A device for determining the radial velocities of stars of known spectral type. One design consists of a spectrometer which focuses a star’s spectrum on to a mask which has slots at the laboratory positions of the stellar absorption lines. The total light transmitted through the mask is measured with a photometer, and the mask is moved until the amount of light collected reaches a minimum, which indicates that the alignment with the target spectrum is exact. The displacement of the mask then gives the radial velocity. Alternatively, a mask can be superimposed digitally (i.e. using a computer) on a normally recorded spectrum, and aligned with the observed absorption lines to determine their offset. In another method, light is passed through an iodine cell before being dispersed; the iodine introduces a few sharp absorption lines in the red part of the spectrum which act as highly accurate markers for measuring radial velocities.