The sensitivity of a telescope indicates the minimum detectable brightness of an object. In other words, sensitivity puts a lower limit on the signal strength that a telescope can clearly pick up.
High sensitivity permits detection of dim objects.
With low sensitivity, only bright objects can be discerned. Many celestial objects shine quite brightly in the radio spectrum. However, the amounts of radio energy detected from them are normally very small. No wonder radiotelescopes have to be extremely sensitive to incoming radio waves. The amount of electromagnetic radiation a telescope collects is proportional to the surface area of its lens, mirror, or dish, often referred to as the collecting area .