The possibilities of multilayer coatings for auroral research

Although the examination of the spectra of aurorae provides a method of estimating temperature at auroral levels, the low dispersion of auroral spectrographs hitherto used and the long exposures required lead to uncertain results. During the past five years it has been established that multilayer coatings are generally more efficient than their silver counterparts for interferometric work. This increase in efficiency is effected by a reduction in the absorption from, for example, 3 per cent for a silver film of 94 per cent reflectivity to 1 per cent for a multilayer coating reflecting 94 per cent of the incident light. Consequently the transparency of a Fabry‐Perot interferometer can be considerably increased by the use of multilayer coatings, so reducing the exposure‐time for recording fringes from an extended source. Several trials on auroral glows have shown that it is possible to record Fabry‐Perot fringes from the 5577 Å radiation with exposures of as little as 20‐min duration, using simply a multilayer interferometer placed in front of an f/2 camera. These investigations are being continued with the intention of deriving temperature distributions from measurements of the fringe halfwidths, and line‐of‐sight motions of the aurorae from displacements of the fringes. Further applications lie in the use of narrow bandwidth (≃ 20 Å at 5500 Å and 60 per cent transmission) all‐dielectric type interference‐filters, particularly for investigating the form of a display in different radiations (e.g. H α , H β ); these filters by relative enhancement of the auroral radiation will make observations possible during moonlight.