  A method for optimal reduction of data taken with multi-fiber spectrographs
is described, based on global correction of their geometrical distortion.
Though it was specifically developed for reducing observations performed at
Palomar Mountain using the Norris fiber spectrograph, this method can be
adapted to other types of multi-object spectrographs such as the multi-slit
ones. Combined with a 2D analytical interpolation of sky-background that
accounts for non-uniform spectral resolution, the Norris software package
achieves very high accuracy in airglow subtraction, even in the near infrared
(7000-9000A) where molecular band-emissions commonly induce strong artefacts
that preclude clean sky subtraction whenever standard image processing
techniques are used. Correlatively, an improvement by a factor of 2 on the
precision of radial velocities is achievable. Throughout the paper possible
improvements to the method are suggested for those devising similar packages
for other instruments.
