page_title: Reflecting telescope
hierarchical_section_title: Reflecting telescope
caption_attribution_description: English: 24 inch convertible Newtonian/Cassegrain reflecting telescope on display in the Franklin Institute rooftop observatory (has since been moved to another location).
context_page_description: A reflecting telescope is a telescope that uses a single or a combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century, by Isaac Newton, as an alternative to the refracting telescope which, at that time, was a design that suffered from severe chromatic aberration. Although reflecting telescopes produce other types of optical aberrations, it is a design that allows for very large diameter objectives. Almost all of the major telescopes used in astronomy research are reflectors. Reflecting telescopes come in many design variations and may employ extra optical elements to improve image quality or place the image in a mechanically advantageous position. Since reflecting telescopes use mirrors, the design is sometimes referred to as a "catoptric" telescope.
  From the time of Newton to the 1800s, the mirror itself was made of metal – usually speculum metal. This type included Newton's first designs and even the largest telescopes of the 19th century, the Leviathan of Parsonstown with a 1.8 meter wide metal mirror.
context_section_description: A reflecting telescope (also called a reflector) is a telescope that uses a single or a combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century, by Isaac Newton, as an alternative to the refracting telescope which, at that time, was a design that suffered from severe chromatic aberration. Although reflecting telescopes produce other types of optical aberrations, it is a design that allows for very large diameter objectives. Almost all of the major telescopes used in astronomy research are reflectors. Reflecting telescopes come in many design variations and may employ extra optical elements to improve image quality or place the image in a mechanically advantageous position. Since reflecting telescopes use mirrors, the design is sometimes referred to as a "catoptric" telescope.
  From the time of Newton to the 1800s, the mirror itself was made of metal –  usually speculum metal. This type included Newton's first designs and even the largest telescopes of the 19th century, the Leviathan of Parsonstown with a 1.8 meter wide metal mirror. In the 19th century a new method using a block of glass coated with very thin layer of silver began to become more popular by the turn of the century. A major turning point in reflecting telescopes was the Paris Observatory 1.2 m of 1878, A.A. Common telescopes which lead to the Crossley and Harvard reflecting telescopes, which helped establish a better reputation for reflecting telescopes as the metal mirror designs were noted for their drawbacks. Chiefly the metal mirrors only reflected about 2/3 of the light and the metal would tarnish. After multiple polishings and tarnishings the mirror could lose its precise figuring needed.
  Reflecting telescopes became extraordinarily popular for astronomy and many famous telescopes such as the Hubble Space Telescope and popular amateur models use this design. In addition, the reflection telescope principle was applied to other wavelengths of light, and for example, X-Ray telescopes also use the reflection principle to make image forming optics.
caption: 24 inch convertible Newtonian/Cassegrain reflecting telescope on display at the Franklin Institute
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