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These are exciting times at the James Clerk Maxwell Telescope! The past year has seen the commissioning of two major new instruments, and two more are expected to arrive shortly. This happy conjunction of instrumental progress is challenging the local staff in their support roles but, at the same time, offers unprecedented opportunities for Canadian astronomers to explore the submillimetre universe. Approximately one-third of the "non-dark" mass in our Milky Way Galaxy exists as cold interstellarmatter in the form of dusty gas clouds that are opaque at optical wavelengths. Significant fractions of the normal mass in most external galaxies also are in the form of cold interstellar media. Such matter is intimately associated with the formation of stars and planetary systems, with the production of dust through stellar mass loss, bipolar outflows, interstellar chemistry, the physics of shocks, starbursting galaxies, the fueling of AGNs and the earliest rapid phase of dust formation in primeval galaxies, to mention just a few current topics in submillimetre astronomical research. Because of its low temperature such matter emits most strongly at submillimetre wavelengths, both as continuum radiation from dust and in the form of thousands of spectral lines arising from the rotational and fine-structure transitions of molecules and atoms. The JCMT, situated at an excellent location, is among the finest submillimetre-wavelength telescopes in the world. Its preeminent position is being strengthened by recent and imminent instrumentation advances. |
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With the arrival of SCUBA at the telescope
things have really changed. As the largest, fastest submillimetre
camera available today, its impact on both observatory operations
and, more significantly, on astronomical activities has been major.
The camera consists of a 37-pixel array optimized for use at a
wavelength of 850um and a 91-pixel
array optimized at 450um. These arrays are simultaneously
imaged on the sky over a 2.3 arc-minute field at resolutions of
13 and 7 arc-seconds. In addition, there are 3 photometric pixels
operating at 1100um, 1350um and 2000um.
The heart of SCUBA, which houses the bolometer arrays, consists of 6.6 kgm of metal cooled by a He-3 dilution refrigerator to 75mK. (Talk about cold-hearted. SCUBA makes the Wicked Witch of the West look like Mother Theresa!) Actually, it is interesting to speculate that the inside of SCUBA, cooled to a tiny fraction of a degree above absolute zero, may be the coldest significant mass of material in the universe. Perhaps there is one remaining physical sense in which the earth is still unique, after centuries of successive demotions from its Ptolemaic position at the centre of the universe? |
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SCUBA is designed to be photon-noise limited. Each of its pixels is about ten times more sensitive than the single detector of its predecessor, UKT14. This improvement, coupled with the size of its arrays, translates into an increase of more than 5000 in mapping speed at 450um for extended sources! Such large single improvements in observing capability occur seldom in a lifetime, and this offers a great opportunity for Canadian astronomers to help push back the frontiers of submillimetre astronomy.
SCUBA can be used in a number of modes, depending upon the source characteristics. The details are available on the SCUBA home page.
For relatively compact sources, so-called "jiggle" mapping provides simultaneous two-colour images of a 2.3 arc-minute field. If you're a submillimetre continuum buff, you'll be impressed by sensitivities of about 6mJy (1-sigma) at 850um after about1 hour, and the current best value of about 0.5 mJy, achieved after binning adjacent pixels following 4 shifts of integration. For those of you who haven't struggled in the past for a whole shift to get a single-point detection of a 30 mJy source, let's just say, "That's good, eh!".
Recently, scan-mapping has been used successfully to provide spectacular images of extended sources Under good sky conditions a region 10 arc-minutes on a side can be mapped to a 1-sigma level of about 70 mJy in 1 hour. Such fields can be combined to form considerably larger mosaics.
Similar mapping modes are available to provide simultaneous images at 350um and 750um.
In addition to the arrays at 350, 450, 750 and 850um, SCUBA provides single-pixel photometry at 1100, 1350 and 2000um. The spectral information so obtained is useful for modeling source temperatures and the frequency-dependence of dust emissivity.
The interstellar magnetic field is one of the key physical parameters affecting a host of astrophysical processes, including the fundamental one of pre-protostellar cloud contraction. At submillimetre wavelengths the dust emission is polarized with the E-vector perpendicular to the field . An advantage of measuring the polarization at submillimetre wavelengths is that the emission is optically thin regardless of the visual extinction. Consequently, it's possible to study the magnetic field direction deep within molecular clouds, where all of the star formation occurs. SCUBA presently offers single-pixel polarization capability using either or both central pixels of the two arrays or any of the photometric bolometers. Within the year it is expected that full-array polarization will be possible.
SCUBA, for all its exciting capabilities as a submillimetre camera, does not provide any dynamical information. This vital third dimension of the "data cube", plus direct access to the physical state of the dominant gas component, must be acquired through spectral line observations. Here, too, the JCMT is making good progress.
The arcane JCMT terminology for designation of the heterodyne instruments has its origin in the chronological order in which receivers were delivered to the telescope. This meant that the lowest-frequency (easiest technology) band received the first, trend-setting letter. Thereafter, the pattern was set. The lowest frequency atmospheric window is A-band (215-270 GHz, 1200um), then comes B-band (315-370 GHz, 870um), C-band (450-500 GHz, 630um), D-band (630-700 GHz, 450um) and E-band (800-900 GHz, 350um). Thus, Receiver B3 is the third receiver at the telescope for the 315-370 GHz window. (It's actually the fourth. There was a RxB3i that isn't usually counted because the i stands for "interim" which meant that it was a stand-in for the real RxB3.)
Applicants for JCMT time should be assured that they are not required to master JCMT "receiverese" as a prerequisite for admission to the observatory. Just call your favourite JCMT Staff Astronomer and tell him the frequency you want to use for that Nobel-Prize-winning first detection of the submillimetre line of Dark Matter. He can do the translation for you.
These receiver designations have been the subject of discussion from time to time. They can be confusing for new-comers and, certainly, the designation RxC2, for example, lacks the appeal of a MONICA, or the colourful imagery that comes to mind with SCUBA or the competition-intimidating connotation of SHARC . But it just seems hard to come up with catchy, uncontrived acronyms for the line receivers.
The newest of the heterodyne receivers is RxB3 , built by HIA's JCMT Group in collaboration with RAL (UK) and SRON (Netherlands). It was commissioned early in 1997. This dual-beam receiver offers quick, remote tuning, wide instantaneous bandwidth and flat baselines . It has a single sideband filter and state-of-the-art sensitivity. This instrument provides an increase in speed of about a factor of 5 over its predecessor, RxB3i. It can be used in the standard JCMT observing modes, including frequency switching and raster mapping.
The oldest of the active receivers at the JCMT is RxA2. Very shortly this venerable old workhorse will be retired, to be replaced by A3i. Those of you who have passed your immersion course in JCMT-speak will understand that this designation means the receiver is designed to cover the 210-275 GHz window, with plans for upgrades in the future. RxA3i is the Phoenix (Hey! Maybe that's what we should call it?) of the JCMT's suite of instruments. It existed in a former life as RxB3i, and will be delivered, after extensive modification by the HIA's JCMT Group, with an environmentally-friendly "Recycled Material" sticker in the form of a green Maple Leaf.
RxA3i will have a tunerless mixer, which will permit lightning-fast tuning (figuratively speaking), and provide enhanced sensitivity and a broader tuning range than A2.
Given that E-band is the shortest-wavelength submillimetre window, you might reasonably infer that this receiver's designation indicates that the JCMT aspires to branch out into x-ray astronomy. Such is not actually the case, although there are ongoing efforts to optimize the JCMT's surface accuracy. "W" in this case stands for "Wide-band" because the receiver will cover both the C- and D-band windows, with two orthogonally polarized beams in each window.
With its dual-beams and improved mixer performance, RxW will greatly enhance the JCMT's capabilities in the two atmospheric windows where the Mauna Kea site is truly advantageous in terms of accessibility and sky transmission. RxW is expected to arrive this spring. It will replace the existing C2 receiver and will provide, for the first time, a resident capability in the important D-band window.
Together these new instruments represent a suite of powerful tools for exploring the submillimetre universe. In addition, there are plans and efforts on many fronts to improve capabilities - the Antenna Surface and Telescope Control SystemUpgrades, an interferometry link with the SMA, a receiver for E-band, an 8-beam spectral-line camera for B-band, and a new autocorrelator spectrometer, ACSIS . ACSIS will serve as the backend of the future for all the JCMT line receivers including the B-band camera. Details on these and other activities can be found on the JCMT home page. So the future looks bright and the coming decade promises to be an exciting one at the JCMT.
Submillimetre astronomy is not always easy - the photons are wimpy (hv is small compared to visible light), the technology of detecting them is challenging (it's not easy to build a mixer block with quarter-height waveguide to operate at 450um) and the atmosphere, on a good night, soaks up half your signal before it enters the dome. But the improvements being made at the JCMT are helping tremendously. So, if you want to know what's really going on inside that star-forming cloud, in the heart of that starbursting galaxy, or back when the universe was young, well, "Come on down and visit the JCMT!" We won't leave the light on for you - that upsets all the optical astronomers on the mountain - but you can be sure there'll be a hot new receiver waiting to give you a cold-hearted welcome.
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Lorne Avery
lorne.avery@jach.hawaii.edu
"Lorne is currently posted as a staff astronomer to the JCMT from the Herzberg Institute of Astrophysics. He is principally involved there in the support of visiting observers and in maintaining Receiver B3." |
