In most telescope applications it is desired that light from a distant object or source be brought to focus where it may be detected, photographed, or measured. Conversely, these systems are often used as collimators or target projectors, where a target or source placed in the focal plane is imaged to some distant point. In many cases, one-mirror optical designs provide an ideal solution. Conic surfaces of revolution possess two focal points between which perfect on-axis imaging can be obtained. In the case of the paraboloid, light from an infinitely distant point source is brought to a perfect, on-axis point image.
Ellipsoids focus light similarly between two finite conjugate positions.
In the Newtonian Telescope, a small diagonal mirror is inserted in the focusing beam to bring it out at a right angle to the incoming beam. This yields a more accessible focused spot, but produces a central obscuration in the aperture which increases the system diffraction spot size. To eliminate obscuration effects, an off-axis section of the primary mirror can be used. Known as a Herschelian Telescope, this design is common in collimation and target projection systems. The Classical Cassegrainian Telescope employs a parabolic primary mirror, and a hyperbolic secondary positioned such that the parabolic and virtual hyperbolic focii coincide.