A telescope's function is to gather the parallel light rays coming from a distant object, and bring them to a focus point, where they can form an image or be fed into a spectrograph. The 200-inch telescope can achieve this focus in two different primary ways, depending on the sort of analysis required.

In the simplest configuration, the light bounces off the concave primary mirror and converges inside the prime focus cage, which is suspended in the middle of the front aperture of the telescope. The light path is demonstrated in this animation (Quicktime, 1.0 Mb). (The illumination of the primary mirror and of the cage are greatly exaggerated.) In the old days, the astronomer would sit inside the prime focus cage all night long, guiding the telescope and controlling the film exposures. Nowadays, small computerized instruments are operated remotely from the data room on the floor of the dome.

The other primary optical configuration places a convex secondary mirror just in front of the prime focus position, reflecting the light back down through a hole in the primary mirror to the Cassegrain focus. The Cassegrain focus is somewhat easier to access, and it can accommodate larger instrument packages.