Astronomy: Chapter 6 Vocabulary
Glossary
Chapter 6
achromatic lens A telescope lens composed of two lenses ground from different kinds of glass and designed to
bring two selected colors to the same focus and correct for chromatic aberration.
active optics
Optical elements whose position or shape is continuously controlled by computers.
adaptive optics
Computer-controlled telescope mirrors that can at least partially compensate for seeing.
alt-azimuth
mounting
A telescope mounting capable of motion parallel to and perpendicular to the horizon.
Angstrom
A unit of distance equal to 10-10 m; often used to measure the wavelength of light.
atmospheric
window
Wavelength regions in which Earth's atmosphere is transparent - at visual, infrared, and radio
wavelengths.
Cassegrain
focus
The optical design of a reflecting telescope in which the secondary mirror reflects light back down
the tube through a hole in the center of the objective mirror.
charge-coupled An electronic device consisting of a large array of light-sensitive elements used to record very faint
device (CCD)
images.
chromatic
aberration
A distortion found in refracting telescopes because lenses focus different colors at slightly different
distances. Images are consequently surrounded by color fringes.
comparison
spectrum
A spectrum of known spectral lines used to identify unknown wavelengths in an object's spectrum.
cosmic ray
A subatomic particle traveling at tremendous velocity that strikes Earth's atmosphere from space.
diffraction
fringe
Blurred fringe surrounding any image caused by the wave properties of light. Because of this, no
image detail smaller than the fringe can be seen.
electromagnetic Changing electric and magnetic fields that travel through space and transfer energy from one
radiation
place to another - for example, light, radio waves, etc.
equatorial
mounting
A telescope mounting that allows motion parallel to and perpendicular to the celestial equator.
eyepiece
A short-focal-length lens used to enlarge the image in a telescope; the lens nearest the eye.
false-color
image
A representation of graphical data in which the colors are altered or added to reveal details.
focal length
The distance from a lens to the point where it focuses parallel rays of light.
frequency
The number of times a given event occurs in a given time; for a wave, the number of cycles that
pass the observer in 1 second.
grating
A piece of material in which numerous microscopic parallel lines are scribed; light encountering a
grating is dispersed to form a spectrum.
infrared
radiation
Electromagnetic radiation with wavelengths intermediate between visible light and radio waves.
interferometry
The observing technique in which separated telescopes combine to produce a virtual telescope
with the resolution of a much larger diameter telescope.
light pollution
The illumination of the night sky by waste light from cities and outdoor lighting, which prevents the
observation of faint objects.
light-gathering
power
The ability of a telescope to collect light; proportional to the area of the telescope objective lens or
mirror.
magnifying
The ability of a telescope to make an image larger.
power
nanometer (nm) A unit of length equal to 10-9 m.
Newtonian
focus
The focal arrangement of a reflecting telescope in which a diagonal mirror reflects light out the side
of the telescope tube for easier access.
objective lens,
mirror
The main optical element in an astronomical telescope. The large lens at the top of the telescope
or large mirror at the bottom.
photon
A quantum of electromagnetic energy; carries an amount of energy that depends inversely on its
wavelength.
polar axis
The axis around which a celestial body rotates.
primary lens,
mirror
The main optical element in an astronomical telescope. The large lens at the top of the telescope
tube or the large mirror at the bottom.
prime focus
The point at which the objective mirror forms an image in a reflecting telescope.
radio
interferometer
Two or more radio telescopes that combine their signals to achieve the resolving power of a larger
telescope.
reflecting
telescope
A telescope that uses a concave mirror to focus light into an image.
refracting
telescope
A telescope that forms images by bending (refracting) light with a lens.
resolving power The ability of a telescope to reveal fine detail; depends on the diameter of the telescope objective.
SchmidtCassegrain
focus
The optical design of a reflecting telescope in which a thin correcting lens is placed at the top of a
Cassegrain telescope.
secondary
mirror
In a reflecting telescope, the mirror that reflects the light to a point of easy observation.
seeing
Atmospheric conditions on a given night. When the atmosphere is unsteady, producing blurred
images, the seeing is said to be poor.
sidereal drive
The motor and gears on a telescope that turn it westward to keep it pointed at a star.
spectrograph
A device that separates light by wavelength to produce a spectrum.
ultraviolet
radiation
Electromagnetic radiation with wavelengths shorter than visible light but longer than X rays.
wavelength
The distance between successive peaks or troughs of a wave; usually represented by