  Numerous sky background subtraction techniques have been developed since the
first implementations of computer-based reduction of spectra. Kurtz & Mink
(2000) described a singular value decomposition-based method which allowed them
to subtract night sky background from multi-fiber spectroscopic observations
without any additional sky observations. We hereby take this approach one step
further with usage of non-negative matrix factorization instead of principal
component analysis and generalize it to 2D spectra. This allows us to generate
approximately 10 times as many valid eigenspectra because of non-negativity. We
apply our method to short-slit spectra of low-mass galaxies originating from
intermediate-resolution Echelle spectrographs (ESI at Keck, MagE at Magellan,
X-Shooter at the VLT) when sources fill the entire slit. We demonstrate its
efficiency even when no offset sky observations were collected.
