Reflecting telescopes took over a century to catch on after Newton built one
Isaac Newton built a practical mirror telescope in 1668, avoiding the colour fringes that plagued lens telescopes. Yet his speculum metal mirrors were hard to make and performed poorly, so it was more than a hundred years before reflectors became popular. Today the largest telescopes on Earth all use mirrors.
Telescopes were less a scientist's invention than an optical craftsman's discovery. Lens theory stretched back to ancient Greece and the medieval Islamic world, but the crucial ingredient was the spectacle trade that began in Venice and Florence in the 13th century and spread to the Netherlands and Germany. The first documented refracting telescope was a 1608 patent application by the Dutch spectacle maker Hans Lippershey, quickly followed by rival claims from Jacob Metius and an unknown third party.
Galileo heard the news and within a year had built improved versions, becoming the first to publish astronomical observations with one. His design paired a convex front lens with a concave eyepiece. Johannes Kepler suggested a convex eyepiece instead. Such lens telescopes suffered from chromatic aberration until the achromatic lens, which combined crown and flint glass; Chester Moor Hall designed the first in 1729, though John Dollond obtained the patent. That allowed shorter tubes and bigger lenses.
Mirror telescopes had theory before practice. Alhazen had shown that curved mirrors can behave like lenses, and after 1608 Galileo and others discussed building one. James Gregory published a notable design in 1663, but no working model followed before Newton's. Improvements came slowly: accurate parabolic mirrors in the 18th century, silvered glass in the 19th, durable aluminium coatings, segmented mirrors and active optics in the 20th. Hybrid catadioptric designs such as the Schmidt camera combine a correcting lens with a mirror for wide sharp fields.
A telescope's power to resolve detail depends on the diameter of its main lens or mirror, and its light gathering on that element's area. Adaptive optics and space telescopes now sidestep the blurring of Earth's atmosphere. In the 2010s cheap computer-linked telescopes let amateurs stack many digital exposures, reaching stars as faint as magnitude 15.
Source: Optical telescope
