The ionization structures of AGNs are determined by the specific ionization mechanisms and the shapes of the ionizing continua. The technique of using emission-line intensity ratios to classify objects and investigate their ionization mechanisms is enhanced significantly by combining high-quality UV and optical spectrophotometry. The HST archive is an excellent source of such data.
The FOS spectra are recalibrated using the latest ‘average inverse sensitivity’ (AIS) calibration. This new method for flux calibrating FOS data (1) normalizes count data from all apertures to the 4”.3 aperture to account for changes of aperture throughput as a function of the Optical Telescope Assembly focus; (2) corrects the data for time-dependent detector sensitivity degradation; and (3) scales the data to the white-dwarf reference scale. The AIS calibration method has been developed over a period of several years, with improvements applied progressively based on observed deficiencies and/or discrepancies of the existing calibrations. Indeed, the final AIS calibration incorporates corrections in the wavelength overlap regions of adjacent gratings derived based on inconsistencies discovered as a result of this study of over 1000 FOS spectra.