Experimental determination of the refractive index spectral and thermal variation and transparency quantification of fluorine fluids for the MezzoCielo telescope

MezzoCielo (or “half of the sky”), novel concept of extremely large field-of-view monocentric optical devices designed to continuously patrol the whole sky, is an optical sphere which requires the filling of its inner volume with special fluid in order to be actually convergent with reasonable focal ratio (for spherical aberration reduction). This fluid has to be characterized by two main properties, namely low refractive index and extremely high transparency in the visible range, but, of course, many other characteristics, such as thermal and chemical stability over time, no toxicity or flammability, compatibility with glass and metals, are also desirable. By virtue of these considerations, two fluorine liquids have been selected: the fluids commercially known as FC-72, perfluorohexane with nd = 1.251 at T = 25°C, and Novec7200, ethoxy-nonafluorobutane with nd = 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.