Illustration of the concept of subaperturing. All panels show a view from the Universe looking down onto the telescope. Left: Each dot is a photon shown as it passes through a diffraction grating. Photons are colored according to their polar angle in this panel. Middle and right: Photons detected in the zeroth and first diffraction order on the detector, respectively. The dispersion direction is from left to right (y coordinate), and the distance from the focal point at (y, z) = (0, 0) is shown. Each photon is shown in the same color as in the left panel. Photons passing the mirror and grating close to z = 0 (dark purple) are spread out along the dispersion direction (y coordinate); photons passing mirror and gratings close to y = 0 are more concentrated along the dispersion direction. Subaperturing means that the mirror and gratings do not cover a full circle, but only regions marked in yellow and green in the left panel, so that the resulting photon distributions on the detector are narrower in dispersion direction, which increases the spectral resolving power.