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Published online by Cambridge University Press: 04 August 2017
Fourier analysis of the observed distribution of spiral tracers (e.g. HII regions) can yield a wealth of information about the spiral structure of a given galaxy. A description of the method has been given by Kalnajs (1975) and, in brief, involves decomposing the observed distribution into components with given angular periodicity m. Component m = 0 corresponds to the axisymmetric part, m = 1 to either a one-armed spiral or an asymmetry, m = 2 to a spiral or a bar, etc. Each component is then analysed into a superposition of logarithmic spirals. This does not imply that the observed spirals are assumed to be logarithmic, but rather that logarithmic spirals are convenient building blocks.