4.12 Key Terms
Active control: a process that measures a sag (like that of a Gemini mirror) many times each second and apply forces at 120 different locations to the back of the mirror to correct the sag. 4.5
Active optical element: flexible mirrors with lots of little actuators to bend them. 4.8
Adaptive optics: systems used with telescopes that can compensate for distortions in an image introduced by the atmosphere, thus resulting in sharper images. 4.8
Ancient observatories: special sites for observing the sky built by many ancient cultures that often had religious and ritual functions as well. 4.2
Aperture: diameter of the primary lens or mirror of a telescope. 4.3
Charge-coupled devices (CCDs): electronic detectors used to record astronomical images which are similar to the detectors used in video camcorders or in digital cameras. 4.8
Dish: a concave metal reflector. 4.9
Eyepiece: magnifying lens used to view the image produced by the objective lens or primary mirror of a telescope. 4.4
Focal length (of a lens): the distance from the lens to the location where the light rays focus, or image, behind the lens. 4.4
Focus (of a lens): the point at which all parallel rays of light converge after refracting due to the curvatures of a lens’ surfaces. 4.4
Glass plates: the prime astronomical detectors used throughout most of the twentieth century, whether for photographing spectra or direct images of celestial objects. 4.8
Hubble Space Telescope (HST): launched in April 1990, this is the largest telescope put into space so far with an aperture of 2.4 metres. 4.10
Infrared Astronomical Satellite (IRAS): the first orbiting infrared observatory, launched in 1983 and built as a joint project by the United States, the Netherlands, and Britain. 4.10
Interference: a technical term for the way that multiple waves interact with each other when they arrive in our instruments, and this interaction allows us to coax more detail out of our observations. 4.9
Interferometer: two or more radio telescopes linked together electronically to sharpen their images. 4.9
Interferometer array: combination of multiple radio dishes to, in effect, work like a large number of two-dish interferometres. 4.9
Light pollution: when the air scatters the glare from lights, producing an illumination that hides the faintest stars and limits the distances that can be probed by telescopes. 4.5
Magnification: the size of the image formed by the lens in a telescope. 4.4
Primary mirror: a concave mirror that serves as the main optical element in reflecting telescopes and that is curved like the inner surface of a sphere, reflecting light in order to form an image. 4.4
Prime focus: location near the front end of the telescope where the mirror reflects the light to form an image. 4.4
Radar is the technique of transmitting radio waves to an object in our solar system and then detecting the radio radiation that the object reflects back. 4.9
Refracting telescope: a telescope based on a refractor design. 4.4
Refractor: a long tube with a large glass lens at one end that is used as the main optical element to form an image. 4.4
Resolution: the precision of detail present in an image: that is, the smallest features that can be distinguished. 4.7
Secondary mirror: used in some reflecting telescopes to allow more light to get through the system. 4.4
Seeing: unsteadiness of Earth’s atmosphere, which blurs telescopic images; good seeing means the atmosphere is steady. 4.5
Transients: phenomena that change quickly, such as exploding stars and chunks of rock that orbit near Earth. 4.11
