Conclusion

The aim of the short descriptions of these radio telescopes has been to highlight some of the original design aspects of major antennas over the last 40 years. We have now entered the era in which extensive metrology and active control of the reflector surface and pointing fluctuations are necessary to maintain performance under operational conditions. The GBT and LMT are examples of this approach. With access to the superb sites in the Atacama Desert and the South Pole, astronomers are proposing to build submillimeter telescopes with a highest frequency well above 1 THz, and larger than existing ones. One example is the Cornell-Caltech-Atacama-Telescope (CCAT). The proposal entails a 25 m diameter reflector of 10 mm rms surface accuracy and a pointing accuracy of 0.35 arcsecond. On the other side of the spectrum, the Square Kilometer Array (SKA), operating in the frequency band 100 MHz - 25 GHz might be realised by some 6000 dishes of approximately 15 m diameter. Here the challenge is not so much surface and pointing accuracy as low production cost. A similar approach of an array of relatively small dishes is being considered for the next generation of NASA's deep-space tracking stations to replace the aging 70 m antennas.

In the meantime, existing telescopes are adopting active elements in order to improve the performance. An example is the new "deformable" subreflector for the Effelsberg telescope to correct for the large-scale deformations of the primary reflector.

Finally, the optical astronomers are designing telescopes in the 25 - 100 m class. Some of these look remarkably similar to radio telescopes, but of course they require a glass reflector with an accuracy and smoothness measured in nanometers instead of micrometers. However, building and commissioning these will require entirely new methods and materials, a possible subject for another book by a different author.

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