9. The next telescopes

Each question that telescopes have helped answer has led to new questions: what is dark matter and dark energy? How did the first galaxies form? Are there habitable, Earth-like exoplanets? To address these questions, a new generation of telescopes are being built. ‘The next telescopes’ describes some of these, including the three extremely large infrared/optical telescopes, equipped with adaptive optics systems, due to start operating in the next decade. Other new telescopes discussed are the Square Kilometre Array, a radio telescope that will soon be the world’s largest scientific instrument, and the James Webb Space Telescope due to be launched in 2018, which is the 100 times more powerful successor to the Hubble Space Telescope.

4. Windows in the sky

The atmosphere influences much of what can be seen through a telescope. Most of the atmosphere lies within 16 km from the Earth’s surface. Further out, the air becomes thinner until it merges with outer space. In the ionosphere—a layer 75–1000 km high—neutral atoms are ionized by solar radiation and high-energy cosmic ray particles arriving from distant parts of the Universe. ‘Windows in the sky’ explains electromagnetic radiation and the electromagnetic spectrum from gamma rays through to visible light and radio waves. Electromagnetic waves are transverse waves that can be polarized. The atmosphere acts as a filter and blocks cosmic electromagnetic radiation. Atmospheric turbulence distorts starlight resulting in ‘twinkling’ stars.

5. Instruments of light

In the eye, detection is done by the retina. In telescopes operating in different parts of the electromagnetic spectrum, light interacts with matter in different ways, and so a variety of detectors is used. ‘Instruments of light’ describes how, first, photographic, and then electronic light detectors have greatly increased the sensitivity of telescopes, and made it possible to record the state of the sky, as well as obtain information on the physical properties of stars and galaxies. It describes photometry, the quantum efficiency of light detectors, charge-coupled devices, and high-energy proton detectors, and explains the processes of diffraction grating and spectroscopy. Telescopes and their detectors are now inseparable.

1. Introduction

A telescope is defined as a ‘far-seeing’ optical instrument used to make distant objects appear to be nearer and larger, but this fails to capture the scope and significance of modern telescopes. The tremendous sensitivity and range of wavelengths with which telescopes now operate has transformed our knowledge of the Universe. The ‘Introduction’ outlines the aims of this VSI as a journey from Galileo Galilei’s telescope to telescopes of the future that will generate so much data that it will far exceed the current capacity of the Internet. It is about the science of telescopes: what they do, how they work, what we have learned by using them, and where they are going next.

8. Space telescopes

The last seven decades have seen telescopes launched into space, vastly enhancing the crispness of the images they produce and expanding the range of observable wavelengths to include UV, X- and gamma-ray wavelengths. ‘Space telescopes’ shows how the information gleaned from them has enabled us to make new discoveries and form much more complete astrophysical models. It documents the development of space telescopes—including the Hubble Space Telescope—and highlights key discoveries, such as cosmic microwave background radiation, the results of which have profound implications for cosmology. The Standard Cosmological Model is known as ‘lambda cold dark matter’, containing three main ingredients: baryonic matter, cold dark matter (the unknown form of gravitating matter), and the mysterious dark energy.

7. The radio sky

‘The radio sky’ considers radio telescopes that can see much longer wavelengths. Cosmic radio waves were first discovered in 1932 by Karl Jansky, with the first radio telescope built in 1937. Technology for radar systems advanced during World War II and then after the war scientists, such as Bernard Lovell and Martin Ryle, made use of the advances in electronics, radio technology, and digital computers, to found radio astronomy. Single-dish antennas—including the Lovell Telescope at Jodrell Bank Observatory—continue to play important roles. To improve angular resolution two antennas need to be operated as an interferometer. These are described along with the discovery of quasars, supermassive black holes, pulsars, and neutron stars.

6. A mirror held up to nature

‘A mirror held up to nature’ looks at some of today’s remarkable optical and infrared large telescopes, highlighting the technologies that made them possible and the science results that have revealed galaxies at vast distances and, closer to home, exoplanets. The quest for telescopes with ever greater light-gathering power never stops. The convergence of several diverse technologies has, in the last few decades, produced the Very Large Telescopes. Three new mirror technologies in the 1970s and 1980s—segmented, honeycomb, and meniscus types—allied with active control of mirror shape (active optics), the use of altazimuth mounts, and computing technology transformed telescope production and their abilities. Adaptive optics is also described.

3. Through the looking glass

The magnifying property of glass lenses has been known about since antiquity, but telescopes were first made in the 16th century, and in the early 17th century Galileo created a telescope powerful enough to see stars sixty-four times fainter than with unaided eyes. ‘Through the looking glass’ outlines the key features of the two main types of telescopes: reflectors and refractors. In order to see deeper into our cosmos, bigger and bigger telescopes have been needed. Three great reflector telescopes are described: George Hale’s 60-inch (1.5 m) Cassegrain Telescope from 1908, the 100-inch Hooker Telescope from 1917, and the 200-inch Hale Telescope that has played a leading role in astronomy and cosmology for at least three decades.

2. Grasping light

‘Grasping light’ begins by explaining the workings of the human eye, which collects light from all directions. It goes on to introduce the main features of light, describing its dual personality: sometimes it behaves like a wave, and sometimes like a particle—a photon. Three models of light are then considered: geometrical optics, wave optics, and photon optics. Two vitally important attributes of telescopes are the light collection area and the diffraction-limited angular resolution. Lenses and mirrors are used to focus the collected light to create an image. An eyepiece is also required to see the image and the attribute of magnification is also explained.