A stereoscopic spectrograph is similar to a normal spectrograph except that (A) it has no slit, and (B) multiple spectral orders (often including the non-dispersed zero order) are collected simultaneously.
[1] The individual images are blurred by the spectral information present in the original data.
The images are recombined using stereoscopic algorithms similar to those used to find ground feature altitudes from parallax in aerial photography.
Each spectral order in the instrument produces an image plane analogous to the view from a camera with a particular look angle through the
data space, and recombining the views allows recovery of (some aspects of) the spectrum at every location in the image.