Unsharp masking
 

Much fine detail is often lost in an astronomical photograph of an extended field because it varies little with respect to the background brightness. Unsharp masking is an image-processing technique that enhances the fine detail. The original picture is processed to obtain a negative mask, such a mask being of low density and slightly out of focus. This mask is then superimposed on the original, resulting in a decrease in the brightness range of the original but without decreasing the fine detail. The tiny brightness variations over the original can then be seen more clearly. The Australian astronomer, David Malin, has produced many beautiful pictures of celestial objects such as the Orion Nebula using unsharp masking, revealing by this method fine detail that otherwise would effectively be lost. A selection of such images is available from the Anglo-Australian Observatory (AAO)^{W 18.1}. This processing technique can also be applied to images recorded in electronic form with the operation undertaken by software rather than by using a physical mask.