Telescopes functions based on two principles 1 Refraction 2 Reflection.
In first type a lens is used to collect light.this is what we call objective
In second type a mirror is used to collect light.This is reflector.
3 main functions of telescope
1 It collect more light
2 It resolves close objects
3 It magnifies.
Pictuire credit Meade com
In the beginning starting with Hans Lippershey and Galileo, the telescopes contained lenses. The problem with lenses is that they can suffer with chromatic aberration, a problem in which different colored images could form at different places so for example Blue and Red colored images could not be formed at the same focus. Isaac Newton revolutionized the use of telescope by replacing Lenses with two mirrors. A primary mirror and a secondary mirror. Light from a distant object first falls onto the curved primary mirror which is reflected to the smaller secondary mirror which focuses light onto an eyepiece. Almost all the modern Telescopes use this technique.