Aberrations: Distortions in the wavefront which result in a blurry image.
Aperture: The diameter of the telescope opening.
Arcminute: A separation on the sky, 1/60th of a degree.
Arcsecond: A separation on the sky, 1/60th of an arcminute.
Bandwidth: The frequency at which the system is able to apply corrections to the distorted wavefronts.
Charge-Coupled Device (CCD): A rectangular grid of detectors which are able to sense photons at an efficiency of nearly 100 percent.
Declination: The latitude of a star, measured from the Earth's equator. It is given in degrees, arcminutes, and arcseconds.
Diffraction: An effect on wavefronts passing though an aperture. Wavefronts passing by the edges of the telescope pupil are bent and result in fringe patterns in the resulting image.
Diffraction limit : The maximum resolution a telescope can achieve.
Diffraction pattern : The interference pattern produced by diffraction. It consists of a central bright source with light and dark circular fringes around it.
Equatorial coordinate system: A coordinate system used to map celestial objects on the sky, very much like latitude and longitude on the Earth.
Fried's parameter (ro ): The size over which a distorted wavefront is still is roughly flat.
Full-width at half maximum (FWHM): The width of a star's image at half its peak.
Guide star: A bright star near the target object which the wavefront sensor uses to measure the distortions in the wavefronts.
Photons: "Packets" that make up light waves.
Pixels: The smallest elements of a CCD detector grid.
Pupil: The net area of wavefront that is collected by the telescope.
Right Ascension: This is like the Earth's longitude. It is measured on a circle starting from a fixed point on the sky; in hours, minutes, and seconds.
Tip-tilt mirror: A simple AO device which adjusts the incoming distorted wavefront so that it is roughly flat. The mirror is moved to counteract the apparent motion of a star.
Wavefronts: Effectively flat waves coming from a distant point source, such as a star.