Professor Alessandro Cacciani (1938-2007) has been known world wide for the invention and development of the Magneto-Optical Filter. Hundreds publications present its applications from solar to stellar helioseismology. Astronomer at the Observatory of Roma (1964-1982), he continued his research at the University of Rome La Sapienza where directed the Solar Laboratory and taught Spectroscopy.
Cacciani
The Magneto-Optical Filter (MOF) is an instrument that gives a really narrow bandwidth, high transmission (almost 50%) and perfect stability. It can work only in a small range of wavelengths, well defined, like Na and K doublets, and can be thought as an high resolution spectrograph within those wavelengths.
It was developed by Alessandro Cacciani at the end of the ’60s Its main use is for the study of the Sun; during the last years it has been adopted also for the analysis of Jupiter oscillations and in Anctartica for long-duration solar observations.
A complete MOF instrumentation is made of two separated unit: the MOF itself and the WS (Wing Selector) that, together, provide accurate Doppler and magnetic measurements. The first unit is composed by two crossed polarizers (P1, P2) and a metallic vapour (Sodium) between them in a longitudinal magnetic field B between 2000 and 4000 Gauss. A schematic diagram of the filter is shown in the following figure.
Fig. 1 Schematic diagram of the MOF filter (top) and spectral behaviour along the optical path. Following the Zeeman rules, the vapour immersed in a magnetic field absorbs two circularly polarized components, leaving the residual light also circularly polarized (in opposite direction). This wavelengths cannot be stopped by the crossed polarizers and are transmitted as a couple narrow bands that can be close at will (depending on the magnetic field strength).
The working principle of the MOF is based on two concurrent effects, namely the Zeeman effect in absorption of Fig. 1 and a sort of Faraday rotation close to the line’s wings called Macaluso-Corbino effect. Both change the polarization in and around the resonance lines, leading to total transmission profile shown in Fig. 2 where the experimental transmission’s profile at different temperature and magnetic field values is shown.
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References and images in http://www.solobskh.ac.at/mof_wg/public_info/