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The spacing between the plates is changed by small steps in order to scan the whole free spectral range (which is the range from one ring to the following one). An image of the interference pattern produced by a galaxy (or nebula) is made for each scanning step so that the observation produces a data cube made of a set of several images. For any pixel in the field one can then plot the change in light intensity observed along the scanning process, thus drawing the profile of the emission line of interest which has been selected (owing to an interference filter). Most often it is the H alpha line of hydrogen which is chosen, because it is the brightest line emitted by galaxies. |