Published in SkyNews magazine
By Alan Dyer and Terence Dickinson
Courtesy of SkyNews Magazine
NEW SKY-WATCHER REFRACTORS AND EQUATORIAL MOUNTS
We test two ED refractors and two mounts - all with performance and prices that seem too good to be true.
The first astronomical telescope, the one Galileo built in 1609 and used to observe Jupiter's moons, the crescent of Venus and the rumpled surface of the Moon, was a refractor. Galileo's best telescope had a single one-inch main lens and another lens element for an eyepiece. By the 18th century, refractor makes were using two or more lens components, known collectively as the objective lens, at the front of the tube to improve telescope performance. This design, called an achromatic refractor, is still used in commercial refractors today.
Although achromatic refractors work well, they have what is known as residual false colour-portions of he blue and red components of light are not properly focused by the objective. This produces a purple fringe around the edge of the moon and a ring around bright stars and planets and tints such targets with a pale yellow-green cast. When the telescope is used for photography, this becomes a significant handicap, especially with digital cameras.
However, during the 1970s, the residual false-colour problem was solved with the development of fluorite objective components, which bring all the colours to essentially perfect focus in a telescope shorter than the traditional achromatic refractor. This type of scope is the apochromatic refractor, also know as a fluorite or an ED refractor, which signifies the type of glass utilized. |
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置顶卡
变色卡
千斤顶
显身卡
