The most frequently used light detector is still the PHOTOGRAPHIC PLATE. It is easy to get the incorrect impression that the advent of the many modern detectors described below has made the photo¬graphic plate obsolete. The quality and simplicity of use of the modern photographic plate ensures that it will be a long time before it is superseded.
The photographic materials used by astronomers are very similar to the black-and-white film used by amateur photographers. Generally astronomers prefer to use photographic plates, rather than film, because of their better resistance to distortion. The EMULSION consists of a large number of tiny GRAINS of silver bromide embedded in a layer of gelatin. This layer is coated very uniformly onto a glass or film base.
Photons incident on the emulsion are scattered by the grains. A few manage to penetrate individual grains, and there the photon gives up much of its energy to an electron inside the grain. The presence of the excited electron is chemically similar to a silver-bromide molecule being broken into silver and bromine atoms. This PHOTOCHEMICAL DISSOCIATION of silver bromide is amplified enormously by the process of development. Developing an emulsion makes the dissociation of the silver and the bromine permanent, so that we see the dark part of the image on a photographic plate. This is simply the dissociated silver. Since the unexposed emulsion is a milky-white colour it helps to remove undeveloped silver bromide molecules, as well as the dissociated bromine atoms, by FIXING the emulsion in, for example, a solution of thiosulphate. Once the plate is dried, the image is permanent and should last for very many years without fading.
Although all emulsions work in the way described above, modern emulsions include a number of features designed to improve the efficiency of the photographic process and the quality of the final image. It is found that grains absorb blue light more efficiently than red light. To improve the spectral response of the emulsion by en¬hancing the absorption of red light, the grains are coated in chemicals which allow red photons to pass into the grains more easily. This process is known as SENSITIZATION. Various kinds of SENSITIZATIONS are used to make the emulsion especially responsive at a particular wavelength.
The absorption process is fairly inefficient – the quantum efficiency of a photographic emulsion is between 0.1 and 1 per cent. This is because many photons are scattered and not absorbed. Photons often pass through the emulsion; these are reflected by the surface of the glass plate. This reflected light can cause the image to be much larger than it ought to be due to HALATION. To minimize this effect, it is usual to provide a special layer (the ANTIHALATION BACKING), which absorbs those photons that pass through the emulsion.
In practice, at least two photons must be absorbed by a grain for it to be developable. During long exposures on faint objects (astronomical exposures can be as long as a few hours), there is often a considerable period of time between the absorption of the individual photons by one grain. It then becomes more likely that the first excited electron loses its energy before the second photon arrives to release a second electron. This means that the effect of the first absorption is lost, and clearly this reduces the efficiency of the process. Because of this, emulsions are specially treated to reduce the effect, which is known as RECIPROCITY FAILURE. The name comes from the fact that if the exposure time is doubled, less than twice the number of photons are recorded.
Modern photographic emulsions are very uniform and can be made large enough to allow the recording of images over consider¬able areas of sky. Plates as big as 35 cm square are used routinely in Schmidt telescopes. They are also comparatively cheap and simple to use requiring very little ancillary equipment at the telescope. Their main disadvantages are associated with their poor sensitivity in comparison with modern electronic detectors. They also suffer from a non-linear variation of their blackening with exposure time.
One of the biggest problems associated with using photographic plates is that of turning them into usable data. Photographic work gives fine pictures which are easy to search for faint objects. They also give a valuable impression of the structure of extended objects such as nebulae and galaxies. However it is relatively difficult to get accurate quantitative data from photographic plates. We start by determining the CHARACTERISTIC CURVE of the emulsion, that is, the relationship between the blackening of the emulsion (or its DENSITY) and the amount of light falling on the emulsion. The plate is then scanned with a MICRODENSITOMETER. a machine which measures the density of the emulsion at a series of points. The measured densities are converted to the corresponding light-flux level and recorded for processing later. In practice, it can take many times longer than the exposure itself to reduce the data on a photographic plate with a microdensitometer. Some modern detectors such as television cameras, are able to record the data directly but they suffer from having a relatively small sensitive field size. For wide-field work we have no option but to use photographic plates.