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A Visit to the
James Clerk Maxwell Telescope
(JCMT)
on top of Mauna Kea, Hawaii, October 2002

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The JCMT (second dome from the left) on its site  on top Mauna Kea at an elevation of 4092m (13425 ft).  On the upper right the Subaru telescope and the domes  of the two Keck telescopes (all optical telescopes)	JCMT

To receive mm and sub-mm radio signals the observer has to eliminate water vapor which strongly absorbs the radiation in these wave lengths. High mountain tops with dry air allow the telescope to be above 97% of all water vapor that is in the atmosphere.
Still a lot of the radiation is absorbed by water molecules in the high atmosphere leaving only a set of 'radio windows' available for the observer. Luckily a lot of scientifically rewarding observation can be made in those windows. Beyond that only very expensive space observatories can detect signals outside these windows.

The telescope mounting is structurally separate from the telescope dome. The yellow line in the ground marks the gap. This prevents vibrations from movements of the dome as from wind loads to be transferred to the telescope which is held perfectly pointed to the celestial object under study.
 

The massive telescope mounting.	The whole dish.	Detail of the support structures for the 276 panels that make up the reflector dish.
View from the top of the dome onto the dish.	The protective Gore-Tex membrane curves around the telescope on the left.	The entire dish under the Gore-Tex screen.

A flash light picture shows reflections from the alignment bolts of the dish. A high tech view!

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Each of the radio windows in the atmosphere is matched with a dedicated receiver at the JCMT. As the signals from space are in a spectral range where the heat of the telescope itself contributes to background noise special techniques are necessary to detect the signals. At first the detectors itself are cooled to nearly absolute zero using liquid helium cooling. (4K)
Next the telescope's secondary mirror can quickly switch between the object under study and nearby blank sky. The difference is then the desired signal from the object.
A major contribution in recent years have been the evolution of receiver technology. When previously the detector (a bolometer) only registered radiation from one signal spot of the sky (1 pixel!). New technologies borrowed from semiconductor manufacturing allow the use of small detector arrays. These are only a few tens to hundred pixels, much less than what is available for optical astronomy, but still a great improvement from before. With those array detectors larger areas of the sky can now be observed using a rastering methodology.
Object of interest in the mm and sub-mm radio astronomy are mostly interstellar dust and molecules. A map of the central portion of the milky way is depicted below.

From the archive of the  JCMT.  The dome is opened for observing. Light from the inside illuminates the Gore-Tex membrane.	Also from the JCMT archives. The dish  is openly visible. The Gore-Tex membrane was rolled up during repair work in September 2000.