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.
