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Also, major photographic surveys, such as those of the National Geographic Society and the Palomar Observatory, can provide a historical base for long-term studies.

Photographic film converted only a few percent of the incident photons into images, whereas CCDs have efficiencies of nearly 100 percent.

A photograph of a particular celestial object may include the images of many other objects that were not of interest when the picture was taken but that become the focus of study years later.

When quasars were discovered in 1963, for example, photographic plates exposed before 1900 and held in the Harvard College Observatory were examined to trace possible changes in position or intensity of the radio object newly identified as quasar 3C 273.

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.

Very Large Array (VLA), operated near Socorro, New Mexico, by the National Radio Astronomy Observatory, 27 movable radio dishes are set out along tracks that extend for nearly 21 km.

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.

This restricts astronomy on Earth’s surface to the near ultraviolet, visible, and radio regions of the electromagnetic spectrum and to some relatively narrow “windows” in the nearer infrared.

Longer infrared wavelengths are strongly absorbed by atmospheric water vapour and carbon dioxide.

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