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The James Clerk Maxwell Telescope


The JCMT dish.

The JCMT is the worlds largest radio telescope capable of working at submillimeter wavelengths (it covers wavelengths between 2mm and 0.3mm). The primary dish has a diameter of 15 meters made up of 276 aluminum panels, each of which is adjustable in order to keep the surface as near to perfection as possible. The dish is supported by a large backing structure and support mount (with a combined weight of 70 tons) which are designed to minimise the flexing of the dish as it is tipped and moved to track the sources it is observing. In order to protect the dish from the weather the entire structure is enclosed in a carousel. During observing the roof and doors are opened revealing the world's largest piece of Gore-Tex which is attached in front of the telescope. This is approximately 97% transparent to millimetre wavelengths and during observing protects the telescope from wind and dust, it also enables the telescope to be pointed at or close to the Sun for observing the inner planets or the Sun itself.


Mauna Kea.

The biggest restriction to observing at submillimeter wavelengths is water vapour in the atmosphere which strongly absorbs the signals from the astronomical sources being observed. In order to minimise this the choice of site is vitally important. Located at the summit of Mauna Kea (4092m) in Hawaii at the highest point in the Pacific ocean the telescope is above 97% of the water in the atmosphere. Due to its distance from sources of both industrial pollution and city light pollution as well as its exceptional weather characteristics Mauna Kea is one of the premier observing sites in the world.


The JCMT has a wide variety of instruments on it in order to make the best possible observations of the many different kinds of object that are studied using the telescope. Despite the excellence of the site chosen the atmosphere is still completely opaque at certain frequencies due to the remaining water vapour above the telescope. Those frequencies where the radio waves come through to the telescope are referred to as 'atmospheric windows'. The JCMT has radio receiver instruments at each of the major atmospheric windows for which it is designed to operate at. In order to minimise the background noise that would otherwise make observing impossible these instruments are cooled to just 4 degrees Kelvin (ie. 4 degrees above absolute zero or -269 degrees Celsius) or even lower.

Two major types of instrument are present on the telescope. Heterodyne instruments are use to study molecular line emission enabling the detection of different types of molecules and the determining how they are moving in space. Continuum instruments detect interstellar dust emission enabling the determination of the mass of objects studied. In addition several devices are available that enhance the operation of the major instruments such as polarimeters which can be used in conjunction with both heterodyne and continuum instruments to determine magnetic field strengths and alignments.

Most instruments are developed specifically for the JCMT and spend their entire life at the telescope, however, the telescope is also open for visiting instruments which may be installed for periods of anywhere between a few weeks and about a year to temporarily enhance operation of the telescope in a specific area.

Major Objects of Study

In many cases radio telescopes are used to study objects that are completely invisible to more traditional optical telescopes. This involves primarily processes where lots of dust is present which obscures any visible light. Such objects include stars in their earliest stages where they are surrounded by gas and dust disks that have not yet coalesced to form planets. Also taking up large amounts of telescope time are observations of extra-galactic objects where studies are looking at overall star formation rates in different types of galaxies ranging from nearby to high Z objects.


The JCMT is operated on behalf of the two partner countries (the UK and Canada) by the Joint Astronomy Centre (JAC) which is based in Hilo, Hawaii. (The JAC also operates the United Kingdom Infrared Telescope, UKIRT). The JAC employes a mixture of local and international staff who operate the telescope. This involves maintaining and repairing the telescope and its instruments, the actual night time operation of the telescope whilst supporting visiting astronomers and writing some of the computer software that is used to operate the telescope and reduce the data it takes.

Using the JCMT

Telescope time is allocated on a percentage basis to each of the partner countries in the same proportion as the financial support provided (55% UK, 25% Canada, 20% Netherlands), this accounts for 90% of the available observing time, the remaining 10% is available for any other astronomers world wide. Astronomers wishing to use the JCMT apply to Telescope Allocation Groups (TAGs) in each country who peer review each project (by sending each written proposal to two other experts in the field, who then provide a report back to the TAG). The TAGs then either approve or reject each project and (if approved) assign it a certain number of telescope hours. The astronomer may then either come to the telescope and personally make the observations or the project may be assigned to a 'flexible' queue which is a list (sorted first by required weather quality and then within each weather band by scientific priority) which will be executed by JAC staff in periods where there are no visiting astronomers at the telescope.

The Future

Although the JCMT was completed in 1987 it is still the largest telescope in its class in the world. Over its lifetime it has had its instrumentation upgraded several times. Further upgrades are currently underway, introducing more sensitive instruments and most significantly instruments capable of making observations at many positions simultaneously (ie. moving from the present 'single pixel' type instrument to taking a 'picture' with an array for many 'pixels'). The JCMT is also pioneering more efficient operational strategies such as flexible scheduling where projects are matched to their required weather bands more accurately than has traditionally been the case. Possible longer term improvements include an upgraded surface (to enable the telescope to work better at higher frequencies). In the very long term these arrays may enable the JCMT to become a survey instrument to complement the increased sensitivity and resolution of future large millimeter interferometer arrays (eg. ALMA).

Contact: JAC outreach. Updated: Mon Apr 1 08:11:28 HST 2013

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