An apparatus consisting of stacked slab waveguides whose outputs are vertically staggered is disclosed. At the input to the stacked waveguides, the entrances to each slab lie in approximately the same vertical plane. A spot which is imaged onto the input will be transformed approximately to a set of staggered rectangles at the output, without substantial loss in brightness, which staggered rectangles can serve as a convenient input to a spectroscopic apparatus. A slit mask can be added to spatially filter the outputs so as to present the desired transverse width in the plane of the spectroscopic apparatus parallel to its dispersion.