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.
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
 

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