  We present a new approach to the sky subtraction for long-slit spectra
suitable for low-surface brightness objects based on the controlled
reconstruction of the night sky spectrum in the Fourier space using twilight or
arc-line frames as references. It can be easily adopted for FLAMINGOS-type
multi-slit data. Compared to existing sky subtraction algorithms, our technique
is taking into account variations of the spectral line spread along the slit
thus qualitatively improving the sky subtraction quality for extended targets.
As an example, we show how the stellar metallicity and stellar velocity
dispersion profiles in the outer disc of the spiral galaxy NGC 5440 are
affected by the sky subtraction quality. Our technique is used in the survey of
early-type galaxies carried out at the Russian 6-m telescope, and it strongly
increases the scientific potential of large amounts of long-slit data for
nearby galaxies available in major data archives.
