Bigger and Better

The quest to "see" the radio universe more clearly has led to the construction of ever-larger radio telescopes in order to improve resolution. Following the collapse of the old 300-ft diameter radio telescope of the NRAO (Figure 15.1) the Robert C. Byrd Green Bank Telescope was constructed to take its place (Figure 15.2). In

Figure 15.1. At 9:43 pill EST on Tuesday the I5lh of November 1988. ihe 300-ft telescope in Green Bank collapsed. The collapse was due to the sudden failure of a key structural element—a large gusset plate in the box girder assembly thai formed the main support for the antenna. This is a photograph of the 300-ft telescope taken on November 16. 1988 after the collapse. The loss of the 300-ft telescope resulted in the Green Bank Telescope project. Figure 15.2, (Photo Richard Porcas, courtesy of the NRAO/ AUI.)

Figure 15.1. At 9:43 pill EST on Tuesday the I5lh of November 1988. ihe 300-ft telescope in Green Bank collapsed. The collapse was due to the sudden failure of a key structural element—a large gusset plate in the box girder assembly thai formed the main support for the antenna. This is a photograph of the 300-ft telescope taken on November 16. 1988 after the collapse. The loss of the 300-ft telescope resulted in the Green Bank Telescope project. Figure 15.2, (Photo Richard Porcas, courtesy of the NRAO/ AUI.)

figure 15.2. The Robert C. Byrd Green Bank Telescope of the NRAO in Green Bank WV This beautiful 330-ft diameter (100-m) dish replaced the old 3(X)-ft that collapsed under its own weight in 1988, The unipod design is meant to reduce unwanted reflections of radio signals from steel beams in the support structure holding up a secondary mirror that then reflects the radio waves to a focus behind the main dish. (Author's photo.)

figure 15.2. The Robert C. Byrd Green Bank Telescope of the NRAO in Green Bank WV This beautiful 330-ft diameter (100-m) dish replaced the old 3(X)-ft that collapsed under its own weight in 1988, The unipod design is meant to reduce unwanted reflections of radio signals from steel beams in the support structure holding up a secondary mirror that then reflects the radio waves to a focus behind the main dish. (Author's photo.)

addition, new breakthroughs in electronics have allowed more sensitive receivers to be built. A flock of new telescopes have recently been designed to operate in previously unexplored parts of the radio spectrum.

The larger the physical (collecting) area of a dish-shaped radio telescope, the greater its sensitivity and the fainter the radio signals it can detect. The I ,000-ft Arecibo dish in Puerto Rico, completed in 1963, covers 20 acres, larger than the-combined area of all the other radio telescopes in the world. It is built in a natural depression in the hills south of Arecibo and required relatively little work to scoop out the terrain so that its enormous reflecting surface could be snugly suspended in the preshaped space.

The smoothness of the surface of any telescope mirror, whether it be an optical or radio telescope, determines how good a reflector it is. The better the reflector, the more of the gathered energy is accurately brought to a focus and the less goes to waste by bouncing off in random directions. The surface irregularities have to be smaller than one eighth of the observing wavelength in order for the mirror to perform well. A radio dish may look rough to the eye but to a radio wave it appears as a shiny mirror. After being resurfaced in the mid-1970s, the Arecibo dish operated down to 6-cm wavelength, which means that over its entire surface the irregularities are smaller than I cm. This was achieved by accurately adjusting each one of 38,778 panels to an accuracy of a few millimeters.

Was this article helpful?

0 0

Post a comment