Astrophysical Techniques

C.R. Kitchin

• Publisher: CRC Press
• Year: 2020
• ISBN:
  9780429956980
• Edition: 7
• Topic: Naturwissenschaften
• Subtopic: Astronomie & Astrophysik

Long used in undergraduate and introductory graduate courses, Astrophysical Techniques, Seventh Edition provides an accessible yet comprehensive account of the innovate instruments, detectors, and techniques employed in astronomy and astrophysics. Emphasizing the underlying unity of all astronomical observations, this popular textbook provides a coherent state-of-the-art account of the instruments and techniques used in current astronomy and astrophysics.

Fully updated throughout, this seventh edition builds upon the sixth edition, covering improved techniques and cutting-edge methods in the field, as well as other exciting new developments in gravitational waves, dark matter and energy, the use of photonics, and astronomy education and outreach, in addition to further detailed discussions on the latest scientific instruments and individual detectors.

The book is written in a very accessible manner, and most of the mathematics is accessible to those who have attended a mathematics course in their final years at school. Nevertheless, the treatment of the topics in general is at a sufficiently high level to be of use to those professionals seeking technical information in areas of astronomy with which they might not be completely familiar.

Key Features:

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• Details the instrumentation and theory of astronomical observations, including radio waves, gamma rays, cosmic rays, neutrinos, gravitational waves and dark matter and energy and more

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• Presents the background theory and operating practice of state-of-the-art detectors and instruments

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• Fully updated to contain the latest technology and research developments

1

Detectors

1.1 OPTICAL DETECTION

1.1.1 INTRODUCTION

In this and the immediately succeeding sections, the emphasis is upon the detection of the radiation or other information carrier and upon the instruments and techniques used to facilitate and optimise that detection. There is inevitably some overlap with other sections and chapters, and some material might arguably be more logically discussed in a different order from the one chosen. In particular in this section, telescopes are included as a necessary adjunct to the detectors themselves. The theory of the formation of an image of a point source by a telescope, which is all that is required for simple detection, takes us most of the way through the theory of imaging extended sources. Both theories are, therefore, discussed together, even though the latter should perhaps be in Chapter 2. There are many other examples such as X-ray spectroscopy and polarimetry that appear in Section 1.3 instead of Sections 4.2 and 5.2. In general, the author has tried to follow the route detection→imaging→ancillary techniques throughout the book but has dealt with items out of this order when it seemed more natural to do so.

The optical region is taken to include the mid/near infrared, the visible and the long-wave ultraviolet (UV) regions (sometimes called the Ultraviolet, Optical and Infrared [UVOIR] region) and, thus, roughly covers the wavelength range from 100 μm to 10 nm (3 THz to 30 PHz, 10 meV to 100 eV).¹ The techniques and physical processes employed for investigations over this region bear at least a generic resemblance to each other and so may conveniently be discussed together.

1.1.2 DETECTOR TYPES

In the optical region, detectors fall into two main groups: thermal and quantum (or photon) detectors. Both these types are incoherent; that is to say, only the amplitude of the electromagnetic wave is detected, and the phase information is lost. Coherent detectors are common at long wavelengths (Section 1.2), where the signal is mixed with that from a local oscillator (heterodyne principle). But only recently have heterodyne techniques been developed for wavelengths as short as the infrared (IR) and applied to astronomy (see Mid Infrared Laser Heterodyne Instrument [MILAHI], for example, Section 1.1.15.4). We may, therefore, safely regard almost all current astronomical optical detectors as still being incoherent in practice. With optical aperture synthesis (Section 2.5), some phase information may be obtained providing that three or more telescopes are used.

In quantum detectors, the individual photons of the optical signal interact directly with the electrons of the detector. Sometimes individual detections are monitored (photon counting), and at other times, the detections are integrated to give an analogue output like that of the thermal detectors. Examples of quantum detectors include the eye, photographic emulsion, photomultiplier, photodiode, charge-coupled devices (CCDs) and many other solid-state detectors.

Thermal detectors, by contrast, detect radiation through the increase in temperature that its absorption causes in the sensing element. They are generally less sensitive and slower in response than quantum detectors, but they have a much broader spectral response. Examples include thermocouples, thermistors, pyroelectric detectors, and bolometers.

1.1.3 THE EYE

This is undoubtedly the most fundamental of detectors to a human astronomer, although it is a way from being the simplest. It is now rarely used for primary detection except, of course, for the billions of people who gaze into the skies for pleasure – and that includes most professional astronomers.

If you live in a city, only a few stars may be visible, even on the clearest of nights. From a city’s suburbs you might see a few tens of stars, but throughout most urban or semi-rural parts of most countries, you will be lucky to see more than a hundred stars with the unaided eye. Because around 5,000 to 6,000 stars should be visible (2,500 to 3,000 at any given moment, of course, because half the sky is below the horizon), thousands of stars are ‘missing’. The ‘missing’ stars, though, have not truly vanished; they have just been swamped by the lights from the city.

In the last three decades or so, many high-publicity campaigns have been initiated to try and reduce light pollution and restore the glories of really dark skies to millions of city dwellers, but regrettably such efforts have had little effect. However, there are still some areas of most countries where light pollution is much reduced. Some of those areas now advertise their dark skies as tourist attractions and so can be found via an internet search. At the time of writing, eleven sites around the world have been identified as ‘International Dark Sky Reserves’. These are areas of at least 700 km² that have exceptional night sky quality and within which further human developments which might increase levels of light pollution are (at least somewhat) controlled. An internet search will quickly list these reserves and any more that may have been recognised recently. If you think you know your constellations but have learnt to identify them only from a ‘normal’ darkish site, then a visit to one of the reserves will amaze you; the constellations will no longer be recognisable because of the hundreds of additional stars that have suddenly become visible (of course, it will need to be a clear night, but that problem remains to be solved!).

Much more detailed coverage of the ways in which eyes and vision work, and how their idiosyncrasies may affect astronomical observations have been included in previous editions of this book. However, most astronomers, both amateur and professional, now use their eyes mainly for monitoring their instruments and for looking at computer VDUs or printouts, so such detailed coverage is no longer appropriate. If needed, further information of how the nature of the human eye affects astronomical observation may be found in the author’s book Telescopes and Techniques (2013), previous editions of Astrophysical Techniques, other introductory and observational astronomy sources and biological and medical sources dealing with the eye and ophthalmology.

One aspect of the eye’s behaviour though, will still be discussed briefly here because it is to be encountered in many aspects of observational astronomy, even today – and that is the use of magnitudes to measure the brightness or luminosity of objects in the sky, especially over the visual region (see also Section 3.1). The magnitude scale is used to measure the brightness of stars and other celestial objects as seen in the sky (i.e., the light energy arriving from the star at the surface of the Earth) and has its roots in eye estimates of stellar brightness made more than 2,000 years ago for Hipparchus’ star catalogue. In that catalogue, the brightest stars were first class, the next brightest were second class, then third class and so on down to the sixth-class stars which could only just be seen.

These classes became roughly our modern stellar magnitudes and so retain the (now unusual) custom of denoting the brighter of two stars by a numerically smaller value of the magnitude i.e., a magnitude 2 star is brighter than a magnitude 3 star, etc.

Additionally, the eye’s response to changes in illumination is approximately logarithmic (Fechner’s law²), so the magnitude scale varies logarithmically with the visual brightness of the stars and not linearly. That is to say; if two sources, A and C are observed to differ in brightness by a certain amount and a third source, B, appears to the eye as being midway in brightness between them, then the energy from B will differ from that from A by the same factor as that of C differs from B. Thus, if we use L to denote the perceived luminosity and E to denote the actual energy at the Earth’s surface of the sources, then for

$L_B=\frac{1}{2}(L_A+L_C)$

(1.1)

we have

$\frac{E_A}{E_B}=\frac{E_B}{E_C}\left[\text{o}\text{r}\log E_B=\frac{1}{2}(\log E_A+\log E_C)\right]$

(1.2)

In other words, a magnitude 2 star is not only brighter than a magnitude 3 star, it is 2.51 times brighter in energy terms. Similarly, a magnitude 3 star is not only brighter than a magnitude 4 star, it is 2.51 times brighter. The magnitude 2 star is thus 6.30 times (2.51 × 2.51) brighter than the magnitude 4 star.³

Fechner’s law and the magnitude scale’s ancient historic background are the reasons for the (awkward) nature of the magnitude scale used by astronomers to measure stellar brightness (Section 3.1).

The faintest stars visible to the dark-adapted naked eye from a good observing site on a clear moonless night are (still) of about magnitude six. This corresponds to the detection of about 3 × 10⁻¹⁵ W or about 8,000 visible light photons per second. Special circumstances or especially sensitive vision may enable this limit to be improved upon by one to one and a half stellar magnitudes (×2 to × 4). Conversely, the normal ageing processes in the eye, such as a decreasing ability to dilate the eye’s pupil and increasing numbers of ‘floaters’, etc., mean that the retina of a 60-year old person receives only about 30% of the amount of light seen by a person half that age.⁴ Eye diseases and problems, such as cataracts, ma...