Cassegrain telescope

Cassegrain telescopes are reflector telescopes that use two curved mirrors to make an image. The big primary mirror gathers and concentrates light. The small secondary mirror then reflects the light through the back of the mirror. The secondary mirror is also curved so that it the light path is longer than the tube.
A "true Cassegrain" uses a primary mirror with a parabola-shaped curve, and a secondary mirror with a hyperbola-shaped curve. This is hard to make. Sphere-shaped curves are easier to make, but the image would be blurry. Catadioptric (lens and mirror) telescopes fix this by putting a special lens in front of the telescope.
Cassegrains are useful both as amateur telescopes which normal people can buy, and for scientists for serious astronomy work.
Types of Cassegrain telescopes.
Lots of telescopes which are similar to cassegrains have been invented. Here are a few.