The intergalactic medium revealed by the numerous absorption lines seen in quasar spectra (the so-called Ly $\alpha$ forest) is believed to trace the potential wells of the dark matter. This is a unique tool to derive the power-spectrum of the initial fluctuations from high resolution and high S/N ratio spectra. One of the major limitations in these studies is the accuracy to which the quasar continuum is known. To investigate this problem, we have observed at intermediate spectral resolution ten bright and high redshift quasars (2.1 $< z_{em} <$ 4.7) that have also been observed with UVES. We shall compare the continuum determinations for both data sets.