two stars) are particularly important as they play a crucial role in the fundamental calibration of PRIMA. The metrology system can be viewed as measuring the distance from the FSU input fibre spatial filter (in an image plane in the laboratory) to this image plane 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. It is interesting to note that this reference surface is best defined by the image of the metrology system retroreflector (RR3) in the mirror RR2. This metrology system image plane is co-located with an image plane of the AT starlight beams. Any difference in the shape (curvature or tilt) of these co-located image planes will lead to a phase error which varies as the stellar images are moved around the image plane (note that there is complete freedom to move both stars around independently through the combined action of the derotator and the tip-tilt correction - the StS pick-off mirrors can ensure that the starlight and metrology beams reach the FSU even if the location of the stellar images moves around in this plane). It is thus critical that the two image planes are aligned at the sub-micron level, and that they have the same curvature.