Refracting telescope

• 1. Introduction

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  In May of 1609, Galileo Galilei was in Venice when word reached him of a curious device being developed in the Netherlands. A spectacle maker from Middelburg named Hans Lippershey had tried to patent it the year before, without success, but news of the invention spread fast enough to cross the Alps. Galileo built his own version and pointed it at the sky. What he found there changed the course of human understanding.

  The device was the refracting telescope, and its story reaches from the workshops of Dutch lens grinders all the way to the moons of distant planets, to the interstellar medium itself. How did a tube with a piece of glass at each end come to unlock so much of the cosmos? And why, after centuries of triumph, did it ultimately give way to a different design altogether?

• 2. The Galilean Design

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  Galileo's most powerful telescope stretched 980 mm in total length and could magnify objects about 30 times. That sounds impressive, but the lens technology of the era forced him to make compromises. He had to install aperture stops, narrowing the diameter of the objective lens to reduce blurring and distortion, and the result was a narrow, imperfect field of view.

  The design Galileo used is now called the Galilean telescope. It paired a convergent plano-convex objective lens with a divergent plano-concave eyepiece. Because the design has no intermediary focus, the image it produces is upright rather than flipped. That mattered for observers scanning the night sky without a map to orient them.

  Despite the blurry edges and limited field, Galileo pressed on. He traced the craters of the Moon, tracked the four largest moons of Jupiter across successive nights, and watched Venus cycle through phases like the Moon does. The telescope that did all of this was a flawed instrument in skilled hands.

• 3. Kepler's Improvement

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  Johannes Kepler proposed a different arrangement in 1611. Instead of the concave eyepiece Galileo used, Kepler substituted a convex lens. The rays of light emerging from this eyepiece converge rather than diverge, which opened up a much wider field of view and gave the observer greater eye relief.

  The trade-off was an inverted image, which mattered less for astronomical work than for watching ships in a harbor. More importantly, the Keplerian design could reach considerably higher magnifications. Lens makers pushed this to remarkable extremes: Johannes Hevelius built a Keplerian telescope with a focal ratio of f/225, an 8-inch objective, and a focal length of 150 feet. Astronomers even built tubeless aerial telescopes that were longer still.

  Huygens built an aerial telescope for the Royal Society of London using a single-element lens measuring 19 centimeters, or 7.5 inches, in diameter. The Keplerian design also allowed a micrometer to be placed at the focal plane, letting astronomers measure the angular size and separation of objects they observed. That capability would prove essential for the century of precise positional astronomy that followed.

• 4. The Achromatic Breakthrough

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  Chester Moore Hall solved a problem that had plagued refractors for generations in 1733. Chromatic aberration, the tendency of a simple lens to bend different colors of light by slightly different amounts, produced halos and color fringing around bright objects. Hall's achromatic lens used two pieces of glass with different dispersion properties, one of crown glass and one of flint glass, to bring red and blue wavelengths into focus at the same plane.

  John Dollond independently invented and patented the same design around 1758, and Dollond achromats became widely popular through the 18th century. A major appeal was that they could be built much shorter than the unwieldy Keplerian designs. Problems with glass manufacturing, however, kept objective diameters below about 4 inches for decades.

  The Swiss optician Pierre-Louis Guinand changed that in the late 19th century by developing methods to produce high-quality glass blanks larger than 4 inches. He passed the technique to his apprentice Joseph von Fraunhofer, who refined it further and developed the Fraunhofer doublet lens design. The result was an era of progressively larger instruments, culminating in the Great Paris Exhibition Telescope of 1900 with a lens 1.25 meters across. Famous doublets of that era include the James Lick telescope at 91 centimeters and the Greenwich 28-inch refractor at 71 centimeters.

• 5. Discoveries and Achievements

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  Asaph Hall discovered Deimos on the 12th of August 1877 at about 07:48 UTC, and Phobos six days later on the 18th of August 1877 at about 09:14 GMT. Both finds were made at the US Naval Observatory in Washington, D.C., using the 26-inch refractor then located at Foggy Bottom. In 1893 that lens was remounted in a new dome, where it remains into the 21st century.

  Edward Emerson Barnard discovered Jupiter's moon Amalthea on the 9th of September 1892, by direct visual observation through the 36-inch refractor at Lick Observatory. No photography, no plate measurement; he simply looked.

  In 1904, observations made with the Great Refractor of Potsdam, a double telescope carrying two doublet lenses, revealed something entirely unexpected. Professor Hartmann studied the binary star Mintaka in Orion and detected calcium in the space between the stars. That was one of the first pieces of evidence for the interstellar medium, the diffuse material threading the galaxy between stellar systems.

  Pluto's discovery came through a different application of the refractor. Astronomers photographed the sky with a 13-inch, three-element astrograph and then compared successive plates in a blink comparator, looking for any dot of light that shifted position against the fixed stars. Titan, the largest moon of Saturn, was found on the 25th of March 1655 by the Dutch astronomer Christiaan Huygens. And in 1861, the 18.5-inch Dearborn refracting telescope revealed that Sirius, the brightest star in the night sky, had a smaller companion orbiting it.

• 6. Limits and Legacy

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  A lens can only be supported at its edge. In very large apertures, gravity causes the center of the glass to sag, distorting the focal point and degrading the image. That physical constraint puts a ceiling on the practical size of a refracting telescope at around 1 meter in diameter.

  Glass also absorbs and reflects light at every air-glass interface it encounters. Some wavelengths, particularly in the ultraviolet and infrared, never pass through at all. These problems mount with aperture, and for large-scale astronomical research, the reflecting telescope, which bounces light off a mirror rather than bending it through glass, has all but replaced the refractor. Mirrors can be made far larger and can be supported across their full surface rather than only at the rim.

  The refractor did not disappear. At Griffith Observatory, the 12-inch Zeiss refractor installed at its 1935 opening has been viewed through by over 7 million people, more than any other telescope in history. The Wide Angle Camera on Voyager 1 and Voyager 2 carried a 6-centimeter refracting lens into deep space in the late 1970s. And modern apochromatic refractors, built with fluorite or extra-low dispersion glass, bring three wavelengths, red, green, and blue, into focus simultaneously, reducing residual color error by an order of magnitude compared to an achromatic lens. The Archenhold Observatory in Germany houses the longest refracting telescope ever built, with a 68-centimeter aperture and a 21-meter focal length, a reminder of how far lens makers were once willing to go.