A Visit to the
James Clerk Maxwell Telescope
(JCMT)
on top of Mauna Kea, Hawaii, October 2002
The JCMT currently is the largest Radio telescope dedicated to millimeter and submillimeter radio astronomy. The range of frequencies covered extends from 0.3mm to about 2mm. The receiver dish has a diameter of 15m and consists of 276 individually adjustable aluminum panels that give it the near perfect parabola shape needed to image the sky in radio waves. The dish is supported by a massive truss support structure that minimizes flexure of the dish when moving across the sky. The moving mass of the telescope is about 70 tons which is tracking the Sky in sub arc second accuracy.
To protect against the elements the dish is mounted inside a free rotating dome which is seismically isolated from the telescope mount. A giant Gore-Tex membrane covers the dome opening. The material is totally (97%) transparent in the radio spectrum and does not interfere with the observation. So the telescope is conveniently protected from wind, rain and dust. It also allows observations very close to the sun or the sun itself. Otherwise the sun would be reflected from the dish and burn the receiver electronics.
Click on the pictures to see a larger version:
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!

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.


Image: JCMT

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.

The three last images with kind permission from the JCMT archives. All other images made by Sibylle Fröhlich and Gert Gottschalk on  October 4th 2002. We would like to thank Bernd Weferling from JCMT for the guided tour.
S. Fröhlich, G. Gottschalk, Oktober 2002