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Time measurements are now based on atomic clocks rather than on Earth’s rotation, and telescopes can be driven continuously to compensate for the planet’s rotation, so as to permit tracking of a given astronomical object.

Although the human eye remains an important astronomical tool, detectors capable of greater sensitivity and more rapid response are needed to observe at visible wavelengths and, especially, to extend observations beyond that region of the electromagnetic spectrum. However, photography still provides a useful archival record.

Having apertures much larger than the pupil of the human eye, telescopes permit the study of faint and distant objects.

In addition, sufficient radiant energy can be collected in short time intervals to permit rapid fluctuations in intensity to be detected.

These distort under their own weight and can be supported only around the perimeter; an appreciable amount of light is lost due to absorption in the glass.

Large-aperture refractors are very long and require large and expensive domes.

Further, with more energy collected, a spectrum can be greatly dispersed and examined in much greater detail.reflectors that use lenses or mirrors, respectively, for their main light-collecting elements (objectives).

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