The VLTI Auxiliary Telescopes

General description

The following photos of a 1/20 scale model built by AMOS illustrate the main conceptual features of the VLTI Auxiliary Telescopes and the "Paranal Railway" system.

ESO Press Photo 25a/98

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Model of an Auxiliary Telescope during Observing Conditions.

The 1.8-m telescope (with an Alt-Az mount, i.e. exactly like the Unit Telescopes) is rigidly anchored to the ground by means of a special interface. The light is directed via a series of mirrors (the Coudé Optical Train) to the bottom of the Telescope. From the Coudé Relay Optics it is sent on to the underground Delay Line Tunnel.

The AT Enclosure (telescope dome) consists of 2 x 3 segments and is here fully open. It protects the lower part of the Telescope structure from strong winds. The Enclosure is supported by the Transporter that also houses electronic cabinets and service modules for liquid cooling, air conditioning, auxiliary power, compressed air, etc.

During astronomical observations, the Enclosure/Transporter is mechanically disconnected from the Telescope and is anchored independently of the Telescope on the rail foundations that rest on soft elastomeric pads. This ensures that vibrations generated by the Enclosure/Transporter or from the ground are not transmitted to the Telescope. At the time of the observations, the Telescope is controlled remotely from the VLTI Control Room.

ESO Press Photo 25b/98

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An Auxiliary Telescope during Relocation from one observing station to another. The Transporter carries the Telescope using a rail network connecting all 30 stations.


The Relocation is a complex operation that involves quite a few steps:
First, the Telescope inside the closed enclosure is undocked from the ground and lifted by jacks located on the Transporter.  The electrical cables to the station are disconnected and power is supplied by the on-board battery set. The Transporter is then unlocked and moved sideways so that the Coudé Relay Optics Box can be lifted up from inside the station. This box can be seen in the photo after it has been lifted on the left side of the Transporter. The lid that protects the station (the white octagon) is then placed on top of the station.
Next, the Transporter is moved along the rails to the chosen station. This movement may include a rail crossing at which the Transporter wheels are first lifted, then rotated by 90° and again placed on the tracks in the perpendicular direction. When reaching the desired station, the station lid is opened, the Coudé Relay Optics is lowered inside the station pit and the Telescope and Transporter are anchored to their respective foundations.
The positioning accuracy of the various mechanisms is so good that no local re-alignment should be necessary, although this can be done remotely from the control room before the next observation.

During the Relocation itself, the control of the Transporter is done locally from a handset by a maximum of two operators.

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Auxiliary Telescope being transported from/to the Mirror for Maintenance.

This will be done during Maintenance activities such as mirror re-coating and overhaul. One hydraulic axle is attached on each side of the Transporter and the complete Telescope & Transporter is lifted and then pulled by a truck up/down the 3 kilometers separating the base camp from the Observatory Platform. On this picture, an open station can be seen with the telescope anchoring devices and the cylindrical pit into which the Coudé Relay Optics is located during observation.

The advantages of the Auxiliary Telescopes

Although the ultimate sensitivity of the VLTI will be obtained when combining the VLT 8.2-m telescopes, the Auxiliary Telescopes constitute an essential element of the VLTI for several reasons:

  • The ATs provide the best imaging capability of VLTI by complementing the array of the four 8.2-m telescopes.  The Auxiliary Telescopes can be placed on any of the 30 possible stations and provide therefore many interferometric baselines. This makes superior interferometric imaging possible.
  • The ATs provide the longest possible baseline of the VLTI (202 meters), fully utilizing the restricted space available on the Paranal mountain platform.
  • The ATs enable full time use of the VLTI facilities. They are 100% dedicated to VLTI, while the 8.2-m UTs are only intermittendly available for interferometric observations.
  • The ATs are used by the "Narrow Angle Astrometry" mode of VLTI (measuring extremely accurate positions of objects in the sky). This requires long baselines as well as regular and long-term monitoring, not achievable with the 8.2-m telescopes.
  • The ATs will make it possible to perform full testing and commissioning of the second generation of VLTI Instruments, without having to make use of valuable light from the 8.2-m telescopes.