Introduction

The VCMs in the VLTI couple the Zernike piston mode to high-order Zernike modes, meaning that the piston mode in the interferometric beam is modulated according to the wavefront corrugations across the aperture plane. This may cause the astrometric phase to depend on the seeing conditions, the performance of the STRAP units or the effects of vibration. For this reason the wavefront corrugations expected immediately before the VCM will be investigated in detail here.