In 1756, John Dollond introduced the first achromatic lenses consisting of a combination of two lenses made from crown and flint glass. By the early 1800s, Joseph Fraunhofer succeeded in applying Dollond's technique to lenses of substantial size. This ushered in the era of giant refracting telescopes which reached its height at the end of the 19th century with the instruments of Alvan Clark, including those at the Lick Observatory and Yerkes Observatory, both of which date back to roughly 1890.
So long as lenses are made of glass, the 40-inch (1-meter) at Yerkes represents the practical limit in size of refractors. Although glass blanks larger than 40 inches in diameter have been cast during the past 50 years, they are seldom, if ever, sufficiently free of internal defects to make satisfactory lenses. Even if an acceptable blank were obtained, the resulting lens, supported only by its edge, would distort so badly from its own weight as to be optically useless.
Before large reflecting telescopes took over in the 20th century, refractors were generally preferred for detailed observations of the planets since, having an enclosed tube, they produced a steadier image.
Related categories TELESCOPE EQUIPMENT AND TECHNIQUES
OPTICS AND OPTICAL PHENOMENA
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