Galileo's first telescopes, 1609-10

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Galileo's first telescopes, 1609-10

Image 1: Galileo's earliest surviving telescopes, 1609-10. Source: Wikimedia (CC 3.0). Higher quality photographs and detailed descriptions are available on the website of the Museo Galileo. The earlier telescope, from late 1609 - early 1610, has a magnifying power of 21x; the later, from ca. 1610, with a magnifying power of 20x, is to dedicated to Grand Duke Cosimo II de' Medici.

Images 2-3: Cannocchiale galileiano, reproduction of one of Galileo's earliest telescopes, 20th century, in the Museo nazionale della scienza e della tecnologia Leonardo da Vinci, Milan; photo by Alessandro Nassiri, 28 Nov. 2013. Source: Wikimedia (CC BY-SA 4.0). A similar replica can be found in the Science Museum London, object no. 1923-668.

Supplementary resources

A series of instructive brief videos and a variety of other explanatory materials on the early history of the telescope are provided by the Museo Galileo.  Most of the videos are complemented by a gallery in which the images used in it are reproduced and briefly described.

  • Galileo's astronomy. This video briefly summarizes the full range of Galileo’s telescopic discoveries. A convenient means of studying each of these discoveries in more detail is provided here.
  • From the workshop to the stars. This video recounts in more detail Galileo’s rapid development of telescopes of 3, 8 and 20 magnifications.
  • The tube and the housing of the optical parts. This video describes how Galileo’s telescopes and other early instruments were constructed.  
  • The Keplerian telescope. This video explains on of the major defects of the Galilean telescope, namely its field of vision, which narrows rapidly as its power of magnification increases. It also explains Kepler’s solution to this problem, which created the dominant form of telescope by the mid-seventeenth century. A further description of the optics of Galilean and Keplerian telescopes is offered here
  • Chromatic aberration. This video explains a second chief defect of the earliest ‘refracting’ telescopes, of both the Galilean and Keplerian type, namely, the different refraction of different wave-lengths of light, which reduced the clarity of the image and the field of vision of the early telescopes still further. Understanding this problem is important for grasping important aspects of the early history of the telescope, including some of the scepticism which first confronted Galileo’s telescope observations, the extreme length of Hevelius’s largest telescope, and the link between Newton’s optical experiments and his design of a telescope based on reflection by mirrors rather than refraction by lenses.
  • Reflecting telescopes: this video takes up the story of the development of reflecting telescopes by Gregory and Newton and extends it into the nineteenth century.
  • The performance of the telescope. This app helps explain the relationship between aperture, magnification, resolving power, and limit magnitude in the early telescopes.
  • A convenient, comparative account of the various kinds of telescope – refracting (Galilean, Keplerian, and terrestrial) and reflecting (Newtonian, Gregorian, Cassegrain and Herschelian) – can be found here.  An interactive application (entitled ‘Build a telescope’) allows the user to experiment with various combinations of lenses.