Final answer:
The convex secondary mirror in a reflecting telescope focuses light down through a hole cut in the concave primary mirror.
Step-by-step explanation:
A convex secondary mirror in a reflecting telescope is used to focus light down through a hole cut in the concave primary mirror. This design is commonly found in Schmidt-Cassegrain telescopes.
In this arrangement, the light-gathering concave mirror reflects the light back towards the convex secondary mirror, which redirects the light through a hole in the primary mirror to an observing station below the telescope.
This allows more light to get through the system and facilitates easy viewing for astronomers.