Resolving Power

Even if a telescope were of perfect optical quality, it would not produce perfectly focused images because of the nature of light itself. A telescope's resolving power is its ability to produce sharp, detailed images under ideal observing conditions.

Resolving power depends directly on the size of the aperture and inversely on the wavelength of the incoming light. For the same light, a 150mm (6-inch) telescope has twice the resolving power of a 75-mm (3-inch) telescope.

Starlight travels in straight lines through empty space, but when waves of starlight pass close to the edge of a lens or mirror, they spread out, in an effect called diffraction, and come to a focus at different spots. Because of diffraction, the image of a star formed by a lens or mirror appears under magnification as

Figure 2.10. Diffraction pattern (image of a star).

a tiny, blurred disk surrounded by faint rings, called a diffraction pattern, instead of as a single point of light (Figure 2.10). Diffraction limits resolving power.

If two stars are close together, their diffraction patterns may overlap so that they look like a single star. Features such as Moon craters and planet markings are also blurred by diffraction.

Resolving power determines the smallest angle between two stars for which separate, recognizable images are produced. The smallest resolvable angle for the human eye is about one minute of arc (1' ), which is the size of an aspirin tablet seen at a distance of 35 m (110 feet).

Explain why what may look like a single star to the eye may resolve into two close neighbor stars in a telescope._

Answer: Resolving power is proportional to aperture, and a telescope's aperture is much larger than the human eye's.

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