A hybrid image-pupil optical reformatter and method for optional use with a spectrometer is disclosed, which performs beam slicing in pupil space and stacks replicas of the input source generated from the pupil beam slices in image space. The optical reformatter comprises a collimator which receives an input light and produces a collimated beam; a first optical element which receives the collimated beam, redirects portions of the collimated beam back toward the collimator as reimaged beams and permits portions of the collimated beam to pass; a second optical element which receives the reimaging beams and redirects the reimaging beams back toward the collimator and the first optical element; to form an output beam comprising the portions of the collimated beams that are not redirected toward the collimator by the first optical element. Also disclosed is the use of the reformatter for reformatting the input light of a spectrometer system, and the use of the reformatter as part of a spectrometer device.