Radio telescopes

Independent arrays of telescopes have been discussed for decades but have generally not been successful, except for radio telescopes, where interferometry is a key virtue, aided by the fact that the individual telescope signals can be amplihed and combined while preserving phase information. This is not practical in the optical, thus there are significant inefficiencies in sensitivity by coherently combining the light from an array of optical telescopes. Instrumentation for an array of telescopes has also been a cause of difficulty. Perhaps the best known successful array has been the VLT with four 8-m telescopes, each with its own suite of science instruments, and the capacity to combine all telescopes together for Interferometric measurements. 

Table 3.2 The collection dish of the JCMT telescope on Hawaii. The 15m diameter JCMT is the largest astronomical telescope in the world designed specifically for sub-millimeter astronomy. (Reproduced by permission of the James Clerk Maxwell Radio Telescope)...

Anticipated next supernova in our Galaxy may be undetectable by the optical instrument due to the Galactic extinction. However, supernovae are now known to be intense radio sources after a year or so of the explosion. Even if the positions are beyond the Galactic center, the radio supernova could be observed using middle size radio telescope. 

Radio telescope arrays (on Earth) Radio waves coming from the Sun... 

Among the most exciting evidence for black holes comes from 10 radio telescopes (collectively known as the Very Long Baseline Array, or VLBA, allowing scientists in 1995 to peer into the spiral galaxy NGC 4258. Researchers mea-... 

During the past fifty years extensive effort in many laboratories has led to enhancements of the simple system, turning microwave spectroscopy into an extremely sensitive and versatile tool. We now review some of these developments. We shall also describe the essential features of a radio telescope because almost thirty diatomic molecular species, many of which would be transient species in the laboratory, have been detected in interstellar gas clouds. Molecular radio astronomy is closely linked with and complementary to laboratory microwave spectroscopy. Or, if you wish, you can reverse the emphasis of that last statement ... 

Superherodyne spectrometers are now not common in laboratory microwave experiments, but superheterodyne detection plays a major role in radio astronomy, as we shall see later. The reasons are obvious one cannot modulate the energy levels of extraterrestrial molecules, and a radio telescope collects radiant energy at all frequencies simultaneously. One does not have a primary monochromatic source of radiation, as in laboratory experiments. 

The first element of a radio telescope is the receiving dish, always the most visually impressive part of the instrument. For the study of spectral lines the ideal dish must satisfy a number of requirements. 

Figure 10.23. Geometry of different focusing systems and detector positions with parabolic radio telescope reflector dishes, (a) Cassegrain, (b) Gregory, (c) Nasmyth, (d) offset Cassegrain [36]. 

Figure 10.24. The principal elements of a radio telescope employing superheterodyne detection for recording frequency-discrete line emissions or absorptions [34].

BALLON. TELESCOPE RADIO TELESCOPE AIRCRAFT. ROCKET ... 

Density profiles. Large radio telescopes have enabled the high spatial resolution mapping of precoUapse clouds and the determination of their interior density structure. While such clouds undoubtedly vary in aU three spatial dimensions, typically the observations are averaged in angle to yield an equivalent, spherically symmetric density profile. These radial-density profiles have shown... 

The International Astronomical Union has established a guide to the naming of planetary features, and for Mercury the convention is that craters are named after artists, musicians, painters, and authors plains are given names corresponding to Mercury in various languages scarps are named after famous ships of scientific discovery, and valleys are named after radio telescopes. 

Neutron stars have also provided the first direct evidence of planets outside the solar system. The pulsar PSR 1257+12 rotates 161 times per second-but its pulses are not evenly spaced, as are the ones from the Crab. Observations made with the giant Arecibo radio telescope showed that some pulses arrive slightly too soon, others just a bit too late. This means something is tugging the pulsar back and forth slightly. Careful measurements showed that a pair of planets is responsible. 

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