Reflecting telescope

Reflecting telescope

Modern telescopes use a concave mirror rather than a lens for the objective. It is rather difficult and expensive to make lenses of large size which form images that are free from any optical defect. Telescopes with mirror objectives are called reflecting telescopes. They have several advantages. Only one surface it to be polished and maintained. Support can be given from the entire back of the mirror rather than only at the rim for lens. Mirrors weigh much less compared to lens. But the one obvious problem with a reflecting telescope is that the objective mirror would focus the light inside the telescope tube. One must have an eye piece inside obstructing some light. This problem could also be overcome by introducing a secondary mirror which would take the light outside the tube for view as shown in the Figure 6.89.