Astronomical photography

(optics) The use of the photographic process to record surface features of celestial objects, their positions and motions (for measurement), and their radiation (photometry) and spectra (spectroscopy). Also known as astrophotography.

The application of the photographic process to astronomy, including monochrome photography and color photography.

Monochrome photography

Monochrome photography was one of the premiere tools of astronomical research during most of the twentieth century. It offered two major advantages: the integration of signal (through time exposure) allowed the accumulation of photons from very faint objects which could not otherwise be seen; and the storage of information in an efficient and permanent form allowed protracted, in-depth study of astronomical objects away from the telescope. Monochrome astronomical photography became a standard technique that was applied to direct imaging, spectroscopy, photometry, polarimetry, and astrometry. See also Astrometry; Photometry; Polarimetry.

Nonetheless, because astronomers were continually searching for increased sensitivity and wavelength coverage, the aggressive attempts to improve photographic emulsions had, by the 1970s, begun to reach the limits of what was reasonably attainable. At the same time a new type of electronic imager, the charge-coupled device (CCD), developed for military applications, began to become available to the scientific community. Space-based imaging applications of the charge-coupled device, particularly the development of the TI 800 × 800-px chip for use in the Galileo spacecraft, stimulated expanded use of this mode of imaging at ground-based observatories. By the end of the twentieth century, charge-coupled devices had become so pervasive in astronomical observatories that it was clear that monochrome astronomical photography was, in many respects, a technique of the past. Rarely used now by professional astronomers, the techniques of high-quality monochrome astrophotography are primarily preserved by amateur astronomers. Some advances are still being achieved, primarily through computer-processing of digitized photographic images.

Color photography

The language of astronomy abounds with references to color, and the concept of color is implicit in many of the measurements that astronomers make. Color index, for example, is a quantity related to the temperature of a star, while the redshift of a galaxy is used to indicate its recessional velocity. More directly, stars may be described as red giants or white dwarfs or even blue stragglers. See also Blue straggler star; Color index; Redshift; Star.

These names reflect the underlying importance of color in astrophysics and cosmology, and though the colors involved are subtle and difficult to distinguish by the eye in its dark-adapted state, special photographic techniques can be used to display them. A realistic representation of the true colors of celestial bodies can reveal new relationships in familiar objects and add an important third dimension to the morphology and brightness information of the more usual monochrome representations.

Special photographic materials are necessary to accommodate the unusual requirements of photography in astronomy. Not only is the amount of incoming radiation to be detected extremely small, but it is accompanied by unwanted light from the night sky (the airglow). The materials must therefore combine extreme sensitivity at long exposures with high contrast. The ability to detect faint objects is ultimately more dependent on the contrast and resolution of the photographic material than on the light grasp of the telescope or available observing time. On the other hand, color films are designed for general use at levels of illumination where high contrast and low-light-level efficiency are unimportant. In addition, these films are intended primarily to reproduce the broadband colors of everyday life, and for this the rather uneven spectral response of their individual layers is unimportant. Unfortunately, gaseous nebulae emit most of their visible radiation in the form of monochromatic emission lines from the ionized elements present. Color films always show gaseous nebulae as red, largely irrespective of the contribution from the green oxygen line, whereas yellow (red + green) would be a more realistic representation in many cases. See also Nebula.

A further problem is the effect of long exposures on the relative sensitivity of the three layers, which are differently affected by low-intensity reciprocity failure. Changes in both sensitivity and contrast of the layers are found, and exposures which are long enough to be astronomically useful often produce severe color-balance distortion.

Low-intensity reciprocity failure of both color and monochrome films is reduced if the long exposure is made at a low temperature. Most experiments have been made with cameras designed for fairly small formats and cooled to about −103°F (−75°C) with solid carbon dioxide.

Some of the techniques which are used for spectroscopic plates may also be applied to color films. Baking both in nitrogen and in forming gas, a 2–4% hydrogen-in-nitrogen mixture, is useful. Films are baked for several hours in a flow of the gas at 150°F (65°C) just prior to exposure and then (preferably) exposed in a nitrogen atmosphere.

These user-applied processes reduce some of the disadvantages of color films for astrophotography, and push development may be used in addition to the above, to increase both speed and contrast. However, the basic problem of uneven spectral response remains. As a result, color films can reproduce only realistic colors of the brighter, continuous-spectrum objects, such as planets, stars, and galaxies. Faint objects and emission nebulae are not well recorded.

An alternative approach is to use the oldest system of color photography, the three-color separation technique. In this system, three exposures are made with combinations of photographic emulsions and filters chosen to record the red, green, and blue parts of the spectrum on separate plates or film. Filter-emulsion passbands are chosen to ensure adequate overlap between adjacent colors so that hues intermediate between red, green, and blue are well recorded. With care in selection of these parameters, coverage of the visible region is much more uniform than is possible with conventional color film.

The same principles are used with digital detectors such as charge-coupled devices, which are much more sensitive than hypersensitized silver-based photographic materials. As yet, they lack the essentially unlimited area and small pixels that give conventional materials their “photographic” characteristics that translate into a distinctive esthetic quality. Charge-coupled-device images are usually combined into three-color images with a computer, and similar methods can be used to combine digitized versions of photographic red, green, and blue exposures.

Two methods are possible to recover the color information in three-color separations. The subtractive process involves imagewise combinations of yellow, magenta, and cyan dyes or pigments and is now rarely practiced outside professional printing applications. Much more flexible in the astronomical application is the additive process, which allows several levels of image manipulation before the monochromes are combined. Additive color photography involves mixing colored light, rather than colored compounds, and its most common manifestation is the color television or computer screen image, which a magnifier shows to be made up of blue, green, and red dots or strips. When used photographically, monochrome positive copies are made by contact copying the three original separation negatives onto a suitable film material. A wide range of image-manipulation techniques can be applied to enhance small or faint features and to adjust the contrast of the original images. See also Electronic display.