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d where sep" stands for the minimum separation in seconds of arc at which two stars of equal magnitude can be observed as two distinct objects separate from each other, in a telescope of a given aperture. The Dawes factor is 4.56, and d represents the diameter of the telescope lens or mirror in inches. So, the larger the telescope, the greater its ability to resolve two stars that are close together; the bigger the aperture, the smaller the separation that can be observed. Once, one of my professional astronomer friends informed me he had requested time on one of the Keck 10-m telescopes for a Jupiter observing run. The reason he wanted it was not its light gathering ability but its resolving power. Of course, resolution can be adversely affected by poor seeing and by optics of poor quality. Seldom will any telescope achieve the theoretical limit of resolution (Table 7.1). Some nights the seeing can be so poor that no telescope, regardless the aperture, would produce acceptable results. But on those nights when seeing is great, a larger telescope will gather more light, allow more magnification, and show more fine detail.

The contrast of features on Jupiter is very subtle; therefore, the desirable telescope for observing Jupiter is one that forms a sharp image with high image contrast. Most astronomers would argue that of all types, the refracting telescope does this best. Refractors make an image by focusing light through a clear lens. The lens is mounted in a lens cell at the front of a tube and light is brought to a focus at the lower end of the tube where an eyepiece or other instrument is attached. A refracting telescope that is well made is a dependable instrument (Fig. 7.1) requiring relatively little maintenance compared to reflecting telescopes. Refractors provide high image contrast since there is no obstruction in the light path as with traditional reflecting telescopes. Observers using high quality refractors have made some of the best planetary drawings I have seen.

However, refractors of large aperture are expensive to make. The objective lenses of early refractors were plagued with a certain amount of chromatic aberration. Because of this chromatic aberration, red and blue fringing was often seen around the edges of bright objects. This caused the color rendition of planets and bright stars to be somewhat unreliable in refractors. In years past, refracting lenses had to be constructed with long focal lengths to eliminate as much chromatic aberration as possible. This resulted in refracting telescopes with focal ratios of f/15 or higher. Really large refractors had optical tubes of great length and weight, requiring large mounts to support them and large structures to house them. Consequently, the 40-in. Yerkes refractor was the largest and the last of the large refractors to be made. Amateurs generally do not have such large instruments; but a 6-in. f/15 refractor still requires a lot of space to use and is not so easily transported.

Recently, especially in the last few years, manufacturers have overcome most of the problems inherent in refracting lenses. Modern high quality refractors use lenses made of special glass that are carefully ground by computerized machines to shorter focal lengths. Modern lenses are able to accurately focus all the different

Table 7.1. Resolution of telescopes

according to Dawes' limit

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