Abstract

In the present Very Large Telescope (VLT) linear array concept four independent telescopes of the 8-m class are coupled together. This offers the unique possibility to operate the telescopes as an optical and/or infrared long base-line interferometer. The first target for an interferometric operation is the infrared range from 5 to 20 μm and later a gradual expansion to shorter wavelengths. In the interferometric mode the gain of the 8-m diam single apertures of the VLT is only given, if adaptive optics is used for a real-time partial or full phase compensation of the degradations due to atmospheric turbulence. When the condition atmospheric coherence area > telescope diameter is not fulfilled, adaptive optics is a technique to restore full coherence over the whole pupil. The gain for the SNR in interferometry goes at least with the inverse cube of the point spread function. Therefore, no other parameter can bring about such an improvement. The correction problem is extremely severe at visible wavelengths due to the high temporal and spatial frequencies of the fluctuations. Instead of several thousand subapertures required in the visible for a full correction, only 100-200 subapertures have to be corrected in the infrared case. Additional to the phasing of the individual telescopes, the whole array has to be phased. In a system with independently mounted telescopes the longitudinal position of the pupil images and the effects of pupil foreshortening have to be corrected in addition to the pure path-length correction.

© 1986 Optical Society of America