1.1 A milestone in the advancement of astronomy
Unaided human eyes, well as they may serve the needs of everyday life, are not very suitable for detailed astronomical observation. First, the eye has a limited sensitivity. A distant source of light, such as a star, will not be seen at all unless the intensity of light from it reaching your eye is above the sensitivity threshold of the retina. Second, the ability of the eye to distinguish fine detail is limited by the finite physical size of the detectors on the retina and by the small aperture of the eye. The ability of a telescope or the eye to distinguish between two objects that are very close to one another is called its (angular) resolution. Limited resolution makes it impossible for human eyes to separate individual distant sources of light that are closer than about 1′ apart, or to discern details of their shape or structure on angular scales finer than this.
The invention of the telescope at the beginning of the seventeenth century was an important milestone in the advancement of astronomy. Here was a simple instrument that at once overcame, to some degree at least, these shortcomings of human eyes. In this section we shall first look at the characteristics of optical elements that may be combined to make telescopes, then we shall consider the main designs of refracting telescopes and reflecting telescopes that have been developed over the past four centuries, and finally look at the key ways of characterising the performance of an astronomical telescope. To clarify the nomenclature, the name ‘refracting telescope’ (or refractor, for short) is used to indicate a telescope in which only lenses are used to form the image; the name ‘reflecting telescope’ (or reflector for short) is used to indicate a telescope in which a curved mirror is used in place of one of the lenses. In any telescope, the optical element that gathers the incoming light is variously referred to as either the objective lens or mirror, or the primary lens or mirror. A subsequent lens used to view the image by eye is referred to as the eyepiece lens.