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book reviews
Radio astronomy booksreviewed by T. Nelson |
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book reviews
Radio astronomy booksreviewed by T. Nelson |
Reviewed by T. Nelson
This is an excellent, well-rounded textbook on radio astronomy, with many problems and equations, a moderately okay index, and appendices on Fourier transforms, one-bit quantization, square law detector response, reciprocity theorem, filled aperture antennas, Hankel transform, and the mutual coherence function. There's also a list of astronomical radio sources that are commonly used for calibration.
Well, if that isn't quite enough to get you excited (yes, they're broadcasting all those old Star Wars movies again!), how about this: the book starts out with electromagnetism (Maxwell's equations, woo-hoo!) and wave polarization. Next are technical chapters on signal processing theory, practical receiver systems, antenna theory, and interferometry. After that there are chapters on emission mechanisms (there are two: thermal and non-thermal, i.e. heating and magnetism). Then, starting on page 333, it talks about radio sources: supernovas, pulsars, stellar wind, neutral hydrogen, and interstellar chemistry (mostly hydrogen and carbon monoxide).
There are lots of other books on the interstellar medium, where radio astronomy is now considered the best tool, and a gazillion books on signal processing and on the Big Bang. What's most unique here is the chapter on detectors and receivers.
I've always wondered why radio astronomers still use those big dishes instead of phased array antennas, which are on a simple, flat surface with no moving parts. The phased arrays they've just started using in 2018 (open access paper from Astronomical Journal here) still use enormously expensive dishes to do the focusing.
This book explains the reason: to get the noise factor down to acceptable levels, radio astronomers have to use superconducting transition edge sensor (TES) bolometers, pseudomorphic high-electron mobility transistors (PHEMTs), and mixers with superconducting-insulating-superconducting (SIS) junctions. These are well described at the level of detail an astronomer would need. Masers give a good noise figure but they're too complicated, and room-temperature dipoles as you'd find in a phased-array antenna can't compete with specialized detectors that require liquid helium.
There's almost nothing about the Sun here. The authors say space weather is an entirely different field, and that daytime and nighttime astronomers speak different languages.
The long wavelengths of radio waves also mean that high resolution radio astronomy relies on interferometry. Many complicated and touchy-looking autocorrelation algorithms are needed to combine signals from antennas separated by thousands of kilometers. Even so, to get rid of artifacts in the image, ordinary deconvolution isn't nearly good enough; on page 271 they show an example of a 6 cm image of radio galaxy Cygnus A. Before processing, it's mostly artifacts. To hammer it into shape, they have to use the maximum entropy method and self-calibration, which relies on known radio sources (hence the list in Appendix G) to clean up the image.
Only after all these technical problems are solved is it possible to do astronomy. Many people also don't realize that any magnetically inert object, such as the Moon, Venus, and asteroids, emits thermal radio waves, but the most interesting signals are produced by magnetic fields (as on Jupiter) or by molecular transitions (as in gas clouds). The many spectral lines from ammonia, carbon monoxide, and other molecules are hard to identify, but most of what's out there is hydrogen, so the king is still the 21-cm hydrogen line, which is known to very high precision. Tune your radio to 1.420405751786(30) GHz, point your antenna “up,” stock up on liquid helium, and don't touch that dial.
dec 20, 2018
Reviewed by T. Nelson
This book consists of 52 articles on various topics in radio astronomy from a conference at Jodrell Bank in 1996. While some of the longer articles are fascinating, most of the articles aren't long enough to say much beyond a basic reporting of major findings.
Achieving high sensitivity is a difficult problem in radio astronomy. Sensitivity and angular resolution are both acutely compromised at radio wavelengths; the first radio images of supernovas at some wavelengths have been acquired only recently. Only a tiny fraction of stars are even detectable with the current generation of radio telescopes. Most imaging of radio sources still requires either interferometry or tedious scanning one point at a time with a dish. Significant progress in this field will probably not only require new technology, but also putting ultra-large antennas out in space.
Most of the articles discuss such topics as circumstellar masers and determining the Hubble constant, and how these findings relate to theories of stellar evolution and stellar nuclear chemistry. Others merely catalog the radio sources that they have observed. The last section gives an overview of the bigger, better, badder antennas that are being planned. Only a few articles discuss technical issues, such as high electron mobility transistors or antenna design. The main value of this book would be to astronomers who missed the meeting--or who slept through it--and need to know which objects their colleagues are studying, their ascension and declination, and their intensities.
dec 17, 2005
Reviewed by T. Nelson
Radio astronomy started with Jansky's 1932 discovery of “star static.” Although it progressed slowly at first (it was some years before they were even able to detect the sun), radio astronomy eventually provided the essential clue, in the form of the microwave background, of the origin of the universe. Radioastronomy is also the only way we have of studying chemistry in space. This awkwardly shaped book, a reprint of the 1986 edition, is spiral-bound, or more properly helix-bound, perhaps in honor of Kraus's invention of the helical antenna. The publication quality is more typical of a 60s era book, with poor quality figures and blurry text. The field has changed dramatically in the 25 years since this book was published. In particular, hot topics like interferometry, which provides the highest resolution of any astronomical observing method, are scarcely mentioned. Modern receiver components like HEMTs are not mentioned at all. But Kraus was an outstanding educator, and Radio Astronomy is still highly regarded for its clear explanation of the mathematical background that is still essential for radioastronomy.
In contrast, An Introduction to Radio Astronomy by Burke and Graham-Smith (reviewed below), is more up to date, but far less rigorous, and has much less detail on antennas (which was Kraus's specialty) and other hardware. An excellent treatise on interferometry is Interferometry and Synthesis in Radio Astronomy by Thompson, Moran, and Swenson.
See also Antennas for All Applications by Kraus and Marhefka, an outstanding book on antennas.
This book describes how radio signals are received and processed in single-aperture radiotelescopes (i.e., dishes) and then moves on to aperture synthesis and interferometry. The authors give the most important equations, deriving some and pulling the rest out of a hat.
Starting with Chapter 8 we start getting some actual astronomy, with nice discussions of polarization, radio sources, and pulsars, and introductions to radio signals from the interstellar medium and, of course, the cosmic microwave background. In some places the science has moved on since this book was written; for example, they write that most of the carbon is in linear chains instead of, as we now know, nanodiamonds and polycyclic aromatic hydrocarbons. But most of the stuff here is still true, and it's a good introduction.
The figures are mostly contour plots, with many graphs and a few low-quality pictures. Watch out: CUP re-released this one in 2008, but it's still only the 2nd edition. A third edition was published in 2014.