Interferometric mode of the European Southern Observatory Very Large Telescope

The interferometric mode of the ESO Very Large Telescope (VLT) permits coherent combination of stellar light beams collected by four telescopes with 8-m diameter and by several auxiliary telescopes of the 2-m class. While the position of the 8-m telescopes is fixed, auxiliary telescopes can be moved on rails, and can operate from 30 distributed on the top of the Observatory site for efficient UV coverage. Coherent beam combination can be achieved with the 8-m telescopes alone, with the auxiliary telescopes alone, or with any combination, up to eight telescopes in total. A distinct feature of the interferometric mode is the high sensitivity due to the 8-m pupil of the main telescopes which will be compensated by adaptive optics in the near-IR spectral regime. The VLT Interferometer (VLTI) part of the VLT Programme is conceived as an evolutionary program where a significant fraction of the interferometer's functionality is funded, and more capability may be added later while experience is gained and further funding becomes available. Major subsystems of the present baseline VLTI include: three auxiliary telescopes, three delay lines which permit combining the light from up to four telescopes, and a laboratory which contains an imaging beam combiner telescope and enough space to accommodate a number of experimental setups. This paper presents a general overview of the recent evolution of the project and its future development.