Great refractor

Lore & Background

The era of great refractors began with Joseph von Fraunhofer's achromatic telescopes in the early 1820s, starting with the Dorpat Great Refractor. This telescope had a 9 Paris inch (about 9.6 in) aperture and introduced the German equatorial mount. Fraunhofer's design became standard for large refractors. Early notable examples include Edward Joshua Cooper's 13.3-inch Cauchoix lens in Ireland (1830s) and the Duke of Northumberland's 11.6-inch Cauchoix lens (1833).

Great refractors were often centerpieces of 19th-century observatories, used alongside instruments like meridian circles, heliometers, astrographs, and comet seekers. They were frequently employed for observing double stars with a filar micrometer and for pioneering astrophotography. The telescopes grew in size through the century, with the 25-inch Newall telescope (1869) becoming the world's largest, later donated to the National Observatory of Athens.

The golden era culminated with the Yerkes 40-inch refractor (1895), which pushed technological limits. Fabricating its two-element achromatic lens required 18 attempts. Refractors reached their practical limit around 1 meter due to lens sagging from gravity, though Alvan G. Clark suggested a 45-inch might be possible. The choice between refractors and reflectors was driven by technology: refractors faced difficulties in making large glass disks, while reflectors used speculum metal that tarnished and required frequent repolishing.

Reader's Guide

Great refractors defined an era of observational astronomy, achieving notable accomplishments such as the discovery of Neptune, the moons of Mars, and various star catalogs. The heliometer, a derivative instrument, measured the first distance to another star by geometric parallax in the mid-1800s. Despite smaller apertures than reflectors, great refractors offered advantages and were popular for astronomy. They were also exhibited at international fairs, like the Trophy Telescope at the 1851 Great Exhibition and the Yerkes Great Refractor at the 1893 World's Fair.

In modern times, many large refractors have become historical items used for public outreach. The 12-inch Zeiss refractor at Griffith Observatory, opened in 1935, has been viewed by over 7 million people—the most of any telescope. However, many have been shut down or moved due to difficulty of use. Older observatories were often located near towns because they served multiple functions: recording weather, determining location, and local time—tasks now performed elsewhere. Physically, the largest refractors were enormous, with moving weights of multiple tons in domes several stories tall, larger than some modern reflecting telescopes.

Did You Know?

• The Fraunhofer 9-inch at Berlin Observatory was used by Johann Gottfried Galle in the discovery of Neptune.
• The Yerkes 40-inch refractor required 18 attempts to fabricate its two-element achromatic lens.
• The Newall telescope, completed in 1869, was the largest refractor in the world and later donated to the National Observatory of Athens.

Origins & the First Refractors

The refracting telescope holds the distinction of being the very first form of optical telescope ever conceived. The earliest documented reference traces back to the Netherlands around 1608, when Hans Lippershey, a spectacle maker based in Middelburg, attempted to secure a patent for his device. The patent application failed, but word of the invention traveled quickly across Europe. By May 1609, Galileo Galilei, who happened to be in Venice, learned of the device and promptly built his own version. He then turned it toward the heavens, making groundbreaking astronomical observations. This early period established the refractor as the foundational design for all subsequent optical telescopes, a role it would maintain through spyglasses, astronomical instruments, and even long-focus camera lenses for centuries to come.

From Galilean to Keplerian: A Design Trade-Off

The two foundational refractor configurations represent a fascinating trade-off between image orientation and optical performance. Galileo's design, dating to roughly 1609, paired a convergent plano-convex objective with a divergent plano-concave eyepiece. Because no intermediate focal point exists, the resulting image remains upright. His most powerful instrument measured 980 millimeters in length and delivered roughly 30 times magnification, though limited lens technology forced him to use aperture stops, yielding narrow, somewhat distorted views. Johannes Kepler improved upon this in 1611 by substituting a convex eyepiece, which produced converging exit rays. This allowed a wider field of view and greater eye relief, but at the cost of an inverted image. The Keplerian design also permitted considerably higher magnifications and the placement of a micrometer at the focal plane for angular measurements. Yet both designs relied on single-element objectives, demanding extremely high focal ratios to suppress aberrations—Hevelius built an f/225 instrument with a 46-meter focal length, and Huygens constructed an aerial telescope with a 19-centimeter lens for the Royal Society of London.

The Achromatic Revolution

For over a century, refractor designers were constrained by chromatic aberration, which forced them into impractically long tubes. The breakthrough came in 1733 when Chester Moore Hall, an English barrister, created the first twin color-corrected lens. Around 1758, John Dollond independently developed and patented a similar achromatic design that became widely popular throughout the 18th century. The principle involved combining two pieces of glass with different dispersion properties—crown and flint glass—each ground and polished on both sides before being assembled into a single objective. This arrangement brought two wavelengths, typically red and blue, into the same focal plane, dramatically reducing both chromatic and spherical aberration. The practical benefit was enormous: refractors could now be built with much shorter focal lengths, making them far more manageable. However, limitations in glass-making technology of the era capped objective diameters at roughly four inches, a constraint that would persist until later advances in optical manufacturing.

Optical Principles & Broader Impact

At its core, a refractor operates by bending light. The objective lens refracts incoming parallel rays so they converge at a focal point, while non-parallel rays meet at a focal plane. The telescope essentially converts one bundle of parallel rays at angle α into a second bundle at angle β, and the ratio β/α defines the angular magnification—equivalently, the ratio of retinal image sizes with and without the instrument. Magnification is simply the focal length of the objective divided by that of the eyepiece. The basic layout remains consistent: a lens at the front, a long tube, and an eyepiece or instrumentation at the rear where the virtual image comes to focus. Beyond astronomy, the same refractive principles have been applied to binoculars, zoom lenses, telephoto lenses, and long-focus camera lenses. Although large refractors dominated popular astronomy in the second half of the 19th century, the reflecting telescope eventually supplanted them for most research purposes, as mirrors can be made in far larger apertures without the chromatic and weight constraints inherent to glass lenses.