Optical telescopes gather light from an object and send it along the focal plane to present the viewer with a real image of the object, as Tammy Plotner explains in a universetoday.com article. Optical telescopes help photographers, stargazers and astronomers spot the details of an object too distant to see in detail with the naked eye. According to Plotner, optical telescopes come in three varieties: refractor telescopes that use lenses, reflector telescopes that use mirrors and catadioptric telescopes that use mirrors with a lens design. Though slightly different in design, all three optical telescopes have the important job of zooming in on distant targets.
Casual stargazers use optical telescopes to view the universe up close. If you see a telescope resting on a tripod in someone's home, most likely it is a refractor, as they are among the most compact optical telescopes. The spy glass, or handheld refractor telescope, is one of the first optical telescopes. As Plotner explains, astronomers Galileo Galilei and Johannes Kepler improved this telescope's design at the turn of the 17th century, and today, amateur astronomers use refractor telescopes to study the skies--or spy on their neighbors across the street. Plotner states that even binoculars are a type of optical telescope.
Photographers sometimes use optical telescope cameras with catadioptric lenses. According to Astronomics.com, a catadioptric telescope uses both mirrors and lenses but folds conveniently so it is portable. Some people even place optical telescope lenses on their iPhones so they can zoom in on targets to take close-up pictures. Many cameras with zoom are essentially optical telescopes, as the mechanisms behind many cameras' zoom lenses are the same as those behind the optical telescope--and both have the same job of magnifying distant objects.
Researchers use sophisticated optical telescopes to study the universe in detail. Many observatories house famous optical telescopes. For instance, the large refractor telescope at the United States Naval Observatory in Washington, D.C., discovered Mars' moons Phobos and Deimos, according to the Space Telescope Science Institute (STScI). Perhaps the most famous optical telescope is the Hubble Space Telescope, which STScI explains is a reflector telescope that has been orbiting Earth since 1990 taking photographs of distant astronomical objects. Hubble's discoveries have significantly aided human understanding of the universe.
Breakthroughs include understanding the age of the universe and a realization that the universe's expansion is accelerating.