Linear vs Logarithmic

Most of the astronomical images that you will see are either a linear or logarithmic translation of the intensity of light which reaches a telescope. The scale represented in an image is a relationship between the actual light intensity being measured and the way it is represented visually. In a linear scale, the steps are evenly spaced, and the units (such as length or intensity) represented between each measure is the same. Logarithmic scales are based on multiplication rather than addition. Each step in a logarithmic plot is defined as a multiple of 10.

The Orion nebula shown on a linear scale
(credit: Steven Gibson)

The Orion nebula shown on a logarithmic scale
(credit: Steven Gibson)

Logarithmic scales are used when the actual light intensities cover such a large range that it would be difficult to represent them on a linear scale. Our eyes respond to light in a logarithmic way and see each doubling of light intensity as equal changes. This helps us see differences over a large range of light intensities. For example, the difference in light intensity between areas lit by sunlight and darker shady areas is very high. If our eyes did not respond in a logarithmic way, we would have difficulty seeing details in the shade. Although astronomical instruments usually gather light in a linear way, astronomical images are often displayed on a logarithmic scale. Logarithmic intensity scales are used in astronomy to show the fainter structures in an image, which would be washed out by high intensity features if shown in on a linear scale.