Glossary

Aberrations:  Distortions in the wavefront which result in a blurry image.

Aperture:  The diameter of the telescope opening.

Arcminute: A separation on the sky which is 1/60th of a degree.

Arcsecond: A separation on the sky which is 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.

Closed-loop system: A adaptive optics system in which the wavefront corrector sends light into the wavefront sensor, rather than the other way around. This means that each correction can be a small, incremental change from the last step.

Declination:  The latitude of a star, much like the latitude measured from 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 cause 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 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.

Magnitudes:  A logarithmic scale which measures the apparent visual brightness of stars. Each integer increase in magnitude corresponds to drop in brightness by a factor of 2.5.

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.

Track time: The time it takes the wavefront-sensor camera to expose an image of the guide star.

Wavefronts:  Flat waves coming  from a distant point source, such as a star.