There are two types of optical telescope: the refracting telescope and the reflecting telescope.
The first person to observe the heavens via a telescope was Galileo in 1609. He constructed his telescope using glass lenses known as refractors which could refract or bend the light passing through them. The modern refractor has two sets of lenses. The objective lens (the front one) forms an image, and then the eyepiece (the lens at the rear) magnifies it. These lenses are positioned within two tubes. The tubes have the capability to slide in and out of each other, which allows the user to focus on the object and receive a sharp image. The image given is shown upside-down, but this does not matter when used for astronomical purposes. The biggest refracting telescope can be found at the Yerkes Observatory in the USA. Its objective lens is more than one metre across.
These days astronomical telescopes are of the reflecting type. These can gather and also focus starlight. It does this by means of mirrors, rather than lenses. The main mirror within the telescope is arced to reflect the light onto one or more of the mirrors. This in turn reflects the light into the eyepiece. Isaac Newton is credited with the invention of the reflecting telescope. He built his first in 1672. This form of telescope can be constructed much larger than the refracting telescope. The biggest has a mirror for light gathering, which is six metres in diameter. It was built in 1976 at Zelenchukskaya within the Caucasus Mountains in Russia. It has been stated that this particular telescope is so sensitive, that it can detect the light of a candle some 25,000 kilometres away.
