Section 5.2: How do telescopes work in the visible and near-visible?
Unaided human eyes are not very suitable for 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 resolve fine image details (acuity) is limited by the physical size of the detectors on the retina and by the small aperture of the eye. This limited resolution makes it impossible for human eyes to separate individual distant sources of light that are too close together, or to make out details of their shape or structure. The invention of the telescope at the beginning of the seventeenth century was an important milestone in the advancement of astronomy. It provided a simple instrument that overcame, at least in part, these shortcomings of human eyes.
Although, in this section, we shall mainly be talking about telescopes that operate in the visible part of the electromagnetic spectrum, the principles may be applied similarly to telescopes that operate in the infrared and ultraviolet regions of the spectrum. These radiations can be focused by telescopes and detected electronically in much the same way as visible light.
The story began in 1608 when a Dutch optician called Hans Lippershey discovered (probably quite accidentally) that a distant object appeared larger when viewed through a combination of two lenses: a relatively weak (long focal length) converging lens facing the object and a strong (short focal length) diverging lens in front of the eye. This combination of lenses was subsequently used by Galileo Galilei for looking at the Moon, the planets and the stars. However, telescopes have come a long way in the last 400 years!