Astronomical Telescope Optics
There are many different mirror assemblies that can lead to a properly focused image. Which of them is used in practise mostly depends on what kind of output one wants to get from a telescope. The most basic setup is the so-called "prime focus" assembly (see below).
Sketch of a parabolic mirror reflecting light into its focal point.
Light is reflected by a parabolic mirror into its prime focus. Placing a receiver/camera there can have the disadvantage that for imaging of extended objects on the sky the focal plane is not flat. This requires either corrective optics or bending the detector plane.
Alternatively, one can use a second mirror, the so-called "secondary" or "subreflector", to reflect light into the secondary (or "Cassegrain" focus) at the base of the primary mirror. This is displayed in the sketch below.
Sketch of a Cassegrain optical assembly.
Besides offering a more planar focal plane, a Cassegrain system has in practise, compared with a prime focus assembly, the additional advantage of easier access to the detector, at the base of the telescope.
These are the two most commonly used assemblies. If one wants to have a longer focal length so as to make use of a big telescope's light power, one would want to focus the rays somewhere much further behind the primary mirror. One way of achieving this, without piercing the primary, is to re-route the light beam along the tipping axis of the telescope to the Coude focus ("Nasmyth" optics).
These and a lot more optical assemblies for telescopes are described in many standard textbooks. A telescope's optics can be chosen quite independent of the way it is mounted.