Introduction

At several places in the VLTI some form of spatial filtering is performed, whereby the light from certain parts of the image plane is selected to continue along one path through the VLTI, and other light is rejected or sent along a different path. Spatial filtering processes such as this 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, as well as modifying the spectral sensitivity distribution (see Section 3.3 for further details of both these effects). As a result, this document will study in detail the expected wavefront corrugations immediately before each VLTI component which performs spatial filtering. The second of these components is the FSU spatial filter, which selects the light from one speckle in the telescope image plane.

Previous studies of the temporal properties of interferometric fringes indicate that the high-frequency fringe motion is dominated by the effects of wavefront corrugations across the aperture plane when the variance in the wavefront phase across the aperture is $\mathrel{\hbox{\rlap{\hbox{\lower4pt\hbox{$\sim$}}}\hbox{$>$}}}1$ radian. For this reason it will be essential to simulate the level of wavefront corrugation expected in order to determing the performance of the fringe tracker.