Telescopes are devices that are used to view the distant objects. They find its use in astronomy and physics. It enables you to view the distant objects by magnifying them. There are many types of telescopes and their prices vary according to the specifications. Many accessories are also available that can be used in conjunction with the telescopes. Small telescopes that are used as toys are also capable of viewing some objects around 50 meters away.

The principle in which the telescope works is very simple. There are two lenses that make up the task of viewing the objects that are at a distance. One of the lenses picks up the light from the object viewed and makes it available at a focus point. Another lens picks up the bright light from the focus point and spreads it out to your retina so that you can view. The lens that picks up the light from the object is called the objective lens or primary mirror. The lens that picks up the light from the focal point is called the eyepiece lens.

The capability of the telescope to collect the light from the object that is viewed and the capability to enlarge the image are the factors that affect the efficiency of the telescope. The capability to collect light from the object depends on the diameter of the lens or mirror, which is otherwise called the aperture. The larger the aperture the more the light it can collect. Enlarging of an image depends on the combination of the lenses that are used. The eyepiece in the telescope performs the magnification.
Some of the world's largest optical telescopes in operation

We say a telescope to be larger based on the aperture size. Based on this we can say that Keck and Keck II are the largest telescopes in operation with an aperture of 10 meters diameter. The Keck telescope is composed of 36 mirror segments. This is located at Mauna Kea, Hawaii. The next largest is Hobby-Eberly located at Mt. Fowlkes, Texas which has an aperture of 9.2 meters. You can get a list of the largest optical telescopes at http://astro.nineplanets.org/bigeyes.html.

The choice of the telescope largely depends on what you want to observe. You can choose compound telescopes and refractor type of telescope for viewing through the urban skies. For the rural skies, you can use compound telescopes and reflectors. They are better than the refractors type of telescopes. Each type has its own advantages and disadvantages. Hence, many people have different telescope for different purposes.