Enclosed Energy Ratio (EER)
The enclosed energy ratio shows how much light is concentrated within a certain radius around the image-center. In a perfect telescope 84% of all the light is concentrated in the airy-disc. The image on the left shows the EER of such a system. On the right another perfect system but now with a 30% obstruction, the airy-disc now only contains around 70% of all the light. The y-axis shows the amount of light (0-1) that is concentrated within the radius on the x-axis. The x-axis will be showing angle, with the airy-disc edge at 1.22 for a non-obstructed system.