When observing invisible light (i.e. electromagnetic radiation outside the visible part of the spectrum), there are some notable differences in the ways that telescopes are designed. For instance, radar dishes such as those in the Skyglow: disdance video above are often constructed with a painted white or grey surface, or even with wire mesh, rather than a highly reflective, precisely engineered mirror. The reason is that when observing the centimetre-to-ten metre wavelength range of radio waves, the reflective surface used to focus a signal down to an antenna does not need to be precisely engineered.
While radio dishes do not require very fine precision engineering in order to focus radio waves, the design of a radio telescope suitable for astronomy comes with a different challenge. That is, as we saw in the previous section, the resolving power of a telescope diminishes with increasing wavelength. Therefore, in order to resolve fine radio signals, telescopes must be built with enormous diameters, such as the (former) 305-m radio telescope at the Arecibo Observatory in Puerto Rico (see Figure 4-9).
In contrast to radio telescopes, the main challenge in observing short-wavelength, high energy radiation such as X-rays lies in gathering and focusing the radiation. Namely, due to their high energies, X-rays cannot be focused using lenses and mirrors since an X-ray photon hits a mirror like a bullet hits a wall, so it is absorbed rather than reflected. For this reason, the Chandra X-ray Observatory, NASA’s flagship X-ray telescope, captures and then focuses X-rays using a set of nested cylindrical mirrors. Rather than attempting to reflect the X-rays, they hit the mirrors at grazing angles. The design of the mirrors is such that they subsequently direct these photons down towards a detector (see animation below).
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