Originally, the application of photography in astronomy, although electronic detectors (CCDs) are now used almost exclusively instead of photographic plates or film; hence astronomers often refer to ‘imaging’ rather than photography. In astronomical imaging, a photographic plate, film, CCD, or other detector is placed at the focus of a telescope instead of an eyepiece. Exposures can last from fractions of a second to many minutes or even hours. During all but the shortest exposures the telescope must be guided accurately on the object being photographed (see guider).

Early photographic plates were insensitive (or ‘slow’) and it was not until the 1880s with the introduction of the dry gelatin plate that astronomical photography began to take off. An early application was in spectroscopy, notably under the guidance of E. C. Pickering at Harvard who realized that spectral lines could be measured accurately and conveniently on photographic plates. In 1887 an ambitious international sky survey, the Carte du Ciel, was initiated with the intention of photographing the entire sky down to 14th magnitude, although it was never completed.

By the early twentieth century, as photographic emulsions became more sensitive and finer-grained, photography supplanted visual observation for almost all professional applications. However, exposures of many hours were still needed to capture images or spectra of faint objects. To cut down exposure times, techniques were developed to make plates plates more light-sensitive, such as hypersensitizing them by exposure to hydrogen or nitrogen gas, or cooling them to sub-zero temperatures.

Once electronic CCDs were introduced in the 1980s, photographic emulsions were almost entirely supplanted. CCDs had the multiple advantages of being more sensitive to light, could record a much wider range of wavelengths, and did not suffer from so-called reciprocity failure (decreasing efficiency with longer exposure times). In addition, after exposure the images could be computer-processed and combined, or ‘stacked’, to improve the final result.

Large modern CCDs for astronomical use contain hundreds of millions or even billions of pixels (picture elements). The detector for the Large Synoptic Survey Telescope contains 3.2 billion pixels and measures 0.64 m on a side, about twice the width of the largest traditional photographic plates.