Reflecting telescopes offer several configurations for attaching scientific instruments. Each configuration has its advantages and drawbacks.
This is the simplest configuration, in which the instruments are mounted above the primary mirror. Light strikes the primary mirror and reflects into the instrument. This was the most common configuration in the early 20th century. In fact, in the largest telescopes, an astronomer would conduct observations from inside a small “cage” at the prime focus. The size of astronomical instruments that can be used at prime focus is limited.
In this configuration, a small mirror is placed in front of the prime focus. Light reflected from the primary mirror hits this secondary mirror and reflects through a small hole in the center of the primary. The scientific instruments are attached to the back of the telescope, where they are easier to maintain and replace. This configuration provides a longer focal length, which provides sharper images of astronomical objects.
This configuration adds a third mirror to the light path to direct light out the side of the telescope. This provides an even longer focal length, which in turn offers even sharper views. It also allows astronomers to connect much larger and heavier instruments to the light path. These instruments can even be located on a different floor from the telescope itself.