aperture

An aperture is an opening, the term being used to describe the light-collecting opening of a telescope, and also used analogously for radio telescopes and telescopes for other parts of the electromagnetic spectrum. An aperture's size (width) is generally listed as a diameter, e.g., in meters, which presumes it is circular, which is generally but not always the case; a distance-measurement termed a telescope's aperture is this diameter. This width affects the telescope's angular resolution, due to diffraction, while the aperture's area corresponds to the telescope's light-collecting ability, i.e., how dim an object it can discern and how long it takes doing so. If a telescope's aperture is not circular, an equivalent aperture may be cited, generally that of a circle with the same area, but for an interferometer-array of telescopes, its longest baseline may be cited as an equivalent aperture, to suggest the width of a single telescope's aperture that would achieve the same angular resolution as the interferometer.

The current technique of apodization consists of deliberately reducing and/or reshaping the aperture (changing a width along some direction) in order to change the resulting diffraction patterns and tease out detail that the particular diffraction by a circular aperture hides.