Figure 2 illustrates the optical path through a simplified reimaging system, using collimator and camera lenses to reimage the focal plane onto a detector. The collimator and camera are drawn as simple lenses; in a real system, they would have to be more complex to avoid aberrations and curvature of field. However, the physical sizes of collimated beams, the calculations of pixel scales and so on are mostly just dependent on focal lengths and f-numbers, and are similar for simple and complex systems.
Diverging beams emerge from each point on the focal surface and are re-collimated; the diameter of a collimated beam is the pupil diameter,
The pupil is the aperture of the system, here the primary mirror. The collimator forms an image of the mirror at the pupil location shown by the vertical dashed line. If we were to put a sheet of paper into the beam at this location, we would see a donut of light, the shape of the primary mirror.