  Modifications were made to the Sloan Digital Sky Survey's Baryonic
Oscillations Spectroscopic Survey (SDSS/BOSS) optical fibres assigned to quasar
targets in order to improve the signal-to-noise ratio in the Lyman-alpha
forest. However, the penalty for these modifications is that quasars observed
in this way require additional flux correction procedures in order to recover
the correct spectral shapes. In this paper we describe such a procedure, based
on the geometry of the problem and other observational parameters. Applying
several correction methods to four SDSS quasars with multiple observations
permits a detailed check on the relative performances of the different flux
correction procedures. We contrast our method (which takes into account a
wavelength dependent seeing profile) with the BOSS pipeline approach (which
does not). Our results provide independent confirmation that the geometric
approach employed in the SDSS pipeline works well, although with room for
improvement. By separating the contributions from four effects we are able to
quantify their relative importance. Most importantly, we demonstrate that
wavelength dependence has a significant impact on the derived spectral shapes
and thus should not be ignored.
