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, in the latter case, changing its shape such that distances across the opening are shorter in some directions. Despite the reduction in the aperture's area an its diameter across some directions, the resulting diffraction pattern can in some cases reveal detail (in specific portions of the focal plane) that would be hidden by the normal aperture. The procedure is to adjust the aperture, then aim the telescope such that additional detail of some targeted source is captured.