As a tool for observing our world, the naked eye does an excellent job. But as an astronomical tool, it leaves much to be desired, even though it was the only tool available to mankind until the early 1600s. 

Limitations of the human eye: 

• Because we evolved as a diurnal (daytime) organism, the pupil, the opening through which light passes, is small, a bit less than a third of an inch (7 mm). 
• Its magnification is limited. 
• The range of frequencies that it can access compared to the entire EM spectrum is also limited; for example, it detects only the visible portion of the spectrum which is rather a narrow portion of the entire EM spectrum. 
• The greatest handicap for the eye as an astronomical tool is the fact that it cannot add up light; in other words, staring at a dim object for any length of time will not make it brighter.

From the moment the telescope was invented it became universally used because it overcomes some of the natural limitations of the naked eye. 

Telescopes are used for the following reasons: 

• They gather light. The larger the lens or mirror, the more light is gathered. For example, a 200 inch (5000 mm) telescope will gather over 500,000 times more light than the naked eye. 
• They magnify the objects we look at. 
• They resolve objects: they are able to make out details that cannot be seen with the unaided eye.

Galileo did not invent telescopes but was he first to use and manufacture them as astronomical tools. In their simplest form, they are simply two lenses which bend light, hence they are called refracting telescopes. Starting with a telescope of approximately nine power, he rapidly improved their magnification to near 30 power. 

Reflecting telescopes 

While the refracting telescope continues to be used to some extent to this day, it was rapidly replaced by the reflecting telescope which uses mirrors rather than lenses, and was made practical by Newton**. There are several powerful reasons for doing this: 

• Each lens in a refracting telescope requires two surfaces to be ground correctly; reflecting telescopes, only one, thereby reducing the chances of distortion. Remember also that the mirror surface is on the front of the glass and because the light does not go through the glass, as in a lens, its path will not be distorted.
• Refracting telescopes’ lenses suffer from chromatic aberration, (the actual focal point of the lens varies for different frequencies.) Mirrors do not.
• Every lens surface loses some of the incoming light, uncoated lenses as much as 30% per lens. This is a tremendous handicap especially if we remember that most astronomical objects are dim to begin with. In contrast, mirrors are 99% efficient in reflecting light and therefore there is little light loss.
• There are also structural reasons for preferring reflecting telescopes. Lenses are heavier in the center and are thinnest where they are mounted along their edges, causing gravitational distortion as they hang in the telescope. In contrast, mirrors are thinner in the center, and thicker along the edges, a structural improvement that is very important in larger telescopes. 
• Reflecting telescopes can be designed for a greater range of frequencies than refracting, which are limited primarily to the visible part of the spectrum. Reflecting telescopes range in frequency from the UV to the Infrared

Satellite Telescopes
In addition to optical telescopes such as the Hubble, we have establishes space platforms that enable us to see the universe in frequencies that are partially or fully blocked by the atmosphere, both in the high frequency range from UV to gamma, and in the infrared. 

With the invention of cameras and photography, astronomers were quick to see the value of a device that would allow the adding up of light in long time exposures. Photography became especially useful with the advent of fine-grained emulsions early in this century. The past two decades have also seen the use of CCD devices (the same ones as on video cams) that are now nearing such sensitivities that virtually not a single photon of light that enters a telescope is lost. 

**Newton is often incorrectly given credit for inventing the reflecting telescope. In fact it was invented by an acquaintance of his, Gregory. Gregory's complicated design was simplified and improved by Newton and it is the latter's design that is still commonly used today.