Fluorine fluid experimental determination of the refractive index, spectral and thermal variation, and transparency quantification of the MezzoCielo telescope

• Published in: Optical materials express 2024-11, Vol.14 (11), p.2542
• Main Authors: Di Rosa, Silvio, Ragazzoni, Roberto, Pelizzo, Maria G., Corso, Alain J., Santi, Giovanni, Arcidiacono, Carmelo, Magrin, Demetrio, Dima, Marco, Farinato, Jacopo, Zaggia, Simone
• Format: Article
• Language: English
• Subjects: Biocompatibility; Field of view; Flammability; Fluorine; Optical properties; Refractivity; Spatial resolution
• ISSN: 2159-3930; 2159-3930
• DOI: 10.1364/OME.531842

The MezzoCielo (or "half of the sky") is a telescope that aims to revolutionize the way astronomical observations are carried out: once operational, it will be able to continuously patrol the whole sky due to its extremely large field of view estimated to be around 10 4 square degrees, showing, at the same time, the spatial resolution required for scientific purposes. It is a monocentric optical device designed on the concept of an optical sphere; the latter requires the filling of its inner volume with special fluid in order to be actually convergent with a reasonable focal ratio (for spherical aberration reduction). This fluid must exhibit two main properties, namely, a low refractive index and an extremely high transparency in the visible range. In addition, many other characteristics are also required: thermal and chemical stability over time, no toxicity or flammability, and compatibility with glass and metals are some examples. On the basis of these considerations, two fluorine liquids have been selected: the fluids commercially known as FC-72, perfluorohexane with n d = 1.251 at T = 25°C, and Novec7200, ethoxy-nonafluorobutane with n d = 1.282 at T = 25°C. In this work, the experimental evaluation of their optical properties (refractive index and transparency) in the visible range and in the thermal interval [-10,+25]°C has been carried out in order to verify the data supplied by the manufacturer and to fully characterize their optical behaviour both spectrally and thermally. All the employed techniques have been firstly applied to commercial pure water, for which extensive literature could be used as reference for the procedures validation.