Lithium niobate double channel Fabry-Perot interferometer for solar corona uses

The use of a tunable etalon in the measurement of spectral profiles of solar corona emission lines is investigated. The etalon is a y-cut plate of lithium niobate. A suitable tuning range for the line is +/-1 A corresponding to an applied voltage of +/-1000 V for a thickness of 0.1 mm. A passband of 0.2 Acan realistically be obtained with an appropriate free spectral range of 6 A. Thermal analysis shows that a temperature excursion of 0.05 degrees C corresponds to a spectral drift of <0.05 FWHM. An emission line profile is obtained by scanning an ordinary-ray passband through the emission line, while measuring the difference between this signal and that from an adjacent extraordinary-ray passband. Using polarization chopping, the scattered-light component due to the sky and instrument is efficiently removed. The extraordinary-ray (reference) passband is insensitive to the applied voltage; hence the reference signal is substantially unaffected by the spectral modulation in the scattered light due to the solar absorption spectrum.