eso8801 — Organisation Release
Strange Building for the ESO New Technology Telescope Arrives at the La Silla Observatory
9 February 1988
Last week, M/S “Cervo" arrived in the harbour of Valparaiso, Chile, with the packaged parts for the building which will house the ESO New Technology Telescope (NTT). Soon thereafter, the 350 ton load was hauled by road to the ESO La Silla observatory in the Atacama desert, some 600 km north of Santiago de Chile. Here, at one of the best astronomical sites on earth, the giant mechanical puzzle will now be put together to form one of the strangest telescope domes ever seen. The 3.5 m New Technology Telescope is the forerunner of the ESO Very Large Telescope (VLT) (see eso8602 and eso8717). When the NTT is ready later this year, it will be the technologically most advanced telescope in the world.
The NTT project
With a prime mirror of almost the same size as that of the ESO 3.6 m telescope (in operation since 1976), the NTT will take full advantage of the excellent observing conditions at La Silla. It will allow observations of very faint and distant objects in the Universe, as well as very detailed studies of the brighter ones. At the same time, it incorporates a large number of technical innovations which will now be tested under realistic conditions. In this way, much valuable experience will be gained for the design and construction of the 16 m VLT.
The realization of the NTT Project started in 1982, at the time when Switzerland and Italy joined ESO. With the entrance fee from these two countries, ESO decided to build the prototype of a new telescope generation, taking full advantage of the most advanced technology in the fields of optics, mechanics and computer science. The goal was to construct the best telescope in the 4 m class, but at only a third of the usual cost. The project would try out new ideas which could later be implemented in more ambitious projects, like the VLT.
The rotating building
The NTT will be mounted in a rotating building with an unusual octagonal shape. It has been designed to ensure maximum exposure of the telescope to the external environment during observation, while protecting the structure from strong winds and dust. Furthermore, the floor of the building is actively cooled and the temperature in the telescope room and in the instrument rooms is maintained at the level of the outside temperature at night. These features will improve the NTT performance, as compared to other telescopes, since there will be less turbulence in the surrounding air and the images of astronomical objects will therefore be sharper.
The exact shape of the building was determined by wind tunnel tests at the Technical University of Aachen.
The building was conceived at ESO and designed and manufactured by a consortium of Italian companies (MECNAFER, Mestre, ZOLLET, Belluno and ANSALDO Componenti, Genova) in close cooperation with a number of European industries. One of these is RKS France who manufactured an extremely precise roller bearing with diameter of no less than 7 m, a key component of the rotating system.
It will take almost six months to complete the erection of the rotating building at La Silla (eso8714).
An unusual mechanical structure
The telescope structure has an alt/az mounting and rotates around one vertical and one horizontal axis, the new trend for optical telescopes. It was mechanically completed in the course of 1987 and after implementation of the electronic hardware and software, it was tested extensively at INNSE, Brescia. This gave complete confirmation of the expected performance. The moving structure of the telescope weighs about 110 tons and is supported by a multi-pad hydrostatic bearing, designed by INNSE to guarantee extremely high rigidity with minimum friction. It is thermally controlled in order not to introduce air disturbances which might degrade the optical quality. The axes of the telescope are guided by a group of water-cooled servodrives, and the absolute position of the telescope by an encoder system which allows pointing on the sky with an accuracy of 1 arcsecond, a figure unsurpassed by any other existing telescope of this size.
The NTT will be dismounted and shipped to Chile within a few months and then erected inside the rotating building. It is expected that the first sky observations begin at the end of 1988.
Here is a summary of the main technological innovations which have been incorporated into the NTT:
- the form of the thin primary mirror and the alignment and focussing of the secondary mirror are computer controlled so that the telescope is always in optimal shape (active optics)
- fast switching of the light beam between two different auxiliary instruments
- alt-azimuth mounting with very high pointing and tracking accuracy
- advanced, flexible and very user-friendly control system
- possibility of remote control from Garching
- open dome structure so that the telescope is fully exposed to the external environment during observation
- rotating compact building.
- Estimated price of the NTT project: 25 million DM
- Size of NTT building: 18 m (high) × 17 m × 17 m
- Weight of Building: 250 tons
- Length of telescope tube: 8 m
- Weight of telescope: 125 tons
- Main mirror material: Zerodur
- Size of main mirror: Diameter 3.58 m; Thickness 24 cm; f/2.2
- Weight of main mirror: 6 tons
- Foci: 2 Nasmyth platforms, f/11, with fixed infrared and visual multi-function instruments
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