We have studied the Residual Blaze Functions (RBF) resulting from division of individual echelle orders by
extracted flat-field in spectra obtained by slit-fed OES spectrograph of 2m telescope of Ondrejov observatory,
Czech Republic. We have eliminated the dependence on target and observation conditions by semiautomatic
fitting of global response function, thus getting the instrument-only dependent part, which may be easily incorporated
into data reduction pipeline. The improvement of reliability of estimation of continuum on spectra of
targets with wide and shallow lines is noticeable and the merging of all orders into the one long spectrum gives
much more reliable results.