Proceedings PaperOptical quality and stability of 1.8-m aluminum mirrors
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Within the framework of the NTT 3.5-m and VLT 8-m telescopes' projects, ESO has been trying to assess the potential interest of Aluminum as a substrate for large astronomical mirrors. After a series of test on 500-mm diameter Aluminum mirrors two 1.8-m diameter Aluminum blanks were ordered and produced according to totally different manufacturing processes. Both blanks were figured within specifications, and successfully underwent a series of thermal cycles aimed at verifying their long-term stability towards relaxation of internal stresses. While in the 8-m range the baseline solution selected by ESO, partly for schedule reasons, was the glass-ceramics substrate, the results obtained so far with Aluminum have been undoubtedly positive. Aluminum is considered as a potentially cost and performance- effective substrate for active telescopes, at least in the four- meter range. Further tests are however still required to validate the technology.