by George Rhee
Fig. 11.5Schematic view of the increase in collecting area of individual telescopes over time. We start with Galileo in 1609 and end with the 10 m diameter Keck telescope and the planned Extremely Large Telescope (ELT) with a planned diameter of 40 m. Improvements in technology have increased the collecting area of the largest telescope by about a factor 10 every 100 years. After 1900 all the telescopes used mirrors to focus the light, with mirrors being made of glass covered by a thin layer of silver
It is not just increased mirror size that has enabled us to study fainter objects. Detector improvements alone have produced an increase in sensitivity of 10,000. The size of detectors has also increased. When CCD detectors were first introduced into astronomy their size was about 1 cm2, whereas in 2011 we are up to 1,500 cm2 detectors. This size increase is achieved by building mosaics of smaller CCD devices. The idea behind this is that we can image a larger area of the sky in one exposure thereby increasing the efficiency of telescopes. Of course the telescopes then produce data at a much higher rate raising issues of data storage and access.
Computing capacity and data storage double every few years. From 1970 to 2000, the total collecting area of telescopes in the world increased by a factor 500 however the CCD detector area went up in the same period by a factor 3,000.
The cost of these tools also increases very roughly as the cube of the mirror diameter. The 8–10 m class telescopes such the Keck Observatory and the ESO-VLT telescopes cost about 100 million dollars. Moving to the 30–40 m class telescopes raises costs to over 2 billion dollars.
Discoveries made by the Keck telescopes include, the detection of extra solar planets, locating and understanding gamma-ray bursts, using supernovae to show that the cosmic expansion is accelerating. Note that none of these developments were foreseen when the Keck telescope was planned in 1985. This is the nature of innovative work. As California musician Mickey Hart put it “Magic won’t happen unless you set a place at the table for it”. In this spirit, Caltech is developing a 30 m telescope in collaboration with the University of California, Canada and Japan. The Carnegie Institution is working on the Giant Magellan Telescope, a 21 m telescope. The European Southern Observatory is developing the ESO Extremely Large Telescope with a 42 m mirror made of nine hundred 1.4 m segments (Fig. 11.6). These telescopes will be designed to compensate for atmospheric turbulence. This is done by observing a reference star or even an artificial laser guide star to measure the atmospheric distortion and correct for it.
Fig. 11.6How the E-ELT may look in the middle of the decade when under construction at Armazones, close to the VLT site at El Paranal in the Atacama Desert, Chile. When completed, the 40m mirror will be the largest optical/near infrared telescope in the world (Credit: ESO/L. Calçada)
Computers as Observational Tools: The Virtual Observatory
Astronomers, thanks to detector and telescope improvements, are drowning in an avalanche of data. To see this we can quantify information in units known as bytes. One byte of data corresponds to one letter of the alphabet stored on a computer hard drive. A book such as this one corresponds to about 1 million bytes (106), the King James bible takes up about 4 million bytes of storage or 4 MB. The Library of Congress contains about 32 million books which adds up to about 32 TB (or 32,000 GB). Coincidentally 32 TB is the amount of data that the Large Synoptic Survey Telescope (LSST) will produce in one night. In a year the LSST will produce an amount of data equivalent to one billion books. Dealing with data bases this large causes multiple problems. One has to store the data, organize the data, access the data and deliver it to the astronomical community in some standard format. These problems have been addressed for the Sloan Digital Sky Survey data. The web interface for obtaining Sloan Survey Data known as the Sky Server is used to retrieve data from the 5 TB digital catalog. The Sky Server has been accessed 400 million times by 1 million distinct users in 6 years. The galaxy zoo team went one step further and invited members of the public to help classify galaxy images after taking some online training and a short test. Some 40 million galaxy images have been classified in this manner (see their website www.galaxyzoo.org if you want to join in the fun).
One key element of experimental science is repeatability. It is essential for scientists to communicate their findings in such a way that the experiment can be repeated or at least check results using the same datasets. Astronomers have developed the International Virtual Observatory Alliance to achieve this. The idea is to form a multiwavelength digital sky that can be searched, visualized, and analyzed in new and innovative ways.
Review: The Next Ten Years
Hubble’s successor, the James Webb Space Telescope will be launched in 2018. This telescope will be more fully described in the next chapter. It should open up a new window into the high redshift universe and detect the first galaxies to form after the Big Bang. By 2021 the Large Synoptic Survey telescope will be up and running. It will use an 8.4 m mirror to image the sky every 4 days. It will be uniquely suited to measuring changes in the sky such as stellar flares and supernova explosions. The telescope will also discover many new objects in our solar system such as asteroids. The Square Kilometer Array radio telescope will start operation in 2020. This telescope will map the neutral hydrogen distribution and search for the imprint of the first stars and galaxies. The plan is for three 30m mirror optical telescopes to be working around 2020. Funding is partially secured. It is reasonable to expect that at least one such telescope will be working in each hemisphere. Very exciting times lie ahead for a new generation of astronomers.
Further Reading
Immanuel Kant versus the Princes of Serendip: Does science evolve through blind chance or intelligent design? S. Glashow. Contributions to Science, 2(2): 251–255, 2002.
What’s the Use of Basic Science? C. Llewellyn Smith, www.jinr.ru/section.asp?sdid=94
The Impact of Astronomy, A. Fabian, Astronomy and Geophysics 51, 3.25–3.30, 2010.
New Worlds, New Horizons in Astronomy and Astrophysics, Committee for a Decadal Survey of Astronomy and Astrophysics; National Research Council, 2010. The National Academies Press.
Telescopes of the Future, Roger Davies, Astronomy and Geophysics, Volume 53, 2012
George RheeAstronomers' UniverseCosmic Dawn2013The Search for the First Stars and Galaxies10.1007/978-1-4614-7813-3_12© Springer Science+Business Media, LLC 2013
12. Tour de Force: The James Webb Telescope
George Rhee1
(1)Department of Physics & Astronomy, University of Nevada, Las Vegas, Nevada, USA
Abstract
The James Webb Space Telescope, known as JWST, is the next big step for observational astronomy. Figure 12.1 shows a scale model of the telescope on the campus of Johns Hopkins University. The telescope has a six and a half meter diameter mirror made of 18 smaller segments. It features a sunshield that is the size of a tennis court. The telescope has to be folded up in order to fit into the Ariane rocket that will launch it. It will take a series of complex movements in space to deploy the full telescope (see the clip on youtube entitled ‘JWST deployment’ for an animation depicting the process). JWST will operate 1 million miles from the Earth, about four times further from the Earth than the Moon is. Unlike the Hubble Space Telescope JWST will be too far away for astronauts to carry out service missions. JWST is designed to find the first stars and galaxies but will also be able to detect earth-like planets. In fact if JWST was located 25 light years from the solar system it would still be able to detect the Earth. The telescope can also peer into the molecular clouds where stars are born and see planets as they form. The telescope is designed to work at infrared wavelengths which is ideal for searching for the first galaxies.
There are other Annapurnas in the lives of men.
Maurice Herzog
The James Webb Space Telescope, known as JWST, is the next big step for observational astronomy. Figure 12.1 shows a scale model of the telescope on the ca
mpus of Johns Hopkins University. The telescope has a six and a half meter diameter mirror made of 18 smaller segments. It features a sunshield that is the size of a tennis court. The telescope has to be folded up in order to fit into the Ariane rocket that will launch it. It will take a series of complex movements in space to deploy the full telescope (see the clip on youtube entitled ‘JWST deployment’ for an animation depicting the process). JWST will operate 1 million miles from the Earth, about four times further from the Earth than the Moon is. Unlike the Hubble Space Telescope JWST will be too far away for astronauts to carry out service missions. JWST is designed to find the first stars and galaxies but will also be able to detect earth-like planets. In fact if JWST was located 25 light years from the solar system it would still be able to detect the Earth. The telescope can also peer into the molecular clouds where stars are born and see planets as they form. The telescope is designed to work at infrared wavelengths which is ideal for searching for the first galaxies.
Fig. 12.1This full-scale model of the James Webb Space Telescope is constructed mainly of aluminum and steel, and is approximately 80 ft long, 40 ft wide and 40 ft tall. In September of 2005, the Webb Telescope team took a group photo with it on the lawn at Goddard Space Flight Center. Seeing the people gathered next to it shows its scale nicely (Credit: NASA)
The idea for JWST (the successor of Hubble) originated in the 1980s before Hubble had even been launched. This is because of the long delay that it takes from having the idea for a telescope to actually having a working telescope. One way to make new discoveries is to make measurements at fainter light levels than have been made before. How faint should one go? In his book Cosmic Discovery, Martin Harwit argues that one has to observe objects 100–1,000 times fainter than current limits to make substantial progress. To achieve this one needs a telescope mirror at least three times larger than the previous one, which is one reason the JWST mirror is 6 m in diameter (versus roughly 2 m for the Hubble Space Telescope). In the 1990s it became clear that infrared detector technology would make it possible to greatly increase the power of telescopes. In the spirit of pushing the envelope to fainter limits it made sense to design a large mirror telescope optimized for the infrared. It was also clear that infrared imaging and spectroscopy would provide the tools for discovering the first stars and galaxies.
Astronomers use infrared observations to detect the most distant galaxies because the neutral hydrogen absorption feature which occurs in the ultraviolet is observed in the infrared for the highest redshifts of interest. The second reason for choosing infrared wavelengths is that infrared light emitted by young stars and planetary disks can escape from dust clouds. Infrared detector technology has improved to the point that the field is ripe for new discoveries. Why do we wish to put this complex technology at high expense in the harsh environment of space? The answer is to get above the Earth’s atmosphere. At infrared wavelengths of 3 μm, the night sky seen from Mauna Kea Observatories (our best ground based sight) is 100 times brighter than the sky brightness at the proposed location in space of JWST. Just as it is difficult to see faint stars in the bright sky when the full Moon is up, it is difficult to see faint objects from the ground in the infrared.
To push the limits of technology in the harsh environment of space is expensive. The Hubble Space Telescope cost about 2 billion dollars to design and develop, the latest plan for JWST estimates costs at about 9 billion dollars with a launch date in 2018. With such large sums at stake the community has to make a strong case for the necessity of such a research tool based on current research problems. However, as we have seen, the most interesting discoveries made with new scientific instruments were often not anticipated by those who built and designed them.
The JWST mirror (Fig. 12.2) has an area 50 times larger than that of the Spitzer infrared space telescope. Infrared mirrors have to be cooled to temperatures well below freezing so that the mirror itself is not a source of radiation. JWST will have four instruments. For imaging there will be a near infrared camera with a large field of view that can see great detail. The second instrument, the near infrared spectrograph will allow scientists to take spectra of up to 100 objects at one time. A spectrograph spreads light out into its constituent wavelengths much like a prism, so that one can measure the intensity of light at various colors. This technique is used to measure galaxy redshifts. The third instrument operates both as an imager and as a spectrograph at mid-infrared wavelengths. The fourth instrument, the tunable filter imager will be able to select specific wavelengths for imaging. JWST will be launched in an Ariane rocket from Kourou in French Guyana.
Fig. 12.2JWST has a 6.5 m diameter primary mirror, which provides a larger collecting area than the mirrors available on the current generation of space telescopes. Hubble’s mirror is a much smaller 2.4 m in diameter. JWST will have a significantly larger field of view than the infrared camera on Hubble (covering more than 15 times the area) and significantly better spatial resolution than is available with the infrared Spitzer Space Telescope (Credit: NASA)
The Scientific Objectives
The four broad mission goals of JWST are; to search for the first stars and galaxies, map the evolution of galaxies, study the formation of stars and planets in the universe today, and, search for earth-like planets that might harbor life.
The first goal is the subject of this book. The dropout technique for searching for high redshift galaxies discussed in Chap. 9 can be extended to the infrared to reach redshifts beyond ten. This requires the ability to image to very faint light levels in the infrared which is exactly what JWST is optimized to do. Observations of the cosmic background radiation suggest that between a redshift of 15 (300 million years after the big bang) and 6 (900 million years after the big bang) the universe became ionized due to the ultraviolet radiation emitted by the first stars and galaxies. We hope that JWST will actually be able to see these objects directly.
The second goal is to understand how the elliptical and spiral galaxies we see around us today were assembled. This can be done by looking further and further into the past (higher redshifts) to see how the properties of galaxies change with time. We would like to know when and where the stars that we see in present day galaxies were formed. All four instruments of JWST can tackle this problem.
The third goal addresses another fundamental area of astronomy; the birth of stars and planetary systems. We need to understand star formation in order to solve the problems of the formation and evolution of galaxies.
The fourth goal is to determine the physical and chemical properties of planetary systems. How do planets form? How common are giant planets? How do giant planets affect the formation of terrestrial planets? The most exciting prospect is finding evidence of life on other planets. The instruments on board JWST should be able to detect carbon dioxide, water and oxygen in planetary atmospheres which may provide indirect evidence of life on such planets.
The Mirrors and the Sunshield
JWST has a sunshield that is about the size of a tennis court (Fig. 12.3). The sunshield is there to protect the telescope from light and heat from the Sun and the Earth. The sunshield consists of five layers, it will be stored for launch and unfurl on the way to the final location of the JWST. The sunshield ensures that the telescopes remains cold.
Fig. 12.3The JWST sunshield is about 22 m by 12 m (69.5 ft × 46.5 ft). It’s almost as big as a Boeing 737 airplane (Credit NASA)
JWST has a primary mirror that is made of many segments. The mirror structure can fold to fit into the payload bay of an Ariane 5 rocket. The 18 mirror segments of JWST are made out of beryllium and coated with gold which is an excellent reflector of infrared light. Segmented mirrors allow the overall shape of the mirror to be changed while the telescope is in space. Each segment is attached to six legs allowing the mirrors to tilt, twist and shift to face the correct direction and position. A pressure pad at the center of each segment can be moved like a piston. Unlike the Hubble tel
escope, the Webb telescope is not enclosed in a tube. The function of the tube is to block out unwanted light. JWST is open to space in order to keep the telescope cool enough for the infrared detectors to work properly. The open design is the only way for a telescope the size of JWST to keep at the right operating temperature. Coolant is used to maintain most infrared telescopes at the low temperatures required for carrying out observations. JWST is so large that many tons of coolant would be needed and the telescope would be unusable once the coolant was used up.
The sunshield divides JWST into a hot and cold side. The hot side of the sunshield is exposed to sunlight and parts of the telescope on that side will get as hot as 185 ∘ Fahrenheit. On the cold side of the sunshield, facing away from the sun, the temperatures will be about − 388 ∘ Fahrenheit, much colder than the coldest recorded temperatures on Earth. The science instruments and mirrors are located here.
NASA has funded a 4 year research program to develop the mirror technology for JWST. Two test mirrors were built, one out of beryllium and one out of glass. The beryllium mirror was selected because it could hold its shape at very cold temperatures.
The telescope must be placed at a special location in orbit for the heat shield to function properly. At this location when viewed from JWST (Fig. 12.4) the Sun, Earth and Moon lie more or less in a line that includes JWST, so the sunshield shields the telescope from radiation emitted by all three objects. L2 is named after Joseph Louis Lagrange who studied the interactions of three objects such as asteroids. He was searching for a stable configuration where three objects could orbit each other while staying in the same relative positions. In our case the three bodies would be the Sun, Earth and JWST. Normally an object circling the Sun beyond the Earth’s orbit would take more than 1 year to orbit the sun. However when we include the effect of the Earth’s gravitational force the extra force means that there is a location L2 where an object beyond the Earth will orbit the Sun in 1 year, so JWST will keep up with the Earth as it goes around the sun. The WMAP satellite was successfully operated at this location.
