December 01, 2022 4 min read

Refracting telescopes are a type of optical telescopes that uses a lens as a goal to create an image. Therefore, they are also called dioptric binoculars. They were originally used in astronomical telescopes but are also use for long focus camera lenses. Magnification is calculated by dividing the focal length of the lens by the focal length of the eyepiece. Long-lens binoculars were popular in the 19th century for most research purpose. Lensed telescopes typically have a lens at the front, then a long tube, and finally an eyepiece or instrument where the view comes into focus. At first the lenses were single-element, but later, a century later, double-element and triple-element lenses appeared.

Lens binoculars are a technology, which has often been applied to other devices, such as zoom lenses, telephoto lenses, or long lens focuses.

The first optical telescopes were refractors. They appeared in 1608 in the Netherlands, by a Middleburg spectacle maker, Hans Lippershey, who eventually tried unsuccessfully to patent them. Of course, world quickly spread to Galileo Galilei, who was in Venice in May 1609, so he built his own version and used it for astronomical explorations.

Refracting telescopes

In terms of design, all refracting telescopes use three same principles. It meant that they used a combination of an objective lens and some type of eyepiece to collect the brighter light that the human eye cannot detect, then focus it and finally show a brighter, clearer, and magnified image.

The design, used by Galileo Galilei in 1609, was named the Galilean telescope. It used a converging lens and a diverging eye lens. Since this type of telescope had no intermediate focus, it did not produce an inverted image, but instead used certain instruments to obtain vertical image. Its most powerful telescope, with a total length of 980 millimeters, magnified objects by a factor of about thirty. However, a design flaw in the narrow field of view and the shape of the objective lens resulted in a distorted and a blurred image. However, despite these flaws, Galileo was good enough to explore the sky. Among other things, he used it to discover the craters of the Moon, the four largest moons of Jupiter and the phase of Venus. Later, in 1611, Johannes Kepler further developed Galileo’s plan. He used a convex lens as an eyepiece instead of a concave one. The advantage of this was that the light coming out of the eyepiece converged, resulting in a much wider field of view and greater relief for the eye, but the image for the viewer was now inverted. Another advantage was that a higher magnification could be achieved. Johannes Hevelius built a telescope with a focal length of forty-six meters based on the plans. And even longer tubeless so-called aerial telescopes were built. Also made it possible to use a micrometer in the focal plane, which decided the angular size and/or the distance between the object seen.

The next big development in refracting lenses was the invention of achromatic lenses. These now corrected for color distortion. Most of the time, two colors merge, blue and red. Also, multielement lenses helped to solve the problems of chromatic aberration. Invented in 1733 by English lawyer Chester Moore Hall, it was not patented until 1758 by John Dollond. Its design consisted of two pieces of glass, and, with the aid of an objective lens, it overcame the need for long focal lengths in refracting telescopes. Both side of the glass were polished and then joined together. These lenses are corrected to focus the two wavelengths in the same plane. Typically, red, or blue. The first double color correcting lens was also made by Chester Moore Hall in 1730. They were very popular in the 18th century. Their main attraction was that they could be shortened. The problem was that the glass lenses could be no more than four inches in diameter. Later, at the end of the 19th century, a Swiss optician, Pierre-Louis Guinand, developed a method of making glass plater larger than four inches and of better quality. He passed it on to his student Joseph von Fraunhofer double lens. These manufacturing techniques led to the large refractors of the 19th century, which gradually grew larger until the 1-meter refractor lenses were eventually replaced in astronomy by reflecting telescopes made of silvered glass. The most famous double refractor telescopes are the James Lick (91 cm) and the Greenwich 28-inch refractor (71 cm). Achromats were famous in astronomy for making star catalogues and needed less maintenance than metal mirrors. The plane Neptune and the moons of Mars were also discovered using achromats. Despite their smaller apertures, they were more often favored by prestige observations. At the end of the 18th century, a longer and larger refractor made its debut every few years, for example the Nice Observatory with a seventy-seven centimeters telescope, the largest of its time, but surpassed within a few years.

Refracting telescopes

Refracting telescopes were most used in astronomy and for terrestrial applications. Many years discoveries of the solar system were made using single refractors. They are widely used in photography and in orbit around the Earth. Single-element refractors have been used to discover the Galilean moons and many other moons in the Solar System. An example is Titan, the moon of Saturn, discovered on 25 March 1655 by the Dutch astronomer Christiaan Huygens. While, in 1861, binocular refractors using the 18-and-half-inch Dearborn telescope saw that the brightest star in the night sky, Sirius, had a smaller companion star. Pluto was discovered by seeing photographs taken with a refracting telescope, an astrograph with a 13-inch, three element objectives.

Research conducted by Dezső Sándor

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