Largest Telescope
Written by Tammy Plotner
Would you like to know what the largest optical telescope on Earth is? That's easy! Right now the The Gran Telescopio CANARIAS (GTC), is the biggest, with a 10.4 meter segmented primary mirror. It is located in one of the top astronomical sites in the Northern Hemisphere – the Observatorio del Roque de los Muchachos (ORM, La Palma, Canary Islands). The "First Light" event took place on July 13, 2007.
The next largest is the two 10 meter Keck and Keck II telescopes located on the summit of Hawaii's dormant Mauna Kea volcano. Each stands eight stories tall and weighs 300 tons, yet operates with nanometer precision. At the heart of each Keck Telescope is a revolutionary primary mirror. Ten meters in diameter, the mirror is composed of 36 hexagonal segments that work in concert as a single piece of reflective glass.
Then comes the 10 meter SALT scope. The South African Astronomical Observatory (SAAO) is the national centre for optical and infrared astronomy in South Africa. The Southern African Large Telescope (SALT) is the largest single optical telescope in the southern hemisphere, with a hexagonal mirror array 11 metres across. Although very similar to the Hobby-Eberly Telescope (HET) in Texas, SALT has a redesigned optical system using more of the mirror array. It will be able to record distant stars, galaxies and quasars a billion times too faint to be seen with the unaided eye – as faint as a candle flame at the distance of the moon.
From there we move down to the 9.2 meter Hobby-Eberly telescope in Mt. Fowlkes, Texas. The HET was designed and constructed with a unique objective: to gather a very large amount of light, specifically for spectroscopy, at extremely low cost. A fixed elevation-axis design, based on the radio telescope at Arecibo, and an innovative system for tracking stars, contributed to an 80% reduction in initial costs compared to optical telescopes of similar size. The primary mirror of the HET is the largest yet constructed, at 11.1 x 9.8 meters. At any given time during observations, only a portion of the mirror is utilized. The HET's 9.2 meter effective aperture makes it currently the world's fourth largest optical telescope. The HET entered its commissioning phase in 1997, and began science operations in October of 1999.
How about the Large Binocular Telescope located in Mt. Graham, Arizona? These twins measure 8.4 meters each! The Large Binocular Telescope uses an elevation over an azimuth mounting. The elevation optical support structure moves on two large C-shaped rings and the compact azimuth platform transmits the loads directly down to the pier. The two 8.4 meter (331 inch) diameter primary mirrors are mounted with a 14.4 meter center-center separation. By using swing arms to rotate the secondary mirrors and their supports, it is possible to switch the telescope from one mode of observation to another very quickly. The short focal length of the primary mirrors (F/1.142) permits a compact, and therefore quite stiff telescope structure.
Measuring in at a modest 8.3 meters in diameter is Suburu. The Subaru telescope is an optical-infrared telescope at the 4,200m (13,460ft.) summit of Mauna Kea on the island of Hawaii. The telescope represents a new generation in telescope design not only because of the size of its primary mirror with an effective aperture of 8.2 meters, but also because of the various revolutionary technologies used to achieve outstanding performance. An active support system that maintains an unprecedentedly high mirror surface accuracy, a new enclosure design to suppress local atmospheric turbulence, an extremely accurate tracking mechanism using magnetic driving systems, seven observational instruments installed at the four foci, and an auto-exchanger system to use the observational instruments effectively are just some of the unique features associated with this telescope. These sophisticated systems have been used and fine-tuned since the telescope's First Light.
There are four 8.2 meter diameter telescopes located in Cerro Paranal, Chile. The VLT offers also the possibility of combining coherently the light from the four UTs to work as an interferometer, the Very Large Telescope Interferometer (VLTI), with its own suite of instruments, ultimately providing imagery at the milli arcsecond level as well as astrometry at 10 micro arcsecond precision. In addition to the 8.2m telescopes the VLTI will soon be complemented with four Auxiliary Telescopes (AT) of 1.8m diameter to improve its imaging capabilities and enable full nighttime use on a year-round basis.
How about the 8.1 meter diameter Gemini telescopes? One is in Mauna Kea, Hawaii (North) and the other is in Cerro Pachon, Chile. The two Gemini Multi-Object Spectrographs (GMOS), one on each telescope, provide 0.36-1.10 µm long-slit and multi-slit spectroscopy and imaging over a 5.5 arcmin field of view. Each GMOS is also equipped with an Integral Field Unit (IFU) making it possible to obtain spectra of an area of about 35 square arcsec with a sampling of 0.2 arcsec. The Nod-and-Shuffle mode, enabling superior sky subtraction, is available with both GMOSs in most spectroscopic modes. The GMOSs were built by a collaboration of the Astronomy Technology Centre at the ROE and the University of Durham in the UK and the Herzberg Institute of Astrophysics in Canada. GMOS-N was delivered in July 2001 with GMOS-S following in December 2002.
Now you know about more than a dozen of Earth's largest telescopes!
Filed under: Astronomy
