By changing air pressure, scanning is accomplished within one free spectral range. This method is well adapted to stellar spectroscopy. The étalon can be placed in the same collimated beam as the grating, but a smaller étalon can be used if it is placed in a separate collimated beam, with some additional light loss. Dielectric coatings with high reflectivity, up to more than 97 per cent increase the free spectral range and decrease the intensity between fringe maxima, but the advantage of such

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... ilms can only be realized if errors in figure of the étalon plates are held well below i/ioo wave length.