Flamsteed Astronomy Society

Today’s big reflecting telescopes

— April 3, 2006

Today’s biggest reflecting telescopes, 8 meters + (Apr 2006)

VLT ‘Very Large Telescope’ 4 x 8.2m European Southern Observatory, Paranal, Chile, can be used as 4 independent telescopes or connected together as an ‘interferometer’ with the equivalent performance of a much larger single instrument.  Use monolithic ‘zerodur’ mirrors which are kept to the correct figure using active optics.

Since 1945 the size of reflecting mirrors has more than doubled :  Keck I of 1993 put telescopes into the 10-meter class.   These new leviathans are made possible by three innovations —

·       Segmented mirrors, constructed from hexagonal mirror segments, each independently positioned by computer-monitoring which controls —

·       Active optics which monitor the mirror’s figure and adjust the segments, and —

·       Adaptive optics which monitor the fluctuations in ’seeing’ caused by movement and currents in the Earth’s atmosphere, and rapidly adjust a flexible secondary mirror to compensate.  Adaptive optics help produce images comparable to those from the Hubble Space Telescope (which uses a 2.5 m primary mirror).

Today there are 13 telescopes worldwide, in the 8+ meter class.  Several are concentrated in the major observatories at Mauna Kea (Hawaii), Cerro Paranal (Chile), and Roque de los Muchachos (La Palma, Canary Is.)

Use the web links from the table, right, to see more information about each telescope.

Some Examples —

Keck Observatory 2 x 9.8m Mauna Kea, Hawaii.  Two fully-steerable telescopes also with interferometer capability.   The primary mirrors are constructed from 36 hexagonal segments which are kept adjusted through active optics.  The Keck Observatory has done much leading development of adaptive optics, which in the Kecks, make adjustments 670 times a second.  Mauna Kea is also the location of the Subaru and Gemini North telescopes, both in the 8+ meter class.

GTC Gran Telescopio Canarias 10.4m Roque de los Muchachos Observatory, La Palma, Canary Is. (ORM).  GTC is similar to the Kecks, a fully-steerable telescope with primary mirror constructed from 36 hex segments.  GTC has yet (4/2006) to achieve first light.  ORM La Palma is also home to the ING Group containing the Isaac Newton telescope (2.1m) and the William Herschel Telescope (4m)

SALT ’Southern African Large Telescope’ 11.1 m SAAO South African Astronomical Observatory, Sutherland site, north of Cape Town, RSA.  SALT which has just achieved first light, is today’s largest telescope as measured by mirror dimensions.  But SALT is not fully-steerable.  It can be turned in azimuth but is fixed at an angle of 37 degrees in altitude permitting it to scan about 70% of the sky.  The instrument is optimised for spectrographic work.  The design is very similar to the Hobby-Eberly Telescope HET, in Texas, also in the 8+ meter class

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