Aperture Efficiency

The ratio of the effective area of a telescope to the geometric area. That is, there is an effective area that is illuminated on a dish of a certain size that is smaller than the actual size of the dish. Losses in the light can happen from imperfections in the surface of the dish, any support structures that are in the way (e.g., toward the receiver), the non-uniform illumination of light on the dish, etc. For example, at the Green Bank Telescope, this is roughly 75 to 80%.