|Activities such as astronomy, nature studies and viewing sports must often be done from a distance. For various reasons we cannot get close enough to the subjects to view them in the detail that is needed. Our eyes are general purpose tools and their resolution is limited, their magnifying properties are minimal and they are limited in how much light that they can gather. We must use optical devices such as telescopes and binoculars to increase our visual range. |
A telescope is an optical device which makes distant objects appear closer. It samples a small area of view, a field, and then magnifies it so that distant objects appear larger. Parallel light rays entering the telescope are focussed to a single point, called the focus or focal point. These focussed rays are then magnified with a very powerful lens, or more commonly a set of lenses, called an eyepiece, to give enlarged views of distant objects. The eyepiece acts in the reverse direction to the telescope lens, taking the focussed rays and sending them to the eye as parallel rays.
A mirror is made by coating the front surface of a concave piece of glass with a reflecting material. Light rays entering the telescope reflect off of the mirror and since they never pass through the glass no false colour is produced.
The surface of the mirror of a high focal ratio reflector can be shaped or figured to that of the surface of a sphere. This works for small reflectors and those with focal ratios of f9 or higher. However, with large reflectors and those with focal ratios of f8 or lower, these spherical mirrors do not bring all of the light rays to the same focal point. The rays from the mirror