A raw spectrum, straight from the HARPS

The light of a star was dispersed according to its colours, or wavelengths, in a very long strip. The optical elements in the spectrograph chop this long spectrum in a series of shorter ones which are positioned one after the other on the two detectors of the instrument. In order to measure the wavelength with a very high accuracy, the light from a laser comb (eso0826) is also dispersed by the spectrograph, and forms a second series of lines just next to the spectrum of the star. The light from the laser comb appears as a series of dots, whose position is well known and serve as reference.