Refracting telescope

Image of a refracting telescope from the Cincinnati Observatory in 1848

A refracting or refractor telescope is a dioptric 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 in other devices such as binoculars and long or telephoto camera lenses.

Invention

Main article: History of the telescope

Refractors were the earliest type of optical telescope. The first practical refracting telescopes appeared in the Netherlands in about 1608, and were credited to three individuals, Hans Lippershey and Zacharias Janssen, spectacle-makers in Middelburg, and Jacob Metius of Alkmaar also known as Jacob Adriaanszoon. Galileo Galilei, happening to be in Venice in about the month of May 1609, heard of the invention and constructed a version of his own. Galileo then communicated the details of his invention to the public, and presented the instrument itself to the Doge Leonardo Donato, sitting in full council.

Dutch telescopes had also appeared in Rome and Naples before Galileo built his first. Galileo had initially heard about the telescope from Fra Paolo Sarpi, his Venetian anti-clerical friend who was in correspondence with an ex-student of Galileo, a French nobleman in Paris, Jacques Badovere. Sarpi was also involved in astronomical work, and he remained a close friend and correspondent of Galileo, despite being attacked by the Varican for his pro-Protestant views -- which played a large part in the Church's later attacks on Galileo.

In making his telescope, Galileo, almost certainly had a rough sketch of the Dutch instrument, but he also had a better knowledge of theoretical optics having spent time during the previous few years involved in optical research, so he was able to build a better instrument. Lens grinders had practical experience, but no theoretical understanding of complex optics.

Recent claims that he personally learned to grind lenses and so made a superior instrument seem to have no foundation. He had direct access to Venetian glass craftsmen, who had 200 years of glass-making and lens-grinding experience and were clearly the best in the world. His lenses have been tested in recent times and shown to be only marginally inferior to those that could be ground today (although they had no correction for colour fringe).

There are two main types of refracting (lens-based) telescopes: the Galilean type and the Keplerian. In either case, a front 'objective lens' creates a 'virtual image' at some distance down the length of the telescope tube, and then the eyepiece lens/lenses cause these light rays to emerge parallel from the eyepiece. When light rays are parallel (ie have no divergence or convergence) the eye can focus them and see the sharp image.

Refracting telescopes also come in many different configurations to correct for image orientation (some have an erector lens to turn the image the right-way-up) and different types have different problems with lens aberrations (color, spherical distortion, comet-tails, etc). Because the image was formed by the bending of light, or refraction, these telescopes are called refracting telescopes or refractors [which distinguishes them from mirror telescopes also called 'reflectors'].

Galilean telescope

The original Dutch design Galileo used in 1609 is commonly called a Galilean telescope. It uses a convex objective lens and a concave eyepiece lens, and this had the advantage of producing upright images. This made the telescope suitable both for terrestrial/maritime use, and for looking at the heavens.

Galileo’s best telescope magnified objects about 20 times. Despite minor flaws and the lack of optic which could handle the various colors of light, the telescope was still good enough for Galileo to explore the sky and see the planets as distant as Saturn as distinct globular objects rather than just specks of light. With his telescope Galileo could identify and draw the phases of Venus, which confirmed that Copernicus was right. Thomas Harriot in England, with a 6-times magnification Dutch-made telescope, was the first to map the mountains and seas on the moon, but he only beat Galileo by a few weeks. Galileo could see the craters on the Moon better than Harriot, and he was able to calculate the height of some mountains. He also saw the four moons orbiting Jupiter (again, he was not the first ... but the best), and saw the rings of Saturn (which he misinterpreted as two companion objects).

Galileo had better telescopes than any other astronomer in Europe for about three years, and during that time he published his famous booklet, Starry Messenger, which became a best-seller and revolutionised our view of the solar system.

Keplerian Telescope

Woodcut illustration of a 45 m (150 ft) focal length Keplerian astronomical refracting telescope built by Johannes Hevelius. From his book Machina coelestis (first part), published in 1673.

The Keplerian Telescope, invented by Johannes Kepler in 1611, is an improvement on Galileo's design. However Kepler apparently never made or used one himself (he had very poor vision, and relied on others for his observations).

It used one or more convex lens as the eyepiece, instead of Galileo's single concave lens, and it positions the eyepieces behind (rather than before) the focal plane of the objective lens. Since the light from the objective lens has already passed through its focal point and is now diverging, the use of these positive eyepiece lenses counteract the diverging rays making them parallel once more, enabling the eye to focus them. The secondary advantage of this arrangement is the eyepiece does not limit the field of view to the extent that it does with the Galilean telescope. However, the image for the viewer is inverted. A third lens system needs to be added for terrestrial telescopes, to invert the inversion.

Considerably higher magnifications can be reached with this design but, to overcome inherent chromatic (color) aberrations the simple objective lens needs to have a very high f-ratio -- focal length to aperture ratio. Johannes Hevelius built one with a 45 m (150 ft) focal length. The design also allows for use of cross-hairs and a micrometer at the focal plane (used to determining the angular size and/or distance between objects observed). By 1650, only a few years after Galileo's death, the Keplerian was the type of telescope universally used by astronomers.

Achromatic refractors

Main article: Achromatic telescope

The theory of the Achromatic refracting lens was invented in 1733 by an English barrister named Chester Moore Hall who discovered that flint glass dispersed colors more widely than crown glass figured to the same lens shape. He had optician George Bass make him some experimental lenses with a positive crown-glass, and a negative flint-glass - but with the crown-glass at a slightly higher power. This meant that the flint-glass would counteract the dispersion, while not entirely cancelling the focal power. This attempt proved to be unsatisfactory because of the precision required.

Leonard Euler had suggested the idea once again in 1748, and the famous lens-maker John Dollond was initially dismissive because the great theorist Isaac Newton had said it was impossible. Dollond wrote an article explaining why it was never going to be possible to correct color problems in lenses, and then received the news about Hall's previous failed attempt from Ball (who had been sworn to secrecy). It turned out that Euler's theories were also wrong, as was Dolland's dismissal of the possibility. Hall had been right, but he didn't have the precision techniques to make it a reality. Flint-glass of optical quality was difficult to find (it was long a British monopoly) and the manufacture required careful calculations and configurations. Dollond retried the idea, and around 1758 he had practically developed and patented the first true achromatic doublet.

When used as an objective lens, the doublet overcame the need for very long focal lengths in refracting telescopes. Such telescopes used an objective comprising a glass 'sandwich' made from glass ("crown" and "flint glass"), each with different dispersions. With these techniques it was possible to limit the effects of chromatism (color confusion and color fringe), which created most of the image defects in telescopes at that time, and these became the main tool of astronomy until the reflecting mirror telescope was developed to a satisfactory level (many years after Newton) with the invention of silvering techniques for glass mirrors.

In making these achromatic doublets, Dollond had to ensure that both sides of each 'singlet' are ground and polished to very high precision. Then the two pieces are joined together using a transparent adhesive (Canadian balsam). Dolland was the master lens-grinder who was the first to be able to match the shapes of two lenses to such perfection that they could be fitted together with no gaps between (which would have produced 'Newton's Rings'). His design relied on the availability of optical quality flint-glass, and this gave England a pre-eminence in astronomy for a number of years.

Achromatic doublets correct color fringe by bringing together the various wavelengths in white light into a single point of focus on the same plane. Without them, the red light rays always focus at a greater distance from the lens than the blue. The other main form of aberration in these lenses, known as Spherical Aberration (which was far less of a problem), was eventually overcome by grinding lenses and mirrors with the edges slightly flattened, changing the lens surfaces from a perfect sphere to that of a parabola. This can't be done with machine or mass-production.

Apochromatic refractors

Main article: Apochromat

Apochromatic refractors are telescopes which have their objectives built with special, extra-low dispersion transparent materials. They are designed to bring all wavelengths into focus in the same plane. The residual color error (tertiary spectrum) can be up to an order of magnitude less than that of an achromatic lens. Such telescopes lenses contain elements of fluorite or special, extra-low dispersion (ED) glass in the objective and produce a very crisp image that is virtually free of chromatic aberration, and are sold in the high-end amateur telescope market. Apochromatic refractors are available with objectives of up to 553 mm in diameter, but most are between 80 and 152 mm.

Technical considerations

The 40-inch (1.02 m) Refractor, at Yerkes Observatory, the largest achromatic refractor ever put into astronomical use.

Refractors have been criticized for their relatively high-degree of residual chromatic and spherical aberration. This affects shorter focal lengths more than longer ones. A 4" f/6 achromatic refractor is likely to show considerable color fringing (generally a purple halo around bright objects). A 4" f/16 will have little color fringing.

In very large apertures, there is also a problem of lens sagging, a result of gravity deforming glass. Since a lens can only be held in place by its edge, the center of a large lens will sag due to gravity, distorting the image it produces. The largest practical lens size in a refracting telescope is around 1 meter.

There is a further problem of glass defects, striae or small air bubbles trapped within the glass. In addition, glass is opaque to certain wavelengths, and even visible light is dimmed by reflection and absorption when it crosses the air-glass interfaces and passes through the glass itself. Most of these problems are avoided or diminished by using reflecting telescopes, which can be made in far larger apertures.

Notable refracting telescopes

• Yerkes Observatory (102 cm)
• Swedish Solar Telescope (100cm)
• Lick Observatory (91cm)
• Paris Observatory (83 cm + 62cm)
• Nice Observatory (76cm)
• Archenhold Observatory (68cm, 21 m focal length - the longest refracting telescope ever built)
• Lowell Observatory (24 in)
• Chabot Space & Science Center (20 in, 8 in)
• Griffith Observatory (12 in)
• Dearborn Observatory (18.5 in)
• Great Paris Exhibition Telescope of 1900 (1.25 m)

• 
  The 76 cm refractor at Nice Observatory.
• 
  20 inch refractor at the Observatories at Chabot Space & Science Center in Oakland, California.
• 
  8 inch refractor at the Observatories at Chabot Space & Science Center in Oakland, California.
• 
  The 68 cm refractor at the Vienna University Observatory.
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  The Great Refractor at the Archenhold Observatory in Berlin.
• 
  Refractor at the Observatory in Aachen, Germany

International Year of Astronomy commemorative coin featuring a refracting telescope.

See also

• International Year of Astronomy commemorative coin
• Reflecting telescopes
• Catadioptric telescopes
• Astrograph
• List of largest optical refracting telescopes
• Henry Fitz, the first American to make refracting telescopes.

External links

• nasa.gov - Build a Telescope
• Making a Galilean Telescope
• BUILDING A GALILEAN TELESCOPE
• OPTICS OF THE SINGLET REFRACTOR: GALILEO vs. KEPLER