Functional Description

The Delay Line (DL) system has two objectives:

• re-imaging the telescope exit pupils to a fixed position inside the interferometric laboratory.
• Compensation of the OPD between two different interferometer arms forms the main objective.

The OPD to be compensated consists of three contributions:

• The Internal (geometrical) OPD corresponding to the differences in optical path lengths measured from the interferometer subapertures (i.e. telescope primary mirrors) to the point of beam combination.The internal OPD is purely static.
• The External (sideral) OPD when observing a source for which the incident wavefront is not parallel to the baseline vector. The external OPD is slowly varying with time according to the diurnal motion of the observed object due to Earth's rotation.
• The OPD due to atmospheric turbulence and micro vibrations of the mechanical structures which varies rapidly

Subsystems of the DL System

The DL system consists of different subsystems. A cat’s-eye optical system, equipped with light-weighted Gold-coated Aluminum mirrors, reflects the light coming from the telescope to the instrument. The tertiary mirror of the cat’s-eye is the Variable Curvature Mirror (VCM) which is used to re-image the telescope exit pupil.

A carriage enables the longitudinal displacement of the cat’s-eye along the u-direction using linear motors and optical scale encoders. Rectified rails provide the guiding of the carriage. A second fine OPL adjustment stage is provided by a Piezo translator on which the tertiary mirror (VCM) is mounted.

A laser metrology system provides accurate measurement of the OPL through the DL measured from a reference plate which is located close to centre of the tunnel.

A control system (LCU and software) ensures proper motion of the DL. Commands of the control system are transferred to the carriage and Piezo element by an IR data link. Locking mechanisms protects the moving system from Earthquakes.

Since the rails are affected by distortions due to temperature changes, a monitoring system DELIRIUM (Delay line rail alignment monitoring system) is permanently installed on-board the DL carriage.DELIRIUM is a rail observer based on capacitive sensors, monitoring the position and altitude of the carriage with respect to a mechanical reference. The profile of the guiding rail is reconstructed and filtered with the calibrated influence functions of the rail supports to generate a vector of corrective commands.

A schematic view of a VLTI DL is shown in Fig.1

For pictures and further information on the VLTI DL, have a look at the press releases.