This sounds more complicated than it is in practice, and a diagram should clarify the stages. Consistency, careful control of exposure and processing and a little experience are needed for success. However, the improvement in detail that the technique can reveal is quite dramatic. Copying with an unsharp mask is useful wherever large scale, low information-content detail contributes to the overall density while obscuring finer structures, for example where structures in the dust lane of Centaurus A is hidden in the bright envelope of the giant elliptical galaxy. This is thus the minimum image density on the plate.
Faint images are therefore at some small density greater than the fog produced by the airglow.
Because the sky density represents a small fraction of the total storage capacity of the material, both the fog and the faint images tend to be in the upper part of the developed layer. If a low density contact copy of a developed plate is made on to high contrast film, using a diffuse light source, the grains of the original are recorded on the copy with greatly increased apparent size, as shown on the diagram. High contrast prints made from this thin-looking but contrasty positive reveal objects which are less than one percent as bright as the night sky, equivalent to a surface brightness of about 28 mag per square arcsecond, or 5 to 6 magnitudes fainter than the Siding Spring night sky. These are some of the faintest astronomical images ever detected. Giant shells around elliptical galaxies and Malin-1 were discovered in this way.