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How does a Cassegrain telescope use its secondary mirror?

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Telescope Aerial view of the Keck Observatory's twin domes, which are opened to reveal the telescopes. Keck II is on the left and Keck I on the right.

In a Cassegrain telescope, the secondary mirror serves to redirect light gathered by the primary mirror to a convenient focal point.

The process functions as follows:
* Light Collection: Parallel rays of light enter the telescope and strike a large concave primary mirror, which reflects them toward its prime focus.1
* Secondary Reflection: Before the light reaches that prime focus, it is intercepted by a small convex secondary mirror.12
* Final Focus: This secondary mirror reflects the light back through a small hole in the center of the primary mirror, bringing the image to a focus located behind the primary mirror.12

By using a hyperboloidal shape for the secondary mirror in conjunction with a paraboloidal primary mirror, the design effectively corrects for spherical aberration, which reduces image blurring. This configuration also allows for a shorter telescope tube relative to the mirror's diameter, making the instrument more compact.132