Abstract
A metrology system is needed to calibrate the internal Optical Path Difference (OPD) within the interferometric array. The objective is to determine the differential optical path between any pair of telescopes of the array with an accuracy of some 100μm. This OPD will be monitored from the Interferometric Laboratory to a retroreflector located on the telescope’s secondary mirror or to a flat mirror located at the folded Nasmyth focus. It will be performed “off-line” every time a change in the interferometer configuration has happened or when thermal disturbances have occurred. A concept has been derived which uses an external light source located in the pupil plane area. In order to verify its applicability, a test bench has been procured and integrated at ESO. This test bench includes a mock-up of two delay lines with translation stages and control units, and metrology units. First results are presented.
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