Stable high-reflection Be/Mg multilayer mirrors for solar astronomy at 304 nm

The He-II (λ=30.4 nm) emission line is one of the spectral channels chosen to study solar corona. This Letter reports on investigations of novel beryllium (Be)/magnesium multilayer coatings which, when incorporated beneath a protective bilayer of aluminium and Be, ensure particularly high-reflection coefficients of up to 56%, a spectral width of Δλ=1.6 nm (λ/Δλ≈20), and high temporal stability.