eso9813 — Communiqué de presse institutionnel
World's Largest Mirror Installed on First VLT Unit Telescope
17 avril 1998
The world's largest optical mirror - with 8.2-m diameter and a surface of more than 50 m 2 , polished to the highest perfection - was today put in place at the First VLT Unit Telescope (UT1) at the Paranal Observatory.
If the present schedule is maintained and no significant technical problems occur, the first "scientific images" will be obtained with the UT1 during the night of May 25 - 26, 1998 (Chilean time). It would then be possible to present the first scientific images less than 24 hours later, in the morning of May 27, at Press Conferences in the ESO members countries and elsewhere that are part of the First Light Event.
The recent work at Paranal has included the first pointing tests of the UT1 , by means of the 20-cm telescope installed on the main structure. Already during the second night of these tests, the pointing error (RMS) was below 10 arcsec, an excellent value at this early stage. Continued improvements of the pointing software will bring this uncertainty further down to a few arcsec, and later to the design goal of 1 arcsec. This high precision will guarantee rapid and correct acquisition of the astronomical objects to be observed while saving precious observing time.
The main event during the past days was the exchange of the 8.2-m concrete dummy mirror with the "real" Zerodur mirror . This operation is documented in the present series of photos and starts with the removal of the M1 Cell with the concrete dummy from the lower end of the UT1.
The M1 cell was transported down the mountain to the Mirror Maintenance Building (MMB) at the Base Camp in the early morning on April 5. After some minor modifications to the Cell had been made, the concrete mirror was lifted up and the Cell was moved out of the building. Then, the concrete mirror was lowered upon its former support and placed near the box with the Zerodur mirror at the nearby storage area. It will be needed again during the coming weeks to test the installation of the Zerodur mirror in the coating unit, already installed in the MMB since last year.
On April 12, the box with the polished Zerodur mirror was placed below the Mirror Handling Device and the precious cargo was lifted off the supports on which it had been resting since it was ready at the REOSC factory in France. 150 interface cups which connect it to the M1 Cell were installed. Then the M1 Cell was placed directly below the mirror which was safely deposited on the computer-controlled supports that will give it the correct shape at the telescope. After further checking and under a protective cover, the M1 Cell with the 8.2-m Zerodur mirror was transported to the telescope and again attached to the bottom end. The mirror is still to be aluminized - this is planned in May.
Following the subsequent installation of the Beryllium secondary mirror (M2) at the top of the telescope tube, the fine optical alignment will start. This involves a careful optimization of the surface of the M1 Zerodur mirror by means of the active supports, and also the exact centering of the large M1 and M2 mirrors. Both are controlled by special software via the VLT Control System VCS). The utmost precision is necessary to guarantee that the sharpest possible images will be obtained in the excellent astroclimate at Paranal.
At the same time, other VCS software work continues to ensure that the heavy telescope can follow the diurnal motion of celestial objects with superior accuracy (optimization of the "pointing model").
Only when the engineers are satisfied that the UT1 is in perfect shape, optically, mechanically as well as electronically, will green light be given to open the shutter of the Test Camera at the Cassegrain focus for the first scientific exposures. That will mark the long-awaited moment of "First Light".
More information about the Very Large Telescope (VLT) project is available on the ESO Web.