Accurate wavelengths for the thorium-neodymium stellar chronometer

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Abstract
The wavelengths of the thorium and neodymuim lines used in the Butcher stellar chronometer have been remeasured, together with those of weak blended lines of iron and nickel. The absolute accuracy achieved ranges from 0.21 mÅ for the thorium line to 1.52 mÅ for the weak iron line. The uncertainty of the wavelengths presented in the literature or deducible from published energy levels is discussed in detail. The database used for astrophysics proves to be much older and much less accurate than is often assumed. New instruments, especially Fourier transform spectrometers, offer the possibility of remeasurement to the higher standards needed to interpret current astrophysical observations.

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