Magneto-Optical Filters

The magneto-optical filter is an extremely narrow band filter made of a cell containing sodium or similar kinds of vapor. The cell is placed in a strong magnetic field in making use of magneto-optical effects in the vapor. The filter has high throughput and wavelength stability, and secure absolute wavelength standard. On the other hand, chemical activity of sodium makes it difficult to fabricate a filter which can be used for a long period. The sodium vapor tends to deposit itself onto the window of the cell and makes it more and more opaque.

In the filter we developed, the cell is heated to about 200 degrees C, so that the cell can be used virtually indefinitely. Thermal distortion in the cell window is avoided by carefully selecting the material of the window, and by placing the cell in a vacuum chamber. We are now testing the cell in actual solar observations in order to confirm its stability.

References

• T. Sakurai et al.: "Construction of Long-Life Magneto-Optical Filters for Helioseismology Observations", in 'Progress of Seismology of the Sun and the Stars', eds. Y.Osaki and H.Shibahashi, Springer Verlag, Lecture Notes in Physics, 367, pp.277-279, 1990