Refracting telescope

Optical diagram of Galilean telescope y – Distant object; y′ – Real image from objective; y″ – Magnified virtual image from eyepiece; D – Entrance pupil diameter; d – Virtual exit pupil diameter; L1 – Objective lens; L2 – Eyepiece lens e – Virtual exit pupil – Telescope equals[5]

The design Galileo Galilei used c. 1609 is commonly called a Galilean telescope.[6] It used a convergent (plano-convex) objective lens and a divergent (plano-concave) eyepiece lens (Galileo, 1610).[7] A Galilean telescope, because the design has no intermediary focus, results in a non-inverted and, with the help of some devices, an upright image.[8]

Galileo's most powerful telescope, with a total length of 980 millimetres (3 ft 3 in),[6] magnified objects about 30 times.[8] Galileo had to work with the poor lens technology of the time, and found he had to use aperture stops to reduce the diameter of the objective lens (increase its focal ratio) to limit aberrations, so his telescope produced blurry and distorted images with a narrow field of view.[8] Despite these flaws, the telescope was still good enough for Galileo to explore the sky. He used it to view craters on the Moon,[9] the four largest moons of Jupiter,[10] and the phases of Venus.[11]

Parallel rays of light from a distant object (y) would be brought to a focus in the focal plane of the objective lens (F′ L1 / y′). The (diverging) eyepiece (L2) lens intercepts these rays and renders them parallel once more. Non-parallel rays of light from the object traveling at an angle α1 to the optical axis travel at a larger angle (α2 > α1) after they passed through the eyepiece. This leads to an increase in the apparent angular size and is responsible for the perceived magnification.

The final image (y″) is a virtual image, located at infinity and is the same way up as the object.

Engraved illustration of a 46 m (150 ft) focal length Keplerian astronomical refracting telescope built by Johannes Hevelius.[12]

The Keplerian telescope, invented by Johannes Kepler in 1611, is an improvement on Galileo's design.[13] It uses a convex lens as the eyepiece instead of Galileo's concave one. The advantage of this arrangement is that the rays of light emerging from the eyepiece are converging. This allows for a much wider field of view and greater eye relief, but the image for the viewer is inverted. Considerably higher magnifications can be reached with this design, but, like the Galilean telescope, it still uses simple single element objective lens so needs to have a very high focal ratio to reduce aberrations[14] (Johannes Hevelius built an unwieldy f/225 telescope with a 200-millimetre (8 in) objective and a 46-metre (150 ft) focal length,[15] and even longer tubeless "aerial telescopes" were constructed). The design also allows for use of a micrometer at the focal plane (to determine the angular size and/or distance between objects observed).

Huygens built an aerial telescope for Royal Society of London with a 19 cm (7.5″) single-element lens.[16]

Alvan Clark polishes the big Yerkes achromatic objective lens, over 1 meter across, in 1896.

This 12 inch refractor is mounted in a dome on a mount that matches the Earth's rotation

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.[17]

Dollond achromats were quite popular in the 18th century.[18][19] A major appeal was they could be made shorter.[19] However, problems with glass making meant that the glass objectives were not made more than about four inches in diameter.[19]

In the late 19th century, the Swiss optician Pierre-Louis Guinand[20] developed a way to make higher quality glass blanks of greater than four inches.[19][21] He passed this technology to his apprentice Joseph von Fraunhofer, who further developed this technology and also developed the Fraunhofer doublet lens design.[19] 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:[22]

The Greenwich 28-inch refractor is a popular tourist attraction in 21st century London

• Alvan Clark
• Brashear[23]
• Chance Brothers
• Cauchoix[24]
• Fraunhofer[25]
• Gautier
• Grubb
• Henry Brothers
• Lerebours[26]
• Tulley[27]

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.[28] The Sheepshanks had a 6.7 inches (17 cm) wide lens, and was the biggest telescope at Greenwich for about twenty years.[29]

An 1840 report from the Observatory noted of the then-new Sheepshanks telescope with the Cauchoix doublet:[30]

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.[31] 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.[31]

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.[32]

The Apochromatic lens usually comprises three elements that bring light of three different frequencies to a common focus

Apochromatic refractors have objectives built with special, extra-low dispersion materials. They are designed to bring three wavelengths (typically red, green, and blue) into focus in the same plane. The residual color error (tertiary spectrum) can be an order of magnitude less than that of an achromatic lens.[33] Such telescopes 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.[34] Due to the special materials needed in the fabrication, apochromatic refractors are usually more expensive than telescopes of other types with a comparable aperture.

In the 18th century, Dollond, a popular maker of doublet telescopes, also made a triplet, although they were not really as popular as the two element telescopes.[19]

One of the famous triplet objectives is the Cooke triplet, noted for being able to correct the Seidal aberrations.[35] It is recognized as one of the most important objective designs in the field of photography.[36][37] The Cooke triplet can correct, with only three elements, for one wavelength, spherical aberration, coma, astigmatism, field curvature, and distortion.[37]