Telescope Basics

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The diameter of a telescope’s objective (main) lens or primary mirror is known as its aperture, which is usually given in inches (and sometimes centimeters) for instruments 4-inch or larger and in millimeters for smaller ones. This is the most important of all a telescope’s parameters; for the larger its light-collecting area, the brighter, sharper, and better-contrasted are the images it forms of celestial objects. The primary driving force behind the construction of ever-bigger professional research telescopes (and also that behind the amazing “Dobsonian revolution” sweeping the amateur astronomy community, discussed in Chapter 5) is the need for more light — for collecting ever more photons. (See the discussion on light-gathering power later in this chapter, and also that about the amazing “photon connection” in Chapter 14.) Commercially available telescopes in use by backyard astronomers today range from small 2- and 3-inch aperture refractors up to 36-inch behemoth reflectors, with the most common sizes being in the 4- to 14-inch size range.