SORAL is led by Dr. Chris Walker; its staff consist of several PhD graduate students and undergraduate students at the University of Arizona.
Former SORAL personnel include these fine folks:
Submillimeter astronomy has only become possible in very recent years. In a sense, it is the last wholly unexplored wavelength frontier. Why is this so difficult?
The atmosphere is opaque at submillimeter wavelengths: Absorption of submillimeter light by water vapor (and to a lesser extent, O2 and O3 molecules) leads to a perpetually "cloudy" submillimeter sky. Minimizing this absorption by building observatories at the highest and driest sites, such as Mt. Graham (Arizona), Mauna Kea (Hawaii), the Atacama desert in Chile, and the South Pole allows us to observe "windows" of partial transparency at submillimeter frequencies during periods of good weather. Below is a plot of the transmission of the atmosphere (0=totally opaque, 1.0=totally transparent) for excellent submillimeter conditions at a very dry site. The dryness of a site is measured in millimeters of Precipitable Water Vapor (PWV). 1 mm of precipitable water vapor means that if you could condense all of the water vapor above you into an ocean, it'd be only 1 mm deep! This is typically 10-20 times drier than a summer day in the American Midwest, for example. Such weather is realized 10-30% of the time during winter months at sites like Mt. Graham. This dryness is only routine at the South Pole, where a 1.7-meter submillimeter telescope named AST/RO operates year-round.
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It requires high accuracy radio antennas:
At these short radio wavelengths, a radio dish must be built to very high accuracy -- a small fraction of the wavelength of light to be observed. The most accurate large submillimeter antenna yet built is the 10-meter Heinrich Hertz Telescope of the Submillimeter Telescope Observatory, shown below. This antenna has a recently-measured RMS surface accuracy of nearly 15 microns, a sixth of the width of a human hair! |
Astronomical signals are faint! Let's illustrate this with an example. A typical FM radio station 20 miles away emitting 50 kW (kilowatts) of power in all directions equally would have a "radio intensity" of less than 0.00001 W/m2 (watts per square meter). In scientific notation, that's 10-5 W/m2. A typical radio source might have a radio intensity of 10-26 W/m2! That's over 20 orders of magnitude (10,000,000,000,000,000,000 times) fainter! If you could sum up the total energy striking you from astronomical radio sources per second, it would equal the amount of energy a fly consumes in taking off. (!!)
Building very sensitive radio receivers at frequencies up to 1 THz is a very difficult challenge in and of itself. There are two fundamental types of detectors used at submillimeter wavelengths:
Bolometers
The first is a thermal detector called a bolometer, borrowed from well-established infrared instrumentation. A bolometer absorbs submillimeter-wavelength photons (packets of light, if you will) and converts that absorbed energy into heat, which is in turn registered by a very sensitive thermometer. These types of detectors only record the intensity of light over a very broad range of wavelengths (large bandwidth). At submillimeter wavelengths, bolometers are built to receive light over an entire atmospheric "window"; say, from 310 GHz to 370 GHz. Most bolometers have a single detecting element -- a single pixel! In order to make an intensity map of an object, you must raster the telescope over the object one pixel at a time. The first bolometer arrays have been constructed to speed this mapping process. Two of the most successful of these instruments are SHARC at the CSO and SCUBA, used on the JCMT. Intense broad band emission at far-infrared and submillimeter wavelengths is generated by warm (10 - 50 Kelvin) dust in atomic and molecular clouds, and is a powerful characteristic of star forming regions, both in our Galaxy and far beyond.
Heterodyne receivers constitute the other major type of light detector at submillimeter wavelengths. This technology stems from longer-wavelength radio observatories, where such receivers are used to the exclusion of all other techniques. Such receivers work on the same principles as your stereo AM/FM tuner, but at much higher frequencies and many orders of magnitudes higher sensitivity. In short, these receivers mix the electromagnetic field of the astronomical photons with a locally-generated electromagnetic field (called the local oscillator). The result is a signal at the difference, or beat, frequency. Radio astronomers call this the intermediate frequency, or IF. The original astronomical signal is therefore downconverted to a much lower frequency (from say, 490 GHz to 4-6 GHz), where more conventional low-noise electronics can amplify and process the signal.
The signal at the intermediate frequency carries both phase and spectral information about the original astronomical signal. This means that it is fairly straightforward to extract the spectrum of the astronomical source, and the phase of the incoming light waves. The latter is especially useful for combining the phase information from several receivers on separate telescopes to create an interferometric signal. In this way, ultra-high angular resolution (and more light-gathering ability) can be obtained. This is commonplace at lower-frequency radio observatories like the Very Large Array (VLA) and has in recent years become possible at millimeter wavelengths such as at the Owens Valley Radio Observatory or IRAM's Plateau de Bure Interferometer or the BIMA array. A tantalizing prospect would be to interferometrically combine the signal from the Heinrich Hertz Telescope (HHT) of the Submillimeter Telescope Observatory (SMTO) atop Mt. Graham, Arizona with submillimeter array receivers on the dual-8.4-meter Large Binocular Telescope (LBT) to produce a good submillimeter interferometer. Long term efforts to do this on a very large scale include the SMA and the MMA.
The light collected by the radio telescope is directed into a feed horn that deposits the energy into a "light-pipe" called a waveguide. The waveguide can be optimized by tuning, i.e. altering the electrical length of the waveguide by tuning stubs called backshorts. An optimized waveguide can deposit the fundamental signal mode to the mixer with low losses. The mixer couples the incoming astronomical signal with the local oscillator signal. The result is a signal at the difference frequency, which can be amplified to yield a signal that can be post-processed by spectrometers or other analysis electronics. Mixers used in SORAL receivers use the properties of superconductors (lossless conduction of electric currents) to produce the desired electrical response to incoming submillimeter light (i.e. detection of the astronomical signal). These are called SIS mixers, standing for Superconductor-Insulator-Superconductor, relating to the "sandwiching" of these elements in such a mixer.
Here's a simplistic idea of how it works: When two superconductors are separated by a thin insulator, superconducting pairs of electrons (called Cooper pairs) can travel readily from one superconductor to the other, since the energy levels of both superconductors are the same.
If you apply a voltage across the pair, you shift the energy levels so that the insulator effectively block current from flowing across the SIS junction.
With judicious choice of this biasing voltage, an incoming submillimeter photon can induce a single superconducting electron to "tunnel" across the insulating barrier to an unfilled energy level on the other superconductor. This is perfect -- we can convert an incoming stream of photons to an electrical current across the junction. We've got an electrical signal to work with now! Life is suddenly looking good...
Unfortunately, the SIS junction will only work if you have those two "S's". Superconducting material with the appropriately small energy bandgaps need to be kept at "very cryogenic" temperatures; typically 0.3 Kelvin (!!) for niobium junctions often used in SIS mixers at submillimeter wavelengths. That's less than a degree above Absolute Zero (-459oF), where atomic and molecular motion ceases! Higher temperatures increase thermal noise and shrink the bandgaps until, at the threshold superconducting temperature, the bandgap is zero and the junction is no longer useful.
Here's a zoomed-in image of an 810 GHz mixer block:
Another mixer becoming more popular recently is a hot-electron bolometer mixer. This heterodyne receiver system uses a bolometer to absorb incoming photons and convert them into an electrical signal. This configuration is limited by a narrow IF bandwidth of only a few MHz, so high bandwidth observations (for extragalactic sources) are not possible. These systems are useful at the highest submillimeter frequencies (>810 GHz) where SIS receivers become more problematic (some popular junction materials cannot be used well above 700 GHz).
What can you do with submillimeter receivers?
