E2-54
Shows how rays are focused by a standard reflecting telescope.
Parallel rays incoming from a star at infinity (red strings) are focused by the large mirror to a focus near the aperture of the telescope. The reflected rays (white strings) are intercepted before their focus by a small mirror, and deflected upwards toward the eyepiece of the telescope. The eyepiece is positioned at its focal distance past the focal plane of the objective (mirror).