The mechanism for the Differential Delay Line (DDL) has been developed by Mecartex on behalf of an European consortium of universities. The Consortium has selected the solution proposed by Mecartex for the realization of a first prototype of the DDL for an instrument of the European Southern Observatory (ESO).
DDL is one subsystem of the PRIMA instrument enabling microsecond-astrometry in the Very Large Telescope Interferometer (VLTI). It is able to reduce the effect of atmospheric disturbance and therefore improve the performance of observations from the Earth with the VLTI. The VLTI is part of the Very Large Telescope, which is operated by ESO and situated on Cerro Paranal in Chile.

The DDL developed by Mecartex is a translation stage based on flexures with a stroke of 70mm, a flatness of 3µm and a tip-tilt error below 2 arcsecond. After correction with the Piezo actuator, the tip-tilt requirements could also be fulfilled.
Four systems based on this solution and one spare have been delivered to the summit of Cerro Paranal and installed as part of the Very Large Telescope Interferometer (VLTI) infrastructure in Chile in August 2008.