Radio Astronomy
It is surprising to many people that radio astronomy does not entail listening for ET to phone home. SETI, (the search for extra terrestrial intelligence), represents a relatively tiny portion of radio astronomy. It would seem that the public perception of radio astronomy conjures up images of astronomers in tight jeans wearing headphones to detect some weak signal buried in the galactic noise. If we apply a brief reality check, we find that radio astronomy is much like optical astronomy, in that telescopes (instruments that detect, image and magnify) are used to observe the cosmos.
The difference is that while optical telescopes present images that are familiar in composition (i.e. they present images at frequencies which we can directly see). Radio telescopes observe the cosmos at much lower frequencies. Most of us have seen the stunning images acquired by the Hubble Space Telescope. To be sure these images not only give us a glimpse of the wonders of the universe, but move us in spirit by generating a sense of awe. Unfortunately the primary sensory input device for us humans (our eyes) is very limited in "bandwidth" (the span of electromagnetic frequencies, or 'colours', to which it is sensitive) and while the images move us, they do not give up their secrets easily. As a consequence much of what is happening in the universe is hidden from our view.
To put it simply, each colour is a different frequency, and most of the colour pallet with which the cosmos is painted is invisible to our eyes. It only makes sense to expand our sensitivity, through instrumentation, to the other frequencies in the electromagnetic spectrum. The radio telescope is one of these instruments. It allows us to observe and image the universe at frequencies below our visual abilities which in turn, reveals much of what is going on in the universe. Because certain radio frequencies pass effortlessly through pesky dust and gas clouds we can now study objects heretofore blocked from our view. Also, since certain gases, molecules, and materials in the universe either absorb or emit 'light' at radio frequencies, these structures can be directly viewed by the radio telescope. This ability not only allows the observer to image these objects, but also allows the observer to gather much more information such as composition, velocity, temperature and mass.
The span of frequencies that makes up the radio spectrum is immense, thus the variety and types of instruments that make up radio telescopes is varied in design, size, and configuration. Lower frequency (10 MHz - 100 MHz (wavelengths of 30 meters to 3 meters)) instruments are generally arrays of antennas similar to "TV antennas" or are stationary reflectors of gigantic proportions with moveable focal points some are over 30 meters high by 500 meters wide. At higher frequencies (100 MHz to 1GHz (wavelengths of 3 meters to 30 cm)) very large parabolic or spherical reflectors are used such as the large spherical "Dish" at Arecibo, Puerto Rico. At Frequencies of (1 GHz to 10 GHz (wavelengths of 30 cm to 30 mm)) medium to large parabolic reflectors are used 5 to 90 meters in diameter.
These reflectors are fully articulated and can observe any object simply by pointing the reflector. For frequencies above 10 GHz (wavelengths of 30 mm to .3 mm) high precision parabolic reflectors are necessary typically 3 to 20 meters in diameter. The reflectors are more like mirrors and are thermally stable as well as supported by complex structures since the surface curvature is held to demanding standards. The surface tolerances of these reflectors are held to plus or minus one hundredth of a millimetre for radio telescopes operating in the millimetre to sub-millimetre wavelength region. Each type of instrument opens a new set of "colours" with which for the astronomer may view the universe.
Optical telescopes give such clear images since the wavelength of visible light is so small in relation to the diameter of the focusing device (mirror or lens). Radio waves having enormous wavelengths by comparison, do not focus into neat "pictures" rather they tend to interfere with one another since the focusing device (reflector) is tiny in relation to the wavelength. To construct a 10 mm wavelength radio telescope with the imaging capabilities of a small 4 inch optical telescope one would need a reflector about 2 km (over 6000 feet!) in diameter, clearly this enormous size is impractical. It might seem at first glance that radio astronomy would be doomed to low detail observations and rather dull data gathering tasks. The fact that light and radio waves tend to interfere with one another gives rise to a technique known as interferometry. Simply put, it allows two or more dish antennas to be placed widely apart or in arrays (such as the VLA "Very Large Array" in New Mexico) to function as if they were one large antenna (Aperture Synthesis). The interference between the signals of each of the receiving antennas, when timing corrections are introduced, allows for image reconstruction using Fourier Transforms. We can now get the resolution of a 2 km antenna by placing several antennas 2 km apart and correlating the data. Using this technique it is possible to obtain milliarcsecond resolution. A milliarcsecond is roughly the equivalent of seeing a quarter in New York from Los Angeles!
With the advent of DSP (Digital Signal Processing), Faster and Smaller Computers, and the introduction of super conducting amplifiers radio astronomy has progressed at a break neck pace. New arrays of antennas are being designed and built. Some contain over a thousand individual antennas all operating in harmony, giving resolutions that rival optical telescopes. Other arrays cover a hectare and one in process covers a square kilometer as a "phased array" giving imaging capabilities not experienced before. The future for radio astronomy looks brighter than ever.
A brief history of Radio Astronomy
To start with, a very short history of Radio Astronomy would be helpful. Radio Astronomy was born in the early 1930s when Karl Jansky, working for Bell Laboratories, was trying to determine the origin of a source of noise that was showing up in receivers operating in the 20 MHz region of the radio spectrum.
Jansky built a steerable antenna and began searching for the source of the noise by taking directional measurements. To his surprise he discovered that this noise was from extraterrestrial sources. Jansky, enthused by his discovery published his work, however the majority of astronomers at the time were decidedly underwhelmed by this discovery and for the most part dismissed it as either irrelevant or simply curious. There were a few inventive individuals who saw the potential for this noise from space.
One of them, Grote Reber, an electronics engineer and avid radio armature, had reviewed Jansky's original discovery had speculated that the signals were of thermal origin (caused by very hot objects) and as such they should be easier to detect at higher frequencies. Since Jansky's original work was done at 20 MHz (about 15 metre wave length) and a beam width of about 25 degrees, Reber wanted to narrow the effective beam width to obtain finer detail. Reber reasoned that he should build his first receiver and antenna to operate at 3000 MHz (10cm wave length) an extraordinary frequency at that time. With his own resources and enthusiasm, Reber built the first parabolic reflector radio telescope. Since this was deemed a private 'extracurricular' activity, Reber received no sponsorship or support. Besides being the first of its kind, it was also a huge structure. Basically built by a single individual it was 9.5 metres (31 feet or 3 stories) in diameter.
The term 'Radio Telescope' had not been coined at the time, however Reber gets the credit for building the first one. Although he did not prove his original hypothesis, his work went on to detail the first radio map of the galactic plane and large portions of the sky. Reber published his work "Cosmic Static" in the late 1930's.
It was the search for static or noise that led to the development of the radio telescope and it is essentially noise from the universe that the radio telescope detects. Buried in this roiling confusion is information that is specific in nature to astronomical objects and phenomena. This noise bears witness to the physical characteristics of the universe. The information is presented as a mixture of signal properties such as frequency, phase, amplitude and in some cases repetitive patterns. Also present is information that can be mathematically assembled into 'radio pictures' of these cosmic objects. Some signals arrive from finely defined sources that can be, by and large, considered as point sources (quasars and pulsars for example).
Other sources cover vast areas and can be thought of as wide field objects. These are clouds of dust and gas, star 'nurseries', galaxies and a plethora of other interesting goodies. To obtain information from these sources, the radio telescope must receive not only specific information but also all the 'noise' from these objects and their surroundings then reject what isn't wanted and record the results.
Radio frequency signals of extraterrestrial origin are extremely weak. As an example, if all the signal energy ever received from all the radio telescopes ever built (viewing objects other than the sun) were combined there would not be enough total energy to melt a single snow flake.
The radio telescope must first concentrate signals gathered over a wide area and focus them into a small area. This is the same principle on which the reflecting optical telescope operates. The term "radio optics" refers to this similarity. Since the term 'light' really means electromagnetic radiation, all the same basic equations, theories and principles are applicable to radio, infrared or visible light. The big difference is that optical telescopes operate at extremely high frequencies and microscopic wavelengths, while their cousins the radio telescopes work at lower frequencies and longer wavelengths. \
Resolution, which can also be expressed as beam width, is a function of the wavelength of the signal and the diameter of the reflector. At optical frequencies (blue-green light 600,000 GHz or a wavelength of .0005 mm) a 1 meter diameter "perfect" mirror will have a beam width of about .00003 degrees. The same mirror operating at a radio frequencies (30 GHz for example with a wavelength of 1 cm) will have a beam width of about 6 Degrees. As can be seen, the beam width for the radio telescope is about 200,000 times wider, thus yielding lower resolution observations. At first the solution to this was to build bigger and bigger reflectors, giving narrower beam widths, and higher resolutions.
By the late 1950's reflectors of 100 meters (300 feet) across were being built. At diameters larger than this, a steerable reflector becomes far to heavy and cumbersome to be effectively used. The big problem is that the surface warps and deforms due to gravity and thus the effectiveness of the reflector is compromised. he one advantage of large reflectors is that with their very large gathering surface area they offer significant signal strength, the down side of this is that they are very expensive to operate, maintain, and build.
Even with the large areas, one still must remember that the beam width is still wide compared to optical instruments. A 100 metre diameter radio telescope, operating at 10 cm wavelength, still only has the individual resolving ability of an optical mirror of about 5 mm (less than 1/4 inch). Even with such seemingly myopic resolution the sheer size of these instruments allows for detection of weak sources billions of light years away. In a later article I will discuss interferometry, a technique by which multiple radio telescopes can be combined to give the effective resolution of a single telescope many miles across. This process changes the apparently fuzzy world of the radio telescope to one of crystal clarity. Modern radio telescope arrays such as the VLA in New Mexico and the Caltech OVRO millimetre array have resolving abilities far beyond even the Hubble telescope.
The temperature of the radio telescope, its reflector, and its receiver are all sources of noise with which the observer must contend. Since everything with a temperature above absolute zero gives off electromagnetic noise in one form or another, and the fact that what a radio telescope 'sees' is essentially electromagnetic noise, the radio telescope needs to be highly selective and reject as much superfluous noise as possible.
One method of counteracting noise is to cool the receiving electronics to a temperature just a few degrees above absolute zero. This eliminates thermally generated noise in the electronics. Once this noise has been removed, the amplified signal of interest is then, selectively amplified again, converted to more manageable frequency bands, divided into a series of adjacent channels and finally processed to detect the relative power or energy of the source along with frequency and phase detection.
Because a radio telescope is so sensitive other methods of reducing noise are used. One is to reduce reflected and thermal noise from the ground. This is why many radio telescopes have a Cassegrain configuration (a secondary mirror reflects the signals back through a hole in the centre of the main reflector). Since the receiving electronics input focus points to the sky, picking up thermal and reflected noise from the ground is avoided.
The final method is to reduce the contributed noise from terrestrial sources. This translated means, move the telescope away from the high density cities to some remote location where the local denizens, i.e. rabbits, moss, and life forms found under rocks do not pollute the radio spectrum. This also usually means placing the telescope in a valley surrounded by mountains so that the terrain blocks a great deal of unwanted radio noise. Add to this the help of the local authorities to declare the surrounding area of the telescope as a 'radio free' zone and you have a reasonably quite observing site. Finally when all this is combined, the effective noise temperature of an entire radio telescope system can be reduced to only a few tens of degrees above absolute zero, (quite an improvement when considered that typical room temperature is about 300 Kelvin).
A signal arriving from a celestial source has now been gathered by a large reflector, concentrated into a small area and fed to a low noise electronic receiver which is isolated from strong external sources, quiet in it's own operation and is highly selective. The next part of the process is to store the information for subsequent processing. Since many of the radio source signals are so weak, it is often necessary for a telescope to stay fixed on target for extended lengths of time to insure sufficient information has been gathered. The result of these long 'exposure times' (to borrow a phrase from photography), results in huge amounts of data. In the early days of radio astronomy, information was recorded on paper, which chart recorders spewed out by the mile, and consequently the astronomer had to inspect visually, by the mile. This was an arduous process and sometimes required months to extract the information.
In the 1960s magnetic tape was substituted for paper and computers were given the task of correlating the information. Today with inexpensive desktop computers, flash analogue to digital converters, and billion operation per second digital signal processing chips, much of the information obtained can be processed in real time. It is the results of the computations on the raw signal data that carries the ultimate useful information. With faster and faster real time processing, the storage of information has shifted from saving the raw incoming signals to saving the derivatives and ultimately to saving only the specific information. This not only reduces the total storage required (raw signals require magnitudes more storage) but allows for faster retrieval of pertinent information since the data has been pre filtered and formatted.
Last, but not least, is the interpretation of the data into a meaningful format. Despite our ability to interpret numbers and form abstract conclusions, we human beings are visually oriented. The information from a radio telescope can indeed be turned into a picture that is easy to understand. However, along with this visual presentation comes volumes of additional information that, when analysed reveals the secret workings of much of the universe. This information is often intangible to our senses. Properties such as phase, coherence, polarisation and subtle frequency variations cannot be discerned from a simple picture. Additional signal processing and receiving techniques must be used to reveal these characteristics. Often, the presentation of these other qualities will be in a visual or pictorial format, but the colours and intensities will demonstrate properties not normally visible. These 'false colour' images present to the mind visualisations of concepts and properties heretofore unobservable.
The radio telescope while not as basically easy to use as a simple optical instrument, actually reveals much more information to the observer. With its ability to cover a much wider portion of the electromagnetic spectrum the radio telescope shows much more of the inner workings of the universe. The intrinsic composition of interstellar clouds, the birth of stars, and the properties of stars whose lives have passed, are all observable with the radio telescope where these mysteries are masked to the optical instruments. Now with the combination of highly accurate optical and radio imaging, the cosmos is beginning to become comprehensible.