James Clerk Maxwell Telescope
||This article may be too technical for most readers to understand. (March 2010)|
The James Clerk Maxwell Telescope (JCMT) is a submillimetre-wavelength telescope at Mauna Kea Observatory in Hawaii. Its primary mirror is 15 metres (16.4 yards) across: it is the largest astronomical telescope that operates in submillimetre wavelengths of the electromagnetic spectrum (far-infrared to microwave). Scientists use it to study our Solar System, interstellar dust and gas, and distant galaxies. JCMT will be shutdown at the end of 2014, if a new operator is not found.
The telescope is near the summit of Mauna Kea at 13,425 feet (4092 m). It is part of the Mauna Kea Observatory. The JCMT has the second-largest telescope mirror on Mauna Kea. (The largest is the VLBA antenna.)
This telescope was combined with the Caltech Submillimeter Observatory next to it to form the first submillimetre interferometer. This success was important in pushing ahead the construction of the Submillimeter Array and the Atacama Large Millimeter Array interferometers.
In the late 1960s, the Astronomy Committee of the UK's Science Research Council (SRC, the forerunner of STFC) considered the importance of astronomical observations at submillimetre and millimetre wavelengths. After a series of proposals and debates, in 1975, the SRC millimetre steering committee concluded that it would be possible to construct a 15-metre diameter telescope capable of observing at wavelengths down to 750 µm. The project, then called the National New Technology Telescope (NNTT), was to be an 80/20 per cent collaboration with the Netherlands Organisation for the Advancement of Science. Site tests were made at Mauna Kea in Hawaii, the Pinaleno Mountains in Arizona, and a site in Chile; and Mauna Kea was chosen. The NNTT is a unique facility, larger and with a more instruments than competing telescopes such as the CSO and SMT.
The final specifications called for the "world's largest telescope optimised for submillimetre wavelengths". It was to be a parabolic 15-metre antenna composed of 276 individually adjustable panels with a surface accuracy of better than 50 µm. It would be an altitude-azimuth mounted Cassegrain telescope with a tertiary mirror to direct the incoming radiation onto a number of different receivers. The antenna and mountings were to be protected from the elements by a co-rotating carousel with a transparent membrane stretched across the carousel aperture. Building work started in 1983 and went well apart from a small delay caused by the hijacking of the ship carrying the telescope across the Pacific by modern-day pirates. The telescope saw first light in 1987. The name for the final facility was changed to the James Clerk Maxwell Telescope.
The JCMT is currently funded under an agreement between the United Kingdom (75 percent) and Canada (25 percent) (until March 31, 2013 this was United Kingdom (55 percent), Canada (25 percent), the Netherlands (20 percent)). The telescope itself is operated by the Joint Astronomy Centre (JAC), from Hilo, Hawaii. The telescope site agreement with the University of Hawaii provides observer accommodation and infrastructure in exchange for open access to international proposals and 10 per cent of the observing time for the University's own projects. Proposals for telescope usage are submitted to one of the national Telescope Allocation Groups (TAGs) and if successful are awarded time in the next six-monthly semester.
Continuum emission is a tracer of star formation in other galaxies and gives astronomers clues to the presence, distance, and evolution history of galaxies other than our own. Within our own galaxy dust emission is associated with stellar nurseries and planet forming solar systems.
Spectral-line observations can be used to identify particular molecules in molecular clouds, study their distribution and chemistry and determine gas velocity gradients across astronomical objects (because of the doppler effect).
The The older continuum single pixel UKT14 bolometer receiver was replaced in the 1990s by the Submillimetre Common-User Bolometer Array (SCUBA). The SCUBA project was greenlighted in 1987 by the JCMT board and was in development for nearly a decade before it saw first light on the telescope. While it was not the first bolometer-array it was "unique in combining an unparallel sensitivity with an extensive wavelength range and field-of-view" .
SCUBA operated simultaneously at wavelengths of 450 and 850 micron (with 91 and 37 pixels, respectively), and was sensitive to the thermal emission from interstellar dust. SCUBA is ranked second only to the Hubble Space Telescope in terms of publication of high-impact astronomical research. SCUBA was retired from service in 2005.
SCUBA-2, another continuum instrument, was commissioned in 2011. This ground-breaking camera consists of large arrays of superconducting transition edge sensors with a mapping speed hundreds of times larger than SCUBA. It has 5120 array elements at both 450 and 850 micron wavelength (10,240 total pixels). It has been conducting the JCMT legacy surveys since November, 2011, including the SCUBA-2 All Sky Survey, and was made available for general astronomical observations in February, 2012.  Two ancillary instruments, FTS-2 and POL-2, add spectroscopic and polarimetric capabilities to SCUBA-2.
Spectral line detectors
The JCMT is also equipped with three heterodyne receivers, which allow submillimetre spectral line observations to be made. The spectral-line mapping capabilities of the JCMT have been greatly enhanced by the commissioning of HARP-B, a 350 GHz, 16 element heterodyne array receiver. HARP-B, and the other heterodyne instruments, can be used in conjunction with the JCMT's new digital autocorrelation spectrometer, ACSIS.
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- W.S. Holland et al., SCUBA: a common-user submillimetre camera operating on the James Clerk Maxwell Telescope, Monthly Notices of the Royal Astronomical Society: Letters Volume 303 Issue 4, Pages 659 - 672, 2002 doi:10.1046/j.1365-8711.1999.02111.x
- Farewell UKIRT