Signatures of metal hydride molecules appear in the optical spectra of cool stars. The observed spectra are used not only for identification of the molecule, but also to assess the abundance of the metal from which the molecule is composed, and to measure the strength of the magnetic field in which the molecule is immersed through the Zeeman splitting of individual spectral lines. Metal hydrides are short-lived radicals, often produced via an electrical discharge, and their steady-state concentrations in a sample are low. High-sensitivity probing techniques, like laser-induced fluorescence, are often appropriate, but (typically much less sensitive) absorption techniques are more useful to assess metal abundances. We describe here a cavity ring-down spectroscopy experiment, usually used to detect absorptions from stable molecules, to collect spectra with very high sensitivity and reproducibility from prototypical metal hydrides NiH and NiD. We have constructed an optical cavity of high finesse (F = 60,000) into which a sputtering source is inserted, and have employed optical fibre and a rigid mounting scheme to keep the ring-down mirrors in alignment during an experiment and between days. We compare our NiH/NiD absorption data with literature results, and highlight some of the strengths and weaknesses of this approach.