PHOTOGRAPHIC EMULSION
The photographic emulsion records photons by undergoing a chemical change (a photochemical effect) that will ultimately deposit silver on a glass plate or acetate film. The photographic plate can be made to respond to different wavelength regions within and beyond either end of the visible spectrum, which makes it much more versatile than the eye. Also, its response over a wavelength interval can be made much more uniform than that of the eye. The photographic plate, like the eye, is nonlinear in its response; it has a rather complicated response depending upon the position in its dynamic range.
The photographic plate has a strong advantage over the eye since it will build up a response by storing the image. Thus time exposures allow the astronomer to collect information on a photographic plate about very faint light sources that cannot be detected by the eye through the same telescope. How faint a star can we photograph? The telescope's aperture sets the initial limit. Ultimately, however, the limit is set by the weak illumination in the night sky. This background interference comes from starlight scattered by the earth's atmosphere and from diffuse radiation in the atmosphere (airglow). Unfortunately, the photographic plate's photon-capturing efficiency is low. The emulsion can record only 1 or 2 percent of the incident photons (those that activate the light-sensitive coating). Facing this inefficiency, astronomers have found other types of radiation detectors to improve the telescope's performance.
