The building of an telescope is an interesting hobby only if the building is the aim of the work (you will get many good knowledges from optics and mechanics). But if you only need a cheap telescope, the cheapest way is to buy it. For example dobsonians from Skywatcher are very cheap an of good quality.

A telescope is a device used to form images of distant objects. The most familiar kind of telescope is an optical telescope, which uses a series of lenses or a curved mirror to focus visible light. An optical telescope which uses lenses is known as a refracting telescope or a refractor; one which uses a mirror is known as a reflecting telescope or a reflector. Besides optical telescopes, astronomers also use telescopes that focus radio waves, X-rays, and other forms of electromagnetic radiation. Telescopes vary in size and sophistication from homemade spyglasses built from cardboard tubes to arrays of house-sized radio telescopes stretching over many miles.

A telescope consists of an optical system (the lenses and/or mirrors) and hardware components to hold the optical system in place and allow it to be maneuvered and focused. Lenses must be made from optical glass, a special kind of glass which is much purer and more uniform than ordinary glass. The most important raw material used to make optical glass is silicon dioxide, which must not contain more than one-tenth of one percent (0.1%) of impurities.