Proceedings PaperLAMA (Large Active Mirrors in Aluminum) programme
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In order to settle conclusively the best technical solutions to obtain metallic blanks which could be delivered in large size (at least 8-rn for ESO), and to avoid any possible difficulties in the manufacturing process for flexible glass meniscus, a European programme called "L.A.M.A." (for Large Active Mirrors in Aluminium) was set up. The programme is divided into two phases. The first one, which ended December 1991, addressed the following items: (1) tests to select the best aluminium alloy, (2) aluminium welding, homogeneity and stability, (3) aluminium high precision machining, (4) nickel coating, (5) polishing of the nickel layer, (6) active optics. Furthermore, tests have been conducted to demonstrate that the quality of the mirrors is not altered at various temperatures and after a large number of aluminising and cleaning cycles (corresponding to about 50 years' life). The mirror shape (whose specifications are fully compliant with those of the VLT, as the programme is conducted in cooperation with ESO) was computed under several causes of deformations: evidencing gravity as the predominent effect, and very low distortions as the high thermal conductivity limits the thermal transverse gradient to 0.025°C (Table 1). Second phase of the programme, so far under preparation, will be devoted to the completion of a complete active system in its cell: aluminium meniscus supported by passive and active actuators, the latter monitoring the surface form, together with the wavefront sensor, the active control loop system and the software. An extension of the partnership is envisaged, with additional European partners (UK). The final step is to provide the astronomical community with a high-tech, up-to-date primary mirror telescope. Thus, aluminium alloys are attractive candidates for large lightweight mirror blanks. They exhibit good thermal properties, are readily available at low cost. Major concerns, generally linked use of aluminium for precision optics are the material's softness - and thereby poor polishabiity - and the doubts as to its long-term stability, which doubts have now been cleared with the positive results obtained with the ESO's 1.8 m mirror prototype. In fact, the development of industrial processes to coat the surface with about one hundred microns of nickel (through a Chemical prOCeSS made by TECNOL), has successfully solved the former problem. Many experiments and tests undertaken have led to the conclusion that possible, ageing-induced, warping effects, if any, can aptly be minimised as they are readily absorbed through active correction. The expertise gained by the polisher during the first phase of the LAMA programme shows that the same standards of quality can be achieved on nickel coating as on glass, even if the technique needs a different approach. Aluminium shows a worthwhile advantage over glass due to its thermal characteristics (greater thermal diffusibility), ensuring homogeneous temperature within the blanck, and allows the mirror to reach the ambient temperature faster (a few minutes as against several hours, eliminating the risk of thermal gradients). This would greatly improve the efficiency of thermal control (mirror seeing), which is crucial in a dome. The possibility of monitoring distortions like the correction of possible long-wave mirror like those due to thermal expansion of thermal gradients inside the substrate by active optics systems is now one of the major dedsion-making criteria in favor of an aluminium blank. Finally, brittleness of glass blanks is the most important drawback when using such material for large mirrors. The risk of failure during manufacturing, handling and carrying (espedally nowadays where observing sites are far from manufacturing plants) often necessitates procurement of a spare to avoid programme schedule jeopardy. In this respect, aluminium blanks with higher mechanical characteristics, shorter manufacturing sequences, no brittleness and lower cost, offer important gains over glass blanks. In addition, the manufacturing procedure can be easily organised in such a way that it allows a step-by-step process; any non conformance can be detected at the outset and immediately corrected.