Apart from the Sun, everything in the Solar System emits the bulk of its energy in the infrared. Of course, in reflected optical light from the Sun we do see the planets, comets, zodiacal dust and asteroids, and much of what we have learned about the Solar System has come from optical telescopes. But to see the full picture and make the connection with other planetary systems, we need the infrared vision of the Solar System. We have seen that the infrared space missions IRAS, ISO and Spitzer made a series of discoveries about the Solar System and other planetary systems. In particular, the discovery by IRAS of dust debris disks opened the way for the exploration of other planetary systems in the process of formation. In this chapter, we focus on the debris disk in our own solar system: the asteroid belt, the zodiacal cloud, the Kuiper Belt, and comets and Centaurs. This leads on naturally to the exciting search for planets around other stars and ultimately for planets like Earth which could sustain life.

The five visible planets – Mercury, Venus, Mars, Jupiter and Saturn – have been known and studied since antiquity. In 1543, Nicolaus Copernicus (1473–1543) gave us our modern picture of the planets, including the Earth, orbiting the Sun in approximately a common plane, the ecliptic plane, with the Moon orbiting the Earth. In 1605, Johannes Kepler (1571–1630) showed that the orbits of the planets were approximately ellipses, and Newton in 1687 explained these orbits in terms of the inverse-square law of gravitation. The seventeenth century also saw the discoveries of moons around Jupiter by Galileo in 1610 and of Saturn’s moon Titan by Huygens in 1655. As we saw in Chapter 2, William Herschel discovered Uranus in 1781, and the eighth planet, Neptune, was discovered by Urbain Leverrier (1811–1877) and Jean-Couch Adams (1819–1892) in 1846. The nineteenth century also saw the discovery of asteroids in orbit between Mars and Jupiter and the discovery of the moons Phobos and Deimos around Mars.

William Herschel (Figure 2.1, Plate II) was one of the greatest astronomers of all time, certainly the greatest since Galileo. His achievements dominated his age in a way that did not happen in the twentieth century, even though there have been some great astronomers in our time. Herschel discovered the first new planet in recorded human history, Uranus; constructed telescopes of a size and quality which were unrivalled in his day; demonstrated the disk-shaped structure of our Milky Way Galaxy; and surveyed the whole northern sky for nebulae and demonstrated that many were star clusters. He showed that the universe was enormously larger than had previously been imagined and introduced the idea that stars are born and die. He thought that some nebulae might be distant star systems similar to the Milky Way, but later doubted this. But above all Herschel discovered that the Sun emits radiation beyond the red end of the spectrum – infrared radiation – and thereby opened up the astronomy of the invisible wavelengths that has been a dominant feature of astronomy since 1960.

Remarkably Herschel was an amateur, self-taught astronomer, and might have remained an obscure amateur but for his discovery of Uranus. His father was a bandsman in the Hanoverian army who also loved astronomy and who imparted his love of music and astronomy to his children. In due course, at the age of fourteen, William also became a bandsman, and father and son later campaigned with the Hanoverian Foot-Guards in the War of the Austrian Succession. Following the French occupation of Hanover, Herschel’s parents were concerned about William’s safety, so in 1757 they got him out of the army and sent him to England, at age eighteen, to seek his fortune as a musician.

The enormous impact of the Caltech Two Micron Survey in 1969 stimulated interest in developing specialized ground-based infrared telescopes. While far-infrared wavelengths are accessible only from space, atmospheric windows can be used in the near-infrared and submillimetre wavelengths from high mountaintop sites. From the late 1970s, large near-infrared and submillimetre telescopes began to be built on high-altitude sites, especially on the 4200-metre dormant volcanic peak of Mauna Kea, Hawaii.

The advent of these new infrared telescopes and the dramatic impact of the new infrared-array detectors in the 1980s generated a profusion of scientific discoveries. These ranged from high-redshift galaxies to studies of luminous dusty galaxies and the effects of gravitational lensing, the black hole in the heart of our Milky Way Galaxy, brown dwarf stars, protostars and protoplanetary systems.

LARGE GROUND-BASED NEAR-INFRARED TELESCOPES: UKIRT, IRTF AND CFHT

The first of the large, specialized infrared telescopes was the United Kingdom’s 3.8-metre UK Infrared Telescope (UKIRT), constructed between 1975 and 1978 on Mauna Kea, Hawaii. UKIRT was designed to be lightweight and cheap. It started work in 1979 and had an immediate impact on near-infrared astronomy. At about the same time, NASA’s 3-metre Infrared Telescope Facility (IRTF), with Eric Becklin as its first director, and the Canada-France-Hawaii 3.6-metre optical/infrared telescope also began work on Mauna Kea.

Pioneers Working from Mountaintops

In this chapter, I describe how pioneers working from mountaintops discovered new types of infrared stars, which turned out to be either dying stars shrouded in dust or stars in the process of being born. The leaders of this infrared revolution were the Americans Frank Low (1933–2009) and Gerry Neugebauer (b.1932). First efforts to map the sky with telescopes on balloons and rockets began the push to observe at longer wavelengths inaccessible from the ground. And theorists began to use the observed distributions of brightness with wavelength in the new infrared sources to model the distribution and properties of the interstellar dust in clouds around stars and spread through interstellar space.

The first pioneer of the modern age of infrared astronomy was Harold Johnson (1921–1980) (Figure 4.1). He and his group, then at the University of Texas at Austin, began in the late 1950s to work with lead sulphide detectors, which they cooled with liquid nitrogen to 77 K (−196 °C). Cooling made the detectors much more sensitive, and over the next few years Johnson’s group observed several thousand stars. Johnson had earlier defined the standard optical observing bands, which are denoted U (ultraviolet), B (blue), V (visual), R (red) and I (infrared).

One of the reasons Herschel and his discovery of infrared light do not resonate more strongly in the history of science is that it took so long for infrared astronomy to develop. During the 150 years after Herschel, from 1800 to 1950, progress was extremely slow. There were two main reasons for this slow progress. Firstly, the night is not ‘dark’ at infrared wavelengths; in fact it is very bright. The Earth’s atmosphere radiates strongly in the infrared as part of the greenhouse process, so even if we had infrared eyes we would have great difficulty picking out the stars against this bright foreground, even at night. The second and main problem was the slow progress in developing infrared detectors. Herschel’s thermometers could detect infrared radiation from the Sun, but to detect anything else something better was needed. In this chapter I describe the slow progress in detector technology through the nineteenth and early twentieth centuries, the detection of infrared radiation from the Moon during the classic expedition of Piazzi Smyth to Tenerife, the efforts between 1870 and 1914 to detect radiation from bright stars, and the crucial discovery in the 1930s of extinction of visible light by interstellar dust.

The first step on the road to improved infrared detectors was made by a German physicist, Thomas Seebeck (1770–1831). In 1821, he made a discovery that led to the invention of the thermocouple. He found that when a metallic strip is constructed of two different metals and then heated, a small electric current is generated in the strip. The thermocouple applies this discovery to the detection of heat, or infrared radiation, by measuring the current generated from a bimetallic strip when heated. It was this device, far more sensitive than the thermometer used by Herschel, that Piazzi Smyth would use to make the next major discovery in infrared astronomy, the detection of infrared radiation from the Moon, the second brightest astronomical object in the sky.

I find the slow emergence of infrared astronomy a moving story. It began with the revolutionary discovery by the self-taught William Herschel in 1800 of invisible radiation from the Sun, the significance of which took so long to be fully appreciated. Reading his methodical and imaginative papers makes his genius clear. Piazzi Smyth made the next step, with detection of infrared radiation from the Moon in 1856. I found his book about his expedition to Tenerife captivating, one of the great works of scientific popularization from the Romantic era. There followed another 50 years of painstaking work trying to detect the brightest stars and planets in infrared light and the slow progress of stellar infrared astronomy in the first half of the twentieth century, culminating in the work of Harold Johnson and his group. In 1930, Robert Trumpler discovered the key ingredient for understanding the infrared sky: interstellar dust.

We then come to the titans of the modern era, Frank Low and Gerry Neugebauer, repeatedly being told they were wasting their time as they tried to push astronomy into the infrared. They and their colleagues from many different groups deserve immense credit for their pioneering work in the 1960s and 1970s. This led to the explosive development of infrared astronomy following the launch of the IRAS satellite in 1983. It was such an exciting time to see the clouds of interstellar dust directly shining at us in infrared light and to find some of the most distant galaxies known at that time, infrared monsters convulsed in huge bursts of star formation. Considering where infrared astronomy had been only a decade or so earlier, it was wonderful to be using IRAS as a cosmological probe, linking the infrared galaxy distribution to the cosmic microwave background radiation left over from the Big Bang itself. IRAS was followed by ever more sophisticated space missions: ISO, Spitzer, Akari and today Herschel. And then there have been the giant ground-based telescopes, working either in the optical and near infrared or at submillimetre wavelengths. To be searching in the 1990s for submillimetre sources one thousand times fainter than those we had been trying to observe only 20 years earlier seemed like magic.

It has been a huge challenge to try to write the whole story of infrared astronomy, from the discovery of infrared radiation from the Sun by William Herschel (1738–1822) in 1800 through to the present day, to the discoveries being made by the space mission named after Herschel. I wanted to make this story accessible to the general reader with some interest in science but no scientific background. At the same time I wanted to make it a full and accurate account. Having lived and worked through the great period of infrared astronomy, from the 1960s to the present, I know many of the major figures whose work is described here, and I wanted to do them justice.

To reach the general reader, I have had to continually simplify the text, moving more complex and detailed material to the notes. Astronomy is a branch of physics, and physics is not an easy subject for someone who has perhaps not even studied it at school. I’ve provided a glossary of technical terms and tried to keep them to a minimum. The notes and very full bibliography allow the interested reader to explore the full details of a major area of science.

I have written about what I know, the science of infrared astronomy, and haven’t attempted to give the full story of the technological developments required to make this science possible. To give some idea of the huge army of people who work to provide the tools for science, an infrared space mission generally involves more than one thousand people in its design and construction. I was drawn into infrared astronomy in the early 1970s by my friend, and colleague at Queen Mary College London, Peter Clegg.

The IRAS surveys had demonstrated the origin of the dipole anisotropy in the cosmic microwave background (CMB) and seemed to have shown that the mean density of the universe could be close to the critical value. They had also shown that there was more structure on large scales than expected in the standard ‘cold dark matter’ model for the universe. In this chapter, I describe how the Cosmic Background Explorer (COBE) mission, launched in November 1989, measured the spectrum of the background radiation with unbelievable accuracy and then went on to detect the expected small fluctuations in the background that would be the precursors of galaxies and clusters of galaxies today. However, these fluctuations turned out to be stronger than expected, confirming the IRAS conclusion that an additional ingredient was needed to explain this structure. The choice seemed to be between massive neutrinos or a cosmological constant. This and many other conundrums about the universe were settled by the WMAP mission, launched in 2001. The beautiful detail of WMAP’s measurements of CMB fluctuations on both small and large angular scales heralded a new era of precision cosmology.

The Cosmic Background Explorer and the Spectrum of the Cosmic Microwave Background

During the IRAS mission of 1983, I recall that Mike Hauser, who took a lead role in developing the IRAS extended emission maps, was already working on the Cosmic Background Explorer mission. As far back as 1974, NASA had received three proposals for cosmological background radiation missions in response to a call for small- or medium-sized astronomical missions, which were known as the ‘Explorer’ series of missions. Although the mission selected on that occasion was IRAS, NASA chose members from each of the three cosmic background proposal teams to get together and propose a joint satellite concept. In 1977, this team converged on the idea of a polar orbiting satellite, COBE, that could be launched by either a Delta rocket or the space shuttle.

All astronomy starts from the night sky, the picture of the universe we see with our own eyes: the stars and constellations, the Milky Way. We have become used to the idea that there is a universe beyond the constellations, revealed by the giant telescopes of the astronomers. We can still imagine the Hubble Space Telescope to be a giant extension of our own eyes, and its images are still images made in visible light. Much harder to grasp is the universe that is revealed in invisible light, such as the infrared radiation that is the subject of this book. I’ve called the book Night Vision for two reasons. Firstly, infrared sensors and binoculars are already widely used to aid seeing in the dark, or night vision. And secondly, my goal is to try to make the infrared universe as familiar to you as the night sky, so that when you look out at the night sky you can also imagine what it would look like with infrared eyes, and therefore see the new vision of the night sky that the infrared gives us.

Above all, infrared astronomy is about the cool, dusty universe. Spread between the stars are tiny grains of dust, similar to sand and soot, and these absorb the light from stars and reradiate the energy as infrared radiation. There are dense clouds of gas and dust within which new stars are forming, and only with infrared light can we peer into them. Dying stars and massive black holes are often shrouded in dust, which shines in the infrared. And the cool bodies of the Solar System, planets, comets and asteroids are mainly radiating infrared light. In the infrared and submillimetre parts of the spectrum, we see galaxies in formation, undergoing violent bouts of star formation often caused by collisions between smaller fragments. And we see the cool glow left over by the hot fireball phase of the Big Bang itself.

This chapter describes the spectroscopic techniques that are used in most near-UV, visual, and IR spectrometers. Spectroscopy in this range is particularly important for astrophysics, and instruments for these wavelengths can be constructed using conventional optical elements, such as lenses, prisms, gratings, and normal-incidence mirrors. These instruments share many common features.

Commercially Available Spectrometers

Because optical spectroscopy plays an important role in many different branches of science, medicine, and industry, many manufacturers of optical instrumentation offer commercially produced spectrometers of different types. Most of these devices are not suited for the low light levels from astronomical sources. However, in addition to some instruments that have been designed specifically for astronomical applications (mainly for amateurs), there are commercial “low light level” general-purpose spectrometers on the market, which can be (and are) operated successfully for low-resolution spectroscopy at small telescopes. These commercially available instruments have the advantage of being complete systems, which include high-quality CCD or IR detectors. In many cases they can be conveniently connected to the USB port of a computer for instrument control and data readout. Usually the light from a telescope must be fed to the spectrometer using an optical fiber (see Section 4.5), but some of these spectrometers can be attached directly to a telescope focus. Observers interested in commercially available spectrometers will have no difficulties finding the addresses of potential suppliers by searching for optical spectrometers in the Internet.

So far, we have discussed two basic types of spectroscopic techniques. At high photon energies, the observations were based on the detection of individual photons. Their frequency was determined either by measuring their energy or by measuring their wavelength by means of optical effects. At low (radio) frequencies, the electromagnetic waves were directly recorded, and their frequency distribution was derived using electronic methods. As noted in Chapter 3 (Equation 3.42), under thermal equilibrium conditions, the detection of individual photons requires photon energies hv > kT. Thus, depending on the detector temperature, the transition between photon detection and radio-astronomical methods is expected to take place at FIR or submillimeter wavelengths. In practice, there exists a significant overlap of frequencies at which both types of detection methods can be used, and for which the preferred technique depends on the specific scientific objective. Moreover, in the submillimeter range it is sometimes of advantage to use bolometers, which record light indirectly by measuring the heat that is produced when photons are absorbed. Because of the choice of methods, special techniques have been developed for astronomical observations at these wavelengths, and sometimes combinations of radio and optical techniques are employed. A good example of the diversity of methods used in the FIR/submillimeter range are the three spectroscopic instruments of the Herschel Space Observatory (see Figure 9.4), which (as will be described later) use three different techniques.

The purpose of this chapter is to give a brief introduction to the special methods of the FIR/submillimeter range and to discuss their relative advantages and drawbacks for practical observations.

Although the grating instruments discussed in the preceding chapter dominate present-day astronomical spectroscopy at optical wavelengths, currently there are several other techniques in use at ground-based and space-based observatories. This chapter outlines the principles, the present applications, and the potential of three of these alternative methods.

Fabry-Perot Techniques

Fabry-Perot (FP) devices are based on the interference of light rays that are multiply reflected between partially transmissive mirrors. In astronomy, the main applications of the FP technique are special spectrometers and narrowband interference filters. FP spectroscopy was developed in the final years of the nineteenth century by the French physicists Charles Fabry and Alfred Perot. The technique was soon applied to astronomy. Alfred Perot himself used this method for measuring motions in the solar atmosphere.

The basic principle of the FP technique is outlined in Figure 5.1. The heart of any FP device is a pair of partially transmissive mirrors, separated by the distance l. In the context of FP devices, such a mirror pair is called an etalon (from the French designation of a standard length). In Figure 5.1, the light rays from the telescope are assumed to enter the space between the mirrors at angles γ to the mirror normals from the left (A). As indicated in the figure, the partially transmissive mirrors result in multiple reflections at the two mirror planes. At each reflection a fraction of the light passes through the mirror, while another fraction is returned into the space between the mirrors.

Radio astronomers typically use coherent receivers, which record and analyze the electromagnetic radiation directly. As was pointed out in Section 1.4, this facilitates spectroscopy at radio wavelengths. Making use of electronic frequency filters or of natural resonance effects, receivers can be built that are sensitive to a narrow frequency range only. If such receivers contain electronic components for which the parameters can be varied, the receivers can be “tuned” to specific frequencies. In this case, spectra can be be obtained by tuning narrowband receivers within a certain wavelength range, or by combining receivers that are tuned to different frequencies. As described in Section 1.4, the first radio spectra were obtained in this way, and commercially available radio spectrometers still use this technique. However, if spectra are obtained by tuning a receiver, the different frequencies are measured sequentially and the duty cycle for a given frequency is inversely proportional to the spectral resolution. Therefore, such spectrometers are inefficient and not well suited for the low radiation levels of faint astronomical radio sources. To study the very low signal levels from cosmic sources, more efficient methods must be used, which allow us to record many frequencies simultaneously. Some of these methods are described in the following sections.

The aim of this chapter is to provide an introduction to the technical background of the different types of radio spectrometers at a level that an observer needs to select the optimal instrument for a given task and to assess the potentials and the limitations of the different methods.