On the definition and practical use of crystal-based azimuthal angles

A diffractometer-independent definition of the azimuthal angle ψ of a Bragg reflection is proposed. After completion of intensity measurement, instrument-dependent and partly redundant quantities such as orientation matrix and setting angles may be replaced by ψ, values without loss of information. Data-treatment software, such as absorption- and anisotropic extinction-correction programs, become instrument independent and completely general. Formulae are given to compute ψ for equatorial and non-equatorial diffraction geometries.