ACCURATE WAVELENGTHS FOR THE THORIUM NEODYMIUM STELLAR CHRONOMETER

The wavelengths of the thorium and neodymium lines used in the Butcher stellar chronometer have been remeasured, together with those of weak blended lines of iron and nickel. The results are presented here; the absolute accuracy achieved ranges from 0.21 m angstrom for the thorium line to 1.52 m angstrom 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.