Scintillation

Phase perturbations in the wavefronts from a star produce fluctuations in the optical amplitude after the light has propagated down through the atmosphere. The phase measured by the FSU corresponds to an integral of the contributions over both AT pupil planes, and this measurement will naturally be weighted by the amplitude at each point in the pupil plane. In the absence of internal seeing within the VLTI and with accurate pupil relay the scintillation in the pupil plane in front of the FSU will match the scintillation across the AT aperture. However, if there is internal seeing within the VLTI away from the pupil plane additional scintillation can be generated at the FSU. Incorrect pupil relay can also modify the level of scintillation at the FSU.

The simulations described in Section 3 were used to investigate the approximate level of scintillation expected at the telescope aperture. Example plots from $4$ timesteps of the simulations were shown in Section 4. Figure 14 shows the optical amplitude as a function of position in the AT aperture plane. It is clear that the amplitude fluctuations are dominated by one of the layers moving from the upper right to the lower left. The dominant layer is the higher one ($5\mbox{ km}$ above the telescope - see Section 3.2).