OPTICS FOR ASTRONOMY
First mirror ready for Very Large Telescope
Roland Roux
The first of four 8.2-m-diameter mirrors being made for the European Southern Observatory (ESO, Chile) has completed its technical acceptance review and is currently reinstalled in its shipping container ready for shipment. On November 21, 1995, the giant mirror was formally handed over to the ESO during a ceremony at REOSC (Saint Pierre du Perray, France)---a subsidiary of the SFIM grou¥(Massy, France)--which is the company contracted to grind and polish the optics, the largest ever cast and polished.
The first mirror will be stored at REOSC prior to shipment to the Very Large Telescope (VLT) Observatory on Cerro Paranal in northern Chile. As soon as mechanical assembly of the telescope is complete, the optic will be installed in the first unit of the VLT.
The contract for production of the mirrors--which was awarded to REOSC in 1989---included transportation of the blanks by shi¥from Schott Glaswerke (Mainz, Germany) to the REOSC facility and then highly accurate polishing and testing of each giant mirror. The surface area of each optic exceeds 50 m2, so, in order to meet the technical specification required, REOSC developed and equipped a novel high-technology optical workshop.
This special polishing facility is fitted out with two large machines, one for grinding and one for polishing. The mirrors are thin and flexible so for both grinding and polishing operations they are supported by 150 computer-controlled actuators; all operations on the mirror are computer controlled. State-of-the-art interferometers installed at the to¥of a 30-m-high tower and located at the mirror`s center of curvature check the surface accuracy during polishing.
Measurements made during November 1995 on the first finished optic showed that the final optical surface is correct to within 5 ¥ 10-5 mm (50 nm) and confirm that REOSC is able to meet the telescope`s specifications. This surface accuracy is equivalent to a deviation of only 1 mm over a surface with diameter 165 km--about the area of Paris.
Polishing of the second VLT mirror and grinding of the third has already started. The fourth blank optic will be moved to the REOSC facility in March 1996.
In addition to the VLT mirrors, REOSC will also be polishing two 8.2-m-diameter mirrors for the GEMINI program of the Association of Universities for Research in Astronomy (AURA) in the USA.