ABSTRACT
A method is described for the automatic relative scaling of spectra in which one or more lines with constant flux are present. The algorithm takes into account differences in spectral resolution, flux factors and spectral shifts. It was developed for the relative calibration of series of optical spectra obtained in variability studies of active galactic nuclei, but it also can be used for other purposes, e.g., the subtraction of an underlying stellar continuum in AGN with a template spectrum. Tests show that a high degree of accuracy is obtained, and that the method is superior to scaling the spectra by eye or measuring the narrow line fluxes.