Radio observatories
If one considers optical telescopes to be our magnifying glasses to see the Universe, radio telescopes might be our hearing aids. They are tuned into radio waves, no different than those received by normal radio receivers. Well yes, just millions of times fainter. For this reason, state-of-the-art technology is used to reach the most distant corners and the faintest emitters in the Universe. For those interested, here are some basics of radio astronomy.
As visible in the graph of the atmospheric transmission, the radio regime ranges from wavelengths of about 1 mm to more than 10 meters. It is quite clear also that it is by far the broadest waveband. This offers us the opportunity to study radiation over a wide range of wavelengths - which in turn results in a large variety of antenna and receiver designs optimized to work in parts of this wide band. The fact that our atmosphere is transparent in the radio regime indicates that we can observe radio waves from the ground.
Here examples of real radio observatories, including short descriptions and photos. Some observatories have one telescope, in which case the telescope name is used as that of the observatory. But you will notice that there are a number of observatories with several telescopes. The links in the following list will lead you to a short introduction to the telescope/observatory, the title of which in (almost) all cases contains a link to the observatory's own homepage. Again, this list is not complete (and not intended to be):
Cm-wavelength single dishes
Although looking quite different from optical telescopes, the surfaces of radio telescopes also reflect electromagnetic radiation and thus serve as mirrors. The difference is the wavelength (or frequency) of the light reflected by these mirrors. Mirrors must be shaped precisely, to better than a tenth of the wavelength of the collected light, in order to be effective. For optical light, this leads to the well-known shiny mirrors. For radio light, a wire mesh with a mesh size of less than 1/10 of the observed wavelength will do; this is visible in the photos of the various telescopes shown here:
- Effelsberg 100-m telescope
- Arecibo 305-m telescope
- Green Bank observatory
- Jodrell Bank observatory
- Parkes 64-m telescope
- Dwingeloo 25-m telescope
- Stockert 25-m telescope
Cm-wavelength interferometers
The same surface accuracy requirements stated above for single dishes apply to interferometers as well. Only instruments observing to very short wavelengths will have smooth surfaces. Arrays currently in operation are:
- Very Large Array (VLA)
- Australia Telescope Compact Array (ATCA)
- Giant Meterwave Radio Telescope (GMRT)
- Westerbork Synthesis Radio Telescope (WSRT)
- Dominion Radio Astrophysical Observatory (DRAO)
Mm-wavelength single dishes
A view at the surfaces of mm-wavelength radio telescopes shows that their dishes are much more accurate. Sometimes it is not so obvious, because they are painted with reflective paint. But others are quite shiny, almost resembling optical mirrors (but not quite yet).
- Pico Veleta 30-m telescope
- Swedish-ESO Submillimetre Telescope (SEST)
- Mopra 22-m telescope
- Heinrich Hertz Telescope, HHT
Mm-wavelength interferometers
Mm interferometers have the same surface accuracies as single dishes operating in the same wavelength range. Current/past mm-wavelength interferometers are:
- Plateau de Bure interferometer
- Owens Valley Radio Observatory (OVRO)
- Australia Telescope Compact Array (ATCA)
A special branch of radio interferometry, which achieves the highest angular resolutions, is Very Long Baseline Interferometry (VLBI).
