Light enters at top through the cryostat window and comes to focus 300 mm inside the cryostat at the focal plane mask wheel. The 3.0" x 3.0" field passes to a focal ratio converter mirror which reimages the telescope exit pupil on a cold stop to baffle the system. The f/256 beams pass through a filter wheel seen near the top of the cold work surface plate in the figure and focus on a reflective image slicer at top-left which directs each slice of the focal plane to a different element of the pupil mirror array at top-right where 29 toroidal mirrors reimage the f/16 focal plane in the form of a 64 mm long "staircase" slit on the field mirror array. The 29 field mirrors direct the beams into the spectrograph so that their pupils all coincide on the grating at bottom-left. The spectrograph uses a Bouwers collimator consisting of a spherical mirror and a concentric refractive shell to correct spherical aberration. The pupil mirror array, field mirror array, spectrograph collimator mirror, and its corrector are all concentric about the image slicer so that each of the 29 channels is optically identical. A refractive camera views the diffracted light from the grating with a 30° Ebert angle and focuses spectra from the 29 slitlets onto the detector at bottom-right.

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