An off-axis optical system having a rectangular aperture stop to control rays of incident electromagnetic radiation passing through the optical system along an optical path is provided. The optical system includes one or more optical surfaces along the optical path, each surface being configured to change a direction of each ray on the surface based on a location of the ray relative to the surface. At least one of the surfaces is conjugate to and has the same shape as the rectangular aperture stop. In one embodiment, each optical surface is shaped to avoid vignetting the rays. In one embodiment, the optical system is a three-mirror anastigmat (TMA) and includes a concave primary mirror to collect and focus the electromagnetic radiation; a curved secondary mirror to reflect the electromagnetic radiation focused by the primary mirror; and a concave tertiary mirror to focus the electromagnetic radiation reflected by the secondary mirror.