<title>Progress in the development of far-ultraviolet etalons</title>

Abstract

We report on a continuing program to develop and test reflecting coatings suitable for use in Fabry-Perot etalons operating in the far ultraviolet region of the spectrum. UV etalons are of particular interest for solar studies as they have the potential to enable one to make wide-field high resolution diagnostic images such as spectroheliograms, dopplergrams, and density maps in isolated spectral lines formed in the upper chromosphere, transition region and lower corona. The performance of the high-efficiency coatings required by a UV etalon is limited both by the availability of suitable materials, and by the uniformity and accuracy of the deposition process. The lack of UV transmitting materials with a wide range of refractive index is especially troublesome. The latter problem may be partially overcome by using a vacuumspaced etalon design. A vacuum-spaced etalon with cultured quartz plates was successfully operated at 160 nm in a previous study. In this study, we investigated a family of coating designs based on the fluoride salts of magnesium and lanthanum, finding that usable etalon performance may be achievable at wavelengths as short as 120 nm. Results of theoretical predictions and the performance of test coatings will be presented.