eso9819 — Organisation Release
First Large VLT Mirror Successfully Coated
21 May 1998
One of the most crucial operations before the VLT UT1 First Light was successfully concluded late last evening at Paranal. For the first time ever, an astronomical mirror with a total surface of more than 50 m 2 has been coated with a thin (less than 0.0001 mm) and highly reflective layer of extremely pure aluminium. The very good result of this critical operation guarantees that the VLT UT1 will now be able to capture efficiently the faintest light from celestial objects.
Following the excellent outcome of the optical alignment reported earlier, cf. eso9805, the VLT UT1 is set to achieve a fine performance, already during the initial stages of operation. The ESO staff at Paranal is optimistic that it will now be possible to carry out the remaining fine-tuning according to the established schedule. This implies that the moment of First Light will occur as planned, i.e. during the night between May 25 and 26.
The coating of the 8.2-m Zerodur mirror was preceded by a careful cleaning of its large optical surface during the afternoon of Tuesday, May 19. A CO 2 -snow spray was used to blow loose dust from the surface. This delicate manual operation took almost 6 hours. From next year, an automatic cleaning device will be installed that will take over this cumbersome work, and ensure a greater degree of uniformity.
The mirror was then installed in the Coating Tank under the Magnetron. The tank was evacuated during 6 hours until the high vacuum needed for the coating was reached. The coating process was started in the morning of Wednesday, May 20, at 6:00 am local time (12:00 hrs in Europe). It was completed within 30 minutes.
After the pressure was back to normal, the tank was opened and the coated mirror was inspected visually. Its reflectivity was measured with a special device at a wavelength of 670 nm (in red light). Near the edge of the mirror, a very high value of 91.7% was found. Even in some small areas near the central hole, where the dust had not been removed completely, the minimum reflectivity was still an impressive 89.9%.
The mirror was then extracted from the coating tank with the Handling Device, raised, and the M1 Cell was brought in position underneath. Finally, the mirror was carefully lowered onto the supports in the Cell.
The M1 cell with the mirror has been moved back to the telescope in the morning of Thursday, May 21.
The electronic images below record various phases of the above process.
More information about the Very Large Telescope (VLT) project is available on the ESO Web.