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Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report F Glossary adaptive optics a process in which distortions (like those from Earth’s atmosphere) are removed from a telescope’s image in real time. First, a wavefront sensor uses a reference star to measure the distortions that are occurring, and the distortions are then removed with a phase corrector. angular resolution the ability of an instrument, such as a telescope, to distinguish objects that are very close to each other. The angular resolution of an instrument is the smallest angular separation at which the instrument can observe two neighboring objects as two separate objects. avionics the onboard electronics used for operating a spacecraft, including communications, navigation, and electronic flight management systems black hole a region of space containing a huge amount of mass compacted into an extremely small volume. A black hole’s gravitational influence is so strong that nothing, not even light, can escape from it. Cepheid variable star a type of pulsating star whose light and energy output vary noticeably over a set period of time. The time period over which the star varies is directly related to its light output or luminosity, making these stars useful standard candles for measuring intergalactic distances. dark energy the residual energy in empty space which is causing the expansion of the universe to accelerate de-scope the reduction or elimination of some objectives, performance requirements, or capabilities compared to those in an earlier baseline plan
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Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report end effector a device or tool connected to the end of a robot arm fine-guidance sensor the targeting devices aboard HST that lock onto “guide stars” and measure their positions relative to the object being viewed. Adjustments based on these precise readings keep Hubble pointed in the right direction. flux the amount of something (such as radiation) passing through a surface per unit time gyroscope a spinning wheel mounted on a non-stationary frame that stabilizes and points a space-based observatory. This spinning wheel resists applied external forces and tends to retain its original orientation in space. Hubble constant a number that expresses the rate at which the universe expands with time. H0 appears to be between 60 and 75 kilometers per second per million parsecs. (One parsec is equal to 3.26 light-years and 3.085678 × 1013 kilometers, or approximately 18 trillion miles.) Milky Way Galaxy The Milky Way, a spiral galaxy, is the home of Earth, the Sun, and the rest of our solar system. orbital debris any man-made object, or portions thereof, in orbit about Earth which no longer serves a useful purpose planetary nebula an expanding shell of glowing gas expelled by a star late in its life proto-solar system matter that is beginning to come together to form a star and its collection of orbiting planets quasar the brightest type of active galactic nucleus, believed to be powered by a supermassive black hole. The word “quasar” is derived from quasi-stellar radio source, because this type of object was first identified as a kind of radio source. ranging device an instrument or instrument system for measuring the distance, for example, between two spacecraft as they approach one another reaction wheel one of four spinning wheels that work by rotating a large flywheel up to 3000 rpm or braking it to exchange momentum with the spacecraft which will make HST turn. The flywheels work together to make the observatory rotate either more rapidly or less rapidly toward a new target. red shift an apparent shift toward longer wavelengths of spectral lines in the radiation emitted by an object caused by motion of the emitting object away from the observer
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Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report spherical aberration an example of optical aberration, a deviation from perfect image formation in which light from different parts of a mirror or lens is brought to different foci supernova the explosive death of a massive star whose energy output causes its expanding gases to glow brightly for weeks or months teleoperation the control of robots from a distance Weibull distribution a general-purpose reliability distribution used to model material strength and times-to-failure of electronic and mechanical components, equipment, or systems
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