Hubble Space Telescope (HST)

An orbiting observatory built and operated jointly by NASA and ESA. After a delay of six years beyond the originally scheduled launch date, it was successfully placed in orbit on 25 April 1990 from the Space Shuttle. However, after extensive testing in the first few weeks, it became apparent that a fault in the optical figuring of the main mirror was introducing spherical aberration and that sharp focusing would be impossible until corrective measures were carried out by a maintenance mission to the telescope. This was a grave disappointment, eliminating the possibility of carrying out the most eagerly awaited observations in the first phase of operation. During the first servicing mission in December 1993, a Space Shuttle crew successfully installed a unit known as COSTAR, which corrected the faulty optics, and they replaced the solar arrays. The concept of the HST was to escape from the limitations imposed by the atmosphere on the quality of image that might otherwise be obtained. It was designed with a 15-year lifetime as an observatory that could be maintained and upgraded in orbit. It is a Ritchey-Chrétien telescope with a main mirror 2.4 metres (94.5 inches) in diameter made of ultra-low-expansion glass. The compact, folded optical system is contained in a tube 13 metres (43 feet) long. The wavelength range of operation is from 110 to 1,100 nanometres. The major elements of the telescope are the optical telescope assembly, the support system module, the fine guidance system (which can be used for astrometric measurements) and a suite of scientific instruments. The pointing accuracy is 0.007 arc seconds. Power is provided from two solar arrays and communication is via the Tracking and Data Relay Satellite System. The five original science instruments, operating in the optical and ultraviolet part of the spectrum, were a faint object camera (FOC), a wide-field and planetary camera (WF/PC), the Goddard high-resolution spectrograph (GHRS), a faint object spectrograph (FOS) and a high speed photometer. However, during the 1993 servicing mission, the high-speed photometer was removed to accommodate the COSTAR corrective unit. The WF/PC was also replaced by an improved version, WFPC-2. A second servicing mission took place in February 1997. Various items of worn-out and outdated hardware, including a fine guidance sensor, were replaced, but the principal change was the substitution of two new instruments in place of the GHRS and FOS. These were the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) and the Space Telescope Imaging Spectrograph (STIS). NICMOS was the first HST instrument designed for infrared observation, giving the telescope an important new capability, and the first needing to be cooled. Its sensitive infrared detectors are maintained at 58 K inside an insulated vessel containing frozen nitrogen, which had been expected to have a lifetime of up to five years. However, a technical problem that came to light after installation is likely to reduce significantly the planned lifetime. The instrument contains three cameras, each with different resolution. It also operates as a spectrograph, a coronagraph and a polarimeter. STIS is a powerful imaging spectrograph covering the ultraviolet, visible and near infrared parts of the spectrum. It is capable of recording simultaneously up to 512 spectra at different locations over an extended object, such as a galaxy. Its performance includes all the major capabilities of both the GHRS and FOS. The third servicing mission is scheduled for December 1999, and operation of the telescope is expected to continue to 2010, beyond the 15 years initially envisaged. The HST has been used to make observations of virtually every kind of celestial object, from planets in the solar system to the most remote galaxies detectable. The science operations are conducted from the Space Telescope Science Institute (STScI) in Baltimore, Maryland.