page_title: Refracting telescope
section_title: Achromatic refractors
hierarchical_section_title: Refracting telescope / Refracting telescope designs / Achromatic refractors
caption_attribution_description: Alvan Graham Clark and his assistant Carl Lundin (right) alongside of the 40-inch Lens.
context_page_description: A refracting telescope is a type of optical telescope that uses a lens as its objective to form an image. The refracting telescope design was originally used in spy glasses and astronomical telescopes but is also used for long focus camera lenses. Although large refracting telescopes were very popular in the second half of the 19th century, for most research purposes, the refracting telescope has been superseded by the reflecting telescope, which allows larger apertures. A refractor's magnification is calculated by dividing the focal length of the objective lens by that of the eyepiece.
  Refracting telescopes typically have a lens at the front, then a long tube, then an eyepiece or instrumentation at the rear, where the telescope view comes to focus. Originally, telescopes had an objective of one element, but a century later, two and even three element lenses were made.
  Refracting telescope is a technology that has often been applied to other optical devices such as binoculars and zoom lenses/telephoto lens/Long-focus lens.
context_section_description: The next major step in the evolution of refracting telescopes was the invention of the achromatic lens, a lens with multiple elements that helped solve problems with chromatic aberration and allowed shorter focal lengths. It was invented in 1733 by an English barrister named Chester Moore Hall, although it was independently invented and patented by John Dollond around 1758. The design overcame the need for very long focal lengths in refracting telescopes by using an objective made of two pieces of glass with different dispersion, 'crown' and 'flint glass', to reduce chromatic and spherical aberration. Each side of each piece is ground and polished, and then the two pieces are assembled together. Achromatic lenses are corrected to bring two wavelengths (typically red and blue) into focus in the same plane.
  Chester More Hall is noted as having made the first twin color corrected lens in 1730.
  Dollond achromats were quite popular in the 18th century. A major appeal was they could be made shorter. However, problems with glass making meant that the glass objectives were not made more than about four inches in diameter.
  In the late 19th century, the glass maker Guinand developed a way to make higher quality glass blanks of greater than four inches. He also passed this technology to his apprentice Fraunhofer, whom further developed this technology and also developed the Fraunhofer doublet lens design. The breakthrough in glass making techniques led to the great refractors of the 19th century, that became progressively larger through the decade, eventually reaching over 1 meter by the end of that century before being superseded by silvered-glass reflecting telescopes in astronomy.
  Noted lens makers of the 19th century include:
  Alvan Clark
  Brashear
  Chance Brothers
  Cauchoix
  Fraunhofer
  Gautier
  Grubb
  Henry Brothers
  Lerebours
  Tulley
  Some famous 19th century doublet refractors are the James Lick telescope (91 cm/36 in) and the Greenwich 28 inch refractor (71 cm). An example of an older refractor is the Shuckburgh telescope (dating to the late 1700s). A famous refractor was the "Trophy Telescope", presented at the 1851 Great Exhibition in London. The era of the 'great refractors' in the 19th century saw large achromatic lenses, culminating with the largest achromatic refractor ever built, the Great Paris Exhibition Telescope of 1900.
  In the Royal Observatory, Greenwich an 1838 instrument named the Sheepshanks telescope includes an objective by Cauchoix. The Sheepshanks had a 6.7 inch (17 cm) wide lens, and was the biggest telescope at Greenwich for about twenty years.
  
  An 1840 report from the Observatory noted of the then-new Sheepshanks telescope with the Cauchoix doublet:
  The power and general goodness of this telescope make it a most welcome addition to the instruments of the observatory
  In the 1900s a noted optics maker was Zeiss. An example of prime achievements of refractors, over 7 million people have been able to view through the 12-inch Zeiss refractor at Griffith Observatory since its opening in 1935; this is the most people to have viewed through any telescope.
  Achromats were popular in astronomy for making star catalogs, and they required less maintenance than metal mirrors. Some famous discoveries using achromats are the planet Neptune and the Moons of Mars.
  The long achromats, despite having smaller aperture than the larger reflectors, were often favoured for "prestige" observatories. In the late 18th century, every few years, a larger and longer refractor would debut.
  For example, the Nice Observatory debuted with 77-centimetre (30.31 in) refractor, the largest at the time, but was surpassed within only a couple of years.
caption: Alvan Clark polishes the big Yerkes achromatic objective lens, over 1 meter across, in 1896.
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