How is collimation done in a telescope ?

Collimation refers to the process of adjusting the different components of the optical system such as  lens, prism and the mirrors.  A telescope which is not collimated will result in blurry images. 

Different types of telescopes have different methods of collimation.

How to collimate a telescope ?

A beginner’s guide to collimation

Collimation of a Newtonian Telescope