HUBBLE SPACE TELESCOPE

(HST) also called Space Telescope the most sophisticated optical observatory ever placed into orbit around the Earth. The Earth's atmosphere obscures ground-based astronomers' view of celestial objects by absorbing or distorting light rays from them. A telescope stationed in outer space is entirely above the atmosphere, however, and receives images of much greater brightness, clarity, and detail than do ground-based telescopes with comparable optics. Hubble Space Telescope in the cargo bay of the orbiting space shuttle Discovery, April 25, After the U.S. Congress had authorized its construction in 1977, the Hubble Space Telescope was built under the supervision of the National Aeronautics and Space Administration (NASA) of the United States and was named after Edwin Hubble, the foremost American astronomer of the 20th century. The HST was placed into orbit about 600 km (370 miles) above the Earth by the crew of the space shuttle Discovery on April 25, 1990 (see photograph). Cutaway of the NASA Hubble Space Telescope, revealing the Optical Telescope Assembly, the heart of Image of MyCn18, a young planetary nebula located about 8,000 light-years away, taken with the Wide The HST is a large reflecting telescope whose mirror optics gather light from celestial objects and direct it into two cameras and two spectrographs. The HST has a 2.4-metre (94-inch) primary mirror, a smaller secondary mirror, and various recording instruments that can detect visible, ultraviolet, and infrared light. The most important of these instruments, the wide-field planetary camera, can take either wide-field or high-resolution images of the planets and of galactic and extragalactic objects. This camera is designed to achieve image resolutions 10 times greater than that of even the largest Earth-based telescope. A faint-object camera can detect an object 50 times fainter than anything observable by any ground-based telescope; and a faint-object spectrograph gathers data on the object's chemical composition. A high-resolution spectrograph receives distant objects' ultraviolet light that cannot reach the Earth because of atmospheric absorption. Hubble Space Telescope being refurbished, December 1993. Astronauts Story About one month after launch, it became apparent that the HST's large primary mirror had been ground to the wrong shape owing to faulty testing procedures by the mirror's manufacturer. The resulting optical defect, spherical aberration, caused the mirror to produce fuzzy rather than sharp images. The HST also developed problems with its gyroscopes and with its solar-power arrays. On Dec. 213, 1993, a mission of the NASA space shuttle Endeavour sought to correct the telescope's optical system and other problems. In five space walks the shuttle astronauts replaced the HST's wide-field planetary camera and installed a new device containing 10 tiny mirrors to correct the light paths from the primary mirror to the other three scientific instruments. The mission proved an unqualified success, and the HST soon began operating at its full potential, returning spectacular photographs of various cosmic phenomena. Hubble Space Telescope image of NGC 604, a nebula in the neighbouring spiral galaxy M33, located Gravitational lens, as observed by the Hubble Space Telescope. In this picture a galactic cluster, about five billion light-years away, produces a tremendous gravitational field that bends light around it. This lens produces multiple copies of a blue galaxy about twice as distant. Four images are visible in a circle surrounding the lens; a fifth is visible near the centre of the picture. Photo AURA/STScI/NASA/JPL (NASA photo # STScI-PRC96-10) Hubble Space Telescope Hubble Space Telescope in the cargo bay of the orbiting space shuttle Discovery, April 25, 1990. National Aeronautics and Space Administration Hubble Space Telescope Hubble Space Telescope being refurbished, December 1993. Astronauts Story Musgrave and Jeffrey Hoffman are seen during the last of their five EVAs. Australia's west coast can be seen in the background. National Space Science Data Center/National Aeronautics and Space Administration Hubble's constant in cosmology, constant of proportionality in the relation between the velocities of remote galaxies and their distances. It expresses the rate at which the universe is expanding. It is denoted by the symbol H and named in honour of Edwin Hubble, the American astronomer who attempted in 1929 to measure its value. With redshifts of distant galaxies measured by Vesto Slipher, also of the United States, and with his own distance estimates of these galaxies, Hubble established the cosmological velocity-distance law: velocity = H distance. According to this law, known as the Hubble law, the greater the distance of a galaxy, the faster it recedes. Derived from theoretical considerations and confirmed by observations, the velocity-distance law has made secure the concept of an expanding universe. Hubble's original value for H was 150 km (93 miles) per second per 1,000,000 light-years. Modern estimates, using more precise distance measurements, place the value of H at between 15 and 30 km (9.3 and 18.6 miles) per second per 1,000,000 light-years. The reciprocal of Hubble's constant lies between 10 billion and 20 billion years, and this cosmic time scale serves as an approximate measure of the age of the universe.