Telescopes

A telescope is an instrument which helps us to observe far away objects in the night sky. Hans Lippershey, a Dutch spectacle maker built the first telescope. Galileo Galilei heard about this telescope and made his own one in 1609. By using his new instrument, he observed the phases of planet Venus, moons of Jupiter (The Galilean Moons), sunspots and rings of Saturn. He also found that the moon has a rough surface.

Telescopes developed rapidly and became popular, and in 1668 Sir Isaac Newton constructed the first reflecting telescope. 

Galileo Galilei observing the heavens with his telescope.

The telescope Galileo constructed is known as a refractor. The lens which collect the star light is called the objective lens and the other lens is known as the eyepiece. Refractors use a large objective lens to have a good, clear image. But in reflectors, we do not need to use a large objective lens. Instead we use a concave mirror to focus the light on the magnifying eye piece. Reflectors tend to be more powerful than refractors as constructing a large mirror is easier than constructing a large lens.