Even Galileo learned a telescope can draw a crowd. Today, refractive telescopes fill many needs.
Refractive telescopes, or refractors, were the first type of telescopes invented. It was a refractive telescope that Galileo first turned on the heavens. Refractive telescopes are built from lenses: a large objective lens that collects light and a smaller eyepiece lens that prepares it for viewing. Lenses bend light in a process called refraction, which is where this style of telescope gets its name. Small refractors are convenient to look through; they keep their alignment well and they resist environmental contamination. Those advantages make them handy for many applications.
Astronomy
A refractive telescope can provide a good introduction to amateur astronomy.
Just as Galileo enjoyed gazing at the moons of Jupiter or the phases of Venus, today's amateur astronomer can enjoy looking at the marvels in the heavens. Today's telescopes are far more powerful and much more accurate than Galileo's. Although most serious amateur astronomers prefer larger aperture reflective telescopes, refractors up to 4 inches in diameter or even larger can give good results. Most reflective telescopes use a companion alignment telescope, which is usually refractive.
Rifle and Spotting Scopes
In situations where there's plenty of light and the telescope is not going to be handled delicately, relatively small aperture refractors are an excellent choice. Rifle scopes tend to have diameters in the 1- to 2-inch range, while spotting scopes used for birdwatching and other nature viewing can be as large as 4 inches.
Night Vision
Advances in detector technology have made it possible to detect images in scenes that appear completely dark to the human eye. Combining night vision detection with a telescope brings other advantages to the table. The light gathering capability of the refractive telescope and the magnification allow distant scenes to be seen in detail, even in the dark.
Surveying
Refracting telescopes are frequently used in surveying.
Transits, theodolites, and survey levels are all essentially slightly different versions of refractive telescopes. These tools are used to plot boundaries, lay out construction lines, and determine elevation changes. They combine the telescope with mounting hardware that makes it possible to get a precise determination of angle.
Laser Rangefinding
Yes, those rangefinders used on the golf course incorporate refractive telescopes.
Lasers can be used to accurately measure distances. To do it accurately over long distances, the laser must be kept from spreading out too much and the target must be accurately identified. Most long distance --- say, longer than 10 meters --- rangefinders incorporate a refractive telescope in the design.
Beam Expanders
Although not often seen in a direct consumer application, optical laboratories and complex optical equipment often have a need to change the size of a beam of light. Especially in laser systems, with a narrow wavelength range, refractive telescopes are perfect for the job.
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