In the present work the part of the quasar UV-optical bump within the wavelength range 1210-1450AA was studied with the help of composite spectra compiled from the samples of SDSS DR7 spectra with the similar spectral index alpha_{lambda} within 1270
-1480 AA. This division allowed to see weak emission lines, which were not detected in previous studies of the quasar composite spectra, but were known from individual optical or composite UV spectra from the Hubble Space Telescope. Although the physical explanation of the difference in spectral indices between quasars and their dependence on quasar parameters is still not clear, it is obvious that this difference has to be taken into account when generating composite spectra, e.g. for redshift measurements. It was also shown that the equivalent width of the emission lines does not depend on the 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 Lyalpha-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.
