An optical system that does not suffer from common optical defects such as coma, astigmatism, or spherical aberration


The diameter of the primary lens or mirror of a telescope; hence, the simplest single measure of the light-gathering power of a telescope


The point at which a body in orbit around the Sun reaches its farthest distance from the Sun


The point at which a body in orbit around Earth reaches its farthest distance from Earth

Arc minute

A unit of angle corresponding to 1/60th of a degree. The full moon is 30 arc minutes in diameter.

Arc second

A unit of angle corresponding to 1/3600th of a degree; 1/60th of an arc minute. An arc second is approximately the size of a penny viewed from about 2.5 miles.


Astronomical instruments that have recently been fabricated using new electronic components called detector arrays that consist of thousands of individual detectors constructed on centimeter-sized wafers of silicon, or other materials


An object orbiting the Sun that is smaller than a major planet (sub-kilometer to about 1,000 km in diameter) but shows no evidence of an atmosphere or other types of activity associated with comets. Most asteroids are located in a belt between Mars and Jupiter ranging from 2.2 to 3.3 AU from the Sun.


Advanced Technology Demonstrator


Astronomical unit: the mean distance between Earth and the Sun, about 150 million km or 93 million miles


Ballistic Missile Defense Organization: the arm of the U.S. Department of Defense charged with developing missile-defense systems


A unit of information used in reference to computers and quantities of data. A byte consists of 8 bits (0s and 1s) and may correspond to a single character or number.


Charge-coupled device: an electronic chip that has in recent decades replaced photographic emulsions as the primary recording medium for astronomical images. The recording portion of the chip is divided into discrete photosensitive elements (pixels) arranged in a rectangular array of rows and columns. The HST’s CCDs have 640,000 pixels arranged in 800 rows and 800 columns. For various applications, some pixels are masked. An astronomical CCD has no masked area. At the end of an exposure, a shutter is closed to keep more light from coming in while the array is being read. The entire array of pixels is shifted, one column at a time, into a so-called column register at the edge, and after each shift the column register is read out slowly, row by row, through amplifiers and other circuitry.

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