Gathering Light

How much you can see with any telescope is governed by its aperture. Larger apertures collect more light, thereby showing fainter objects and finer detail. however, large lenses are expensive to make, are heavy, and require longer tubes. As a result, for telescopes with apertures greater than about 4 inches (100 mm), amateur astronomers usually turn to reflectors, which use mirrors, and catadioptrics, a hybrid design. All large professional telescopes are reflectors.

saturn through A

3-in (76-mm) aperture saturn through A

12-in (304-mm) aperture zooming in

Computer-controlled telescopes are now highly popular. This is an 8-in (200-mm) Schmidt-Cassegrain telescope on a GOTO mount, which automatically finds and tracks objects (see p.143).

eyepieces and magnification

Telescopes have interchangeable eyepieces, and the magnification depends on the focal length of the eyepiece used. Shorter focal lengths produce greater magnification. Each eyepiece is marked with its focal length in millimeters, and this must be divided into the telescope's focal length to find the resulting magnifying power. For example, on a telescope with a focal length of 1,200 mm, a 25-mm focal-length eyepiece will produce a power of 48 times. On the same telescope, a 10-mm eyepiece will give a magnification of 120 times. In practice, the highest magnification usable on a given telescope before the image becomes faint and indistinct is twice the aperture in millimeters.

normal wiDE-ANGLE

eyepiece eyepiece normal wiDE-ANGLE

eyepiece eyepiece types of eyepiece

Medium-power eyepieces are best for planetary and lunar studies, while the highest powers should be reserved for close double stars and planetary detail. Wide-angle eyepieces are useful for objects such as star clusters, nebulae, and galaxies since they give a wider field of view than a normal eyepiece.

observation cm

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