Final Tests before MELIPAL "First Light"
Secondary Mirror Now in Place
21 January 2000
Happy expectations are again growing at ESO's Paranal Observatory. For the third time in less than two years, the special moment of "First Light" for an 8.2-metre VLT Unit Telescope is getting close -- now is the turn of MELIPAL.
Yesterday morning (January 20), the secondary mirror for MELIPAL was successfully coated with a thin layer of highly reflecting Aluminium. For this, the light-weight Beryllium mirror (about 50 kg) with a diameter of 1.1-metre and covered with a layer of Nickel was carefully cleaned. It was subsequently placed in the large coating plant in which a target of exceedingly pure Aluminium is bombarded by fast argon ions. In this "sputtering" process, large numbers of Aluminium atoms are released from the target and deposited on the mirror.
After about 15 minutes, the Aluminium layer had reached the desired thickness, 80 nm (=0.00008 mm), or about 600 Aluminium atoms on top of each other. When the plant was opened, the reflectivity was measured as 91%, an excellent value. The equally successful coating of MELIPAL's main 8.2-m mirror was done in the same plant in late 1999, c.f. ESO PR Photos 42a-ad/99.
In the evening of January 20, the mirror was placed in its protective box and transported to the telescope enclosure. The mechanical structure of MELIPAL was tilted towards the horizon for easy access to the M2-Unit, the electro-mechanical support system for the secondary mirror at the top end. After removal of the M2 dummy mirror, the newly coated Beryllium mirror was cautiously mounted on its support. This delicate operation went smoothly, without any problems.
With the optical mirrors in place, a further series of careful tests and thorough checks of all telescope functions will now follow. This will include extremely accurate balancing of the 450-tonnes telescope frame on its hydrostatic oil bearings, as well as precise adjustment of the various motions. Everything is controlled and monitored by computers and the ESO engineers will endeavour to tune the performance of the entire telescope to a high level of perfection, already at this stage.
When the "First Light" observations are made some days from now, the light from the chosen celestial objects will be registered by the VLT Test Camera at the Cassegrain Focus. This comparatively simple instrument was also used for the "First Light" for ANTU and KUEYEN . It is mounted on the telescope's optical axis within the M1 Mirror Cell, just behind the main mirror.
One or more of the astronomical images that will be obtained during MELIPAL "First Light" event will be made available on the web right after this important milestone of the VLT project.
A series of digital photos, most of which were obtained on January 20, illustrate the work on the M2 mirror described above.