Refracting Telescopes

When a "Telescope" term come in mind, we always think of that tube with the big lens and the small one we look in. Though its inventor may be lost in history, this early kind of telescope is called a "Galilean" or simple refractor. The Galilean refractor consists of two lenses: a convex (curved outward) lens held in front of a concave (curved inward) lens a certain distance away. As you know, the telescope’s front lens is called the objective, while the other is referred to as the eyepiece, or ocular.

The Galilean refractor placed the concave eyepiece before the objective’s prime focus; this produced an upright, extremely narrow field of view, like today’s inexpensive opera glasses. Not long after Galileo made his first telescope, Johannes Kepler improved on the idea by simply swapping the concave eyepiece for a double convex lens, placing it behind the prime focus. The Keplerian refractor proved to be far superior to Galileo’s instrument. The modern refracting telescope continues to be based on Kepler’s design. The fact that the view is upside down is of little consequence to astronomers because there is no up and down in space; for terrestrial viewing, extra lenses may be added to flip the image a second time, reinverting the scene back.Unfortunately, both the Galilean and the Keplerian designs have several optical deficiencies. Chief among these is chromatic aberration. As you may know, when we look at any white-light source, we are not actually looking at a single wavelength of light but rather a collection of wavelengths mixed together. To prove this for yourself, shine sunlight through a prism. The light going in is refracted within the prism, exiting not as a unit but instead broken up, forming a rainbowlike spectrum. Each color of the spectrum has its own unique wavelength. If you use a lens instead of a prism, each color will focus at a slightly different point. The net result is a zone of focus, rather than a point. Through such a telescope, everything appears blurry and surrounded by halos of color. This effect is called chromatic aberration.

Another problem of simple refractors is spherical aberration. In this instance, the curvature of the objective lens causes the rays of light entering around its edges to focus at a slightly different place than those striking the center. Once again, the light focuses within a range rather than at a single point, making the telescope incapable of producing a clear, razor-sharp image. Modifying the inner and outer curves of the lens proved somewhat helpful. Experiments showed that both defects could be reduced (but not totally eliminated) by increasing the focal length—that is, decreasing the curvature— of the objective lens. And so, in an effort to improve image quality, the refractor became longer . . . and longer . . . and even longer! The longest refractor on record was constructed by Johannes Hevelius in Denmark during the latter part of the seventeenth century; it measured about one hundred and fifty feet from objective to eyepiece and required a complex sling system suspended high above the ground on a wooden mast to hold it in place! Can you imagine the effort it must have taken to swing around such a monster just to look at the Moon or a bright planet? Surely, there had to be a better way. In an effort to combat these imperfections, Chester Hall developed a two element achromatic lens in 1733. Hall learned that by using two matching lenses made of different types of glass, aberrations could be greatly reduced. In an achromatic lens, the outer element is usually made of crown glass, while the inner element is typically flint glass. Crown glass has a lower dispersion effect and therefore bends light rays less than flint glass, which has a higher dispersion. The convergence of light passing through the crown-glass lens is compensated by its divergence through the flint-glass lens, resulting in greatly dampened aberrations. Ironically, though Hall made several telescopes using this arrangement, the idea of an achromatic objective did not catch on for another quarter century.

In 1758, John Dollond reacquainted the scientific community with Hall’s idea when he was granted a patent for a two-element aberration-suppressing lens. Though quality glass was hard to come by for both of these pioneers, it appears that Dollond was more successful at producing a high-quality instrument. Perhaps that is why history records John Dollond, rather than Chester

Hall, as the father of the modern refractor. Regardless of who first devised it, this new and improved design has come to be called the achromatic refractor, with the compound objective simply labeled an achromat. Though the methodology for improving the refractor was now known, the problem of getting high-quality glass (especially flint glass) persisted. In 1780, Pierre Louis Guinard, a Swiss bell maker, began experimenting with various casting techniques in an attempt to improve the glass-making process. It took him close to 20 years, but Guinard’s efforts ultimately paid off, for he learned the secret of producing flawless optical disks as big as roughly 6 inches in diameter.Later, Guinard was to team up with Joseph von Fraunhofer, inventor of the spectroscope.

While studying under Guinard’s guidance imented by slightly modifying the lens curves suggested by Dollond, which resulted in the highest-quality objective yet created. In Fraunhofer’s design, the front surface is strongly convex. The two central surfaces differ slightly from each other, requiring a narrow air space between the elements, while the innermost surface is almost perfectly flat. These innovations bring two wavelengths of light across the lens’s full diameter to a common focus, thereby greatly reducing chromatic and spherical aberration. The world’s largest refractor is the 40-inch f /19 telescope at Yerkes Observatory in Williams Bay, Wisconsin. This mighty instrument was constructed by Alvan Clark and Sons, Inc., America’s premier telescope maker of the nineteenth century. Other examples of the Clarks’ exceptional skill include the 36-inch at Lick Observatory in California, the 26-inch at the U.S. Naval Observatory in Washington, D.C., and many smaller refractors at universities and colleges worldwide. Even today, Clark refractors are considered to be among the finest available. The most advanced modern refractors offer features that the Clarks could not have imagined. Apochromatic refractors effectively eliminate just about all aberrations common to their Galilean, Keplerian, and achromatic cousins.