Thermo-mechanical design and birefringence analysis for the Mezzocielo assembly

MezzoCielo represents a novel type of monocentric optical instrument devoted to whole-sky monitoring due to its extremely large field of view. The latter property, combined with the need to realize relative wide optical spheres (with diameters up to few meters) and the limitations related to the manufacturing of large optical elements having high performance, requires the adoption of a segmented structure, presenting, for instance, a Platonic solid-like shape, composed by several identical lenses, supported by a metallic frame along the edges of the solid itself. In this paper, the main aspects concerning the sizing of such frame (chosen to be dodecahedral) and a thermo-mechanical analysis of the lenses support assembly, both analytical and numerical, will be presented. In particular, it will be shown how the lenses are able to operate with little temperature difference across their volume independently from the surrounding conditions and the way in which the telescope can withstand external low temperatures without manifesting high thermal stresses, while maintaining, at the same time, constant focal length. Subsequently, a birefringence investigation completes the opto-mechanical analysis and it will be employed as a further instrument to select the more appropriate lens shape. Eventually, the same analysis is repeated for all the twelve lenses of the dodecahedral structure.