Composite spectra of quasars with different UV spectral index

The composite spectra of quasars are widely used as templates for redshift determination, as well as for measurements of the mean transmission in Ly\alpha-forest studies, and for investigation of general spectral properties of quasars. Possibility of composite spectra utilisation in these fields is related to remarkable similarity of quasar spectra in UV-optical range. But despite of general similarity in spectral shapes, they differ in several parameters, one of which is the spectral index. In the present paper we study the possible effects, related to neglect of this difference. We compiled 16 composite spectra from subsamples of individual SDSS DR7 quasar spectra with different spectral indices \alpha_\lambda within the wavelength range 1270-1480 \AA, and show that (i) the redshifts measured for a test sample of high signal-to-noise ratio quasar spectra using these composites as templates appear to be systematically higher than those calculated with a traditional template, compiled from spectra with different \alpha_\lambda, with 1.5 times smaller errors in the former case; (ii) the difference in \alpha_\lambda in individual spectra used for compilation of composites can yield the mean transmission uncertainty up to 20%; (iii) a number of emission lines indistinguishable in ordinary composites, but seen in individual high-resolution spectra, can be detected in such composites. It is also shown, that there is no dependence of \alpha_\lambda on quasar luminosity in SDSS u, g, r and i bands, and monochromatic luminosity at 1450 \AA.