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# Long description

Two graphs are shown, indicating how the amount of light falling on the detector changes with the distance from the centre of the light source. In each graph, the horizontal axis shows the distance in arcseconds, from 0 to 12.5. The vertical axis shows the brightness, in magnitudes per square arcsecond, from 17 at the top to 28 at the bottom.

The left-hand graph shows the distribution for a star like that in the left-hand panel of Figure 3. Although the star is a point source, the graph doesn’t drop to faint magnitudes immediately, but does so after about 2 arcseconds, because of the blurring effects of the telescope and atmosphere, as mentioned in the text.

The right-hand graph shows a similar figure for an extended source, similar to the galaxy in Figure 3. Its brightness also decreases with distance, but much more slowly, so that it is still brighter than the background of the image by the right-hand edge of the graph, at 12.5 arcseconds from the centre.