Chapter 5

Chapter 5 Telescopes Astronomers like to think big, and really big telescopes are now on the drawing board. This artist’s conception for the European Southern Observatory shows ELT—the Extremely Large Telescope. With a mirror diameter of nearly 40 m, ELT would combine unrivaled light-gathering power with the ability to examine cosmic objects with unprecedented detail. This largest telescope in the world will be built on Cerro Armazones, a 3000-m mountaintop in Chile’s Atacama Desert. (ESO/L. Calcada)

Units of Chapter 5 5.1 Optical Telescopes 5.2 Telescope Size 5.3 Images and Detectors 5.4 High-Resolution Astronomy 5.5 Radio Astronomy 5.6 Interferometry 5.7 Space-Based Astronomy Discovery 5-1 The Alma Array 5.8 Full-Spectrum Coverage

5.1 Optical Telescopes Refracting lens Figure 5-2 Refracting Lens. (a) Refraction by a prism changes the direction of a light ray by an amount that depends on the angle between the prism’s faces. When the angle between the faces is large, the deflection is large; when the angle is small, so is the deflection. (b) A lens can be thought of as a series of prisms.

5.1 Optical Telescopes Images can be formed through reflection or refraction Reflecting mirror Figure 5-3 Reflecting Mirror. (a) Reflection of light from a flat mirror occurs when light is deflected, depending on its angle of incidence. (b) Curved mirrors focus to a single point all rays of light arriving parallel to the mirror axis. The arrows indicate the directions of the incoming and reflected rays.

5.1 Optical Telescopes Reflecting and refracting telescopes Figure 5-5 Refractors and Reflectors. Comparison of (a) refracting and (b) reflecting telescopes. Both types are used to gather and focus electromagnetic radiation—to be observed by human eyes or recorded on photographs or in computers. In both cases, the image formed at the focus is viewed with a small magnifying lens called an eyepiece.

5.1 Optical Telescopes Modern telescopes are all reflectors: Light traveling through lens is refracted differently depending on wavelength Some light traveling through lens is absorbed Large lens can be very heavy, and can only be supported at edge A lens needs two optically acceptable surfaces; mirror needs only one

5.1 Optical Telescopes Types of reflecting telescopes Figure 5-6 Reflecting Telescopes. Four reflecting telescope designs: (a) prime focus, (b) Newtonian focus, (c) Cassegrain focus, and (d) Nasmyth/coudé focus. Each design uses a primary mirror at the bottom of the telescope to capture radiation, which is then directed along different paths for analysis. Notice that the secondary mirrors shown in (c) and (d) are actually slightly diverging, so that they move the focus outside the telescope.

5.1 Optical Telescopes The Keck telescope, a modern research telescope Figure 5-7 Keck Telescope. (a) The two 10-m telescopes of the Keck Observatory. (b) Artist’s illustration of the telescope, the path taken by an incoming beam of starlight, and some of the locations where instruments may be placed. (c) One of the 10-m mirrors. (The odd shape is explained in Section 5.3.) Note the technician in orange coveralls at center. (W. M. Keck Observatory)

5.1 Optical Telescopes The Hubble Space Telescope has a variety of detectors Figure 5-8a Hubble Space Telescope. (a) This “see-through” diagram displays some of HST’s hardware surrounding its main mirror (in light blue).

5.1 Optical Telescopes The Hubble Space Telescope’s main mirror is 2.4 m in diameter and is designed for visible, infrared, and ultraviolet radiation Here we compare the best ground-based image of M101, bottom, with the Hubble image on the top Figure 5-8b Hubble Space Telescope. (b) These two images compare the majestic spiral galaxy M101 as observed with the large Mayall telescope on Kitt Peak Mountain (bottom) and with the Hubble telescope in orbit (top). (D. Berry; AURA/NASA)

5.2 Telescope Size Light-gathering power: Improves detail Brightness proportional to square of radius of mirror Photo (b) was taken with a telescope twice the size of the telescope that took photo (a) Figure 5-9 Sensitivity. Telescope size affects the image of a cosmic source, in this case the Andromeda Galaxy. Both photographs had the same exposure time, but image (b) was taken with a telescope twice the size of that used to make image (a). Fainter detail can be seen as the diameter of the telescope mirror increases because larger telescopes are able to collect more photons per unit time, greatly extending our view of the universe. (Adapted from AURA)

5.2 Telescope Size Resolving power: When better, can distinguish objects that are closer together Resolution is proportional to wavelength and inversely proportional to telescope size—bigger is better! Figure 5-12 Resolving Power. Two comparably bright light sources become progressively clearer when viewed at finer and finer angular resolution. When the angular resolution is much poorer than the separation of the objects, as in (a), the objects appear as a single fuzzy “blob.” As the resolution improves, through (b) and (c), the two sources become discernible as separate objects.

5.2 Telescope Size Effect of improving resolution: (a) 10′; (b) 1′; (c) 5″; (d) 1″ Figure 5-13 Resolution. Detail becomes clearer in the Andromeda Galaxy as the angular resolution improves some 600 times, from (a) 10', to (b) 1', (c) 5", and (d) 1". The resolution of the human eye is approximately that of part (b)—if only our eyes were sensitive enough to see this view. (Adapted from AURA)

5.3 Images and Detectors Image acquisition: Charge-coupled devices (CCDs) are electronic devices, which can be quickly read out and reset Figure 5-14 CCD Chip. A charge-coupled device (CCD) consists of hundreds of millions of tiny light-sensitive cells called pixels. Light striking a pixel causes an electrical charge to build up on it. By electronically reading out the charge on each pixel, a computer can reconstruct the pattern of light—the image—falling on the chip. (a) Detail of a CCD array. (b) A CCD chip mounted for use at the focus of a telescope. (c) Typical data from the chip consist of an array of numbers, running from 0 to 9 in this simplified example. Each number represents the intensity of the radiation striking that particular pixel. (d) When interpreted as intensity levels on a computer screen, an image of the field of view results. (MIT Lincoln Lab; AURA)

5.3 Images and Detectors Image processing by computers can sharpen images Figure 5-15 Image Processing. (a) Ground-based view of the star cluster R136, a group of stars in the Large Magellanic Cloud (a nearby galaxy). (b) The “raw” image of this same region as seen by the Hubble Space Telescope in 1990, before its first repair mission. (c) The same image after computer processing that partly compensated for imperfections in the mirror. (d) The same region as seen by the repaired HST in 1994, here observed at a somewhat shorter (bluer) wavelength. (AURA/NASA)

5.4 High-Resolution Astronomy Atmospheric blurring is due to air movements Figure 5-16 Atmospheric Turbulence. Light rays from a distant star strike a telescope detector at slightly different locations because of turbulence in Earth’s atmosphere. Over time, the light covers a roughly circular region on the detector, and even the pointlike image of a star is recorded as a small disk, called the seeing disk.

5.4 High-Resolution Astronomy Solutions: Put telescopes on mountaintops, especially in deserts Put telescopes in space Figure 5-17 European Southern Observatory. Located in the Andes Mountains of Chile, the European Southern Observatory at La Silla is run by a consortium of European nations. Numerous domes house optical telescopes of different sizes, each with varied support equipment, making this one of the most versatile observatories south of the equator. The largest telescope at La Silla—the square building to the right of center—is the New Technology Telescope, a 3.5-m active optics device. (ESO)

5.4 High-Resolution Astronomy Active optics: Control mirrors based on temperature and orientation Figure 5-18 Active Optics These false-color infrared photographs of part of the star cluster R136—the same object shown in Figure 5.15—contrast the resolution obtained (a) without and (b) with an active optics system. Both images were taken with the New Technology Telescope shown in Figure 5.17. (ESO)

5.4 High-Resolution Astronomy Adaptive optics: Track atmospheric changes with laser; adjust mirrors in real time Figure 5-19 Adaptive Optics System. In this daytime photo, a test is being conducted at the Lick Observatory 3-m Shane telescope in California. A laser is used to create an “artificial star” (light reflected from the atmosphere back into the telescope) to improve guiding. The laser beam probes the atmosphere above the telescope, allowing tiny computer-controlled changes to be made in the shape of the mirror surface thousands of times each second. (Lick Observatory)

5.4 High-Resolution Astronomy These images show the improvements possible with adaptive optics Figure 5-20 Adaptive Optics in Action (a) The star cluster NGC 6934 is resolved to a little less than 1" in this uncorrected visible-light image (left) from the 8-m Gemini North telescope in Hawaii. With adaptive optics applied (right), the resolution in the infrared is improved by nearly a factor of 10, allowing more stars to be seen much more clearly. (b) These visible-light images of the double star Castor were acquired at a military observatory on Mount Haleakala in Maui, Hawaii. The uncorrected image (left) is blurred over several arc seconds, giving only a hint of its binary nature. With adaptive optics (right), the resolution is improved to 0.1", and the two stars are clearly separated. (NOAO; MIT Lincoln Laboratory)

5.5 Radio Astronomy Radio telescopes Similar to optical reflecting telescopes Prime focus Less sensitive to imperfections (due to longer wavelength); can be made very large Figure 5-21 Radio Telescope. (a) The 105-m-diameter device at the National Radio Astronomy Observatory in Green Bank, West Virginia, is 150 m tall—taller than the Statue of Liberty and nearly as tall as the Washington Monument. (b) Schematic diagram of the telescope shows the path taken by an incoming beam of radio radiation (colored blue). (NRAO)

5.5 Radio Astronomy Largest radio telescope is the 300-m dish at Arecibo Figure 5-22 Arecibo Observatory. An aerial photograph of the 300-m-diameter dish at the National Astronomy and Ionospheric Center near Arecibo, Puerto Rico. The left inset shows a close-up of the radio receivers hanging high above the dish. The right inset shows technicians adjusting the dish surface to make it smoother. (D. Parker/T. Acevedo/NAIC; Cornell)

5.5 Radio Astronomy Longer wavelength means poor angular resolution Advantages of radio astronomy: Can observe 24 hours a day Clouds, rain, and snow don’t interfere Observations at an entirely different frequency; get totally different information Figure 5-24 Orion Nebula in Radio and Visible. The Orion Nebula is a star-forming region about 1500 light-years from Earth. (The nebula is located in the constellation Orion and can be seen in Figure 1.8.) The bright regions in this photograph are stars and clouds of glowing gas. The dark regions are not empty, but their visible emission is obscured by interstellar matter. Superimposed on the optical image is a radio contour map (blue lines) of the same region. Each curve of the contour map represents a different intensity of radio emission. The resolution of the optical image is about 1"; that of the radio map is 1'. (Background photo: AURA)

5.6 Interferometry Interferometry: Combines information from several widely spread radio telescopes as if it came from a single dish Resolution will be that of dish whose diameter = largest separation between dishes Figure 5-25 VLA Interferometer. (a) This large interferometer, located on the Plain of San Augustin in New Mexico, comprises 27 separate dishes spread along a Y-shaped pattern about 30 km across. The most sensitive radio device in the world, it is called the Very Large Array, or VLA for short. (b) A close-up view from ground level shows how some of the VLA dishes are mounted on railroad tracks so that they can be repositioned easily. (NRAO)

5.6 Interferometry Interferometry involves combining signals from two receivers; the amount of interference depends on the direction of the signal Figure 5-26 Interferometry. Two detectors, A and B, record different signals from the same incoming wave because of the time it takes the radiation to traverse the distance between them. When the signals are combined, the amount of interference depends on the wave’s direction of motion, providing a means of measuring the position of the source in the sky. Here, the dark-blue waves come from a source high in the sky and interfere destructively when captured by antennas A and B. But when the same source has moved because of Earth’s rotation (light-blue waves), the interference can be constructive.

5.6 Interferometry Can get radio images whose resolution is close to optical Interferometry can also be done with visible light but is much more difficult due to shorter wavelengths Figure 5-27 Radio–Optical Comparison. (a) ALMA radio “image” (or radiograph) of the colliding Antennae Galaxies observed at radio frequencies with an angular resolution of a few arc seconds. (b) Visible-light photograph of that same galaxy, made with the Hubble telescope and displayed on the same scale as (a). (ESO/NAOJ/NRAO; STScI)

5.7 Space-Based Astronomy Infrared radiation can produce an image where visible radiation is blocked; generally can use optical telescope mirrors and lenses Figure 5-29 Smog Revealed. An optical photograph (a) taken near San Jose, California, and an infrared photo (b) of the same area taken at the same time. Infrared radiation of long wavelength can penetrate smog much better than short-wavelength visible light. The same advantage pertains to astronomical observations: An optical view (c) of an especially dusty part of the central region of the Orion Nebula is more clearly revealed in this infrared image (d) showing a cluster of stars behind the obscuring dust. (Lick Observatory; NASA)

5.7 Space-Based Astronomy Infrared telescopes can also be in space. This pair of images compares the Eagle Nebula in infrared (top), taken by the Herschel Space Observatory, with the same region in visible light. Figure 5.31 Infrared–Optical Comparison. (a) This infrared image of the Eagle Nebula was acquired with the Herschel Space Observatory. In this false-colored image, colors denote dust temperatures, descending from blue to red. (b) The same nebula in visible light shows clearly just how much of the region is obscured by the dust within it. (ESA; ESO)

5.7 Space-Based Astronomy The Spitzer Space Telescope, an infrared telescope, is in orbit around the Sun. These are some of its images. Figure 5-30 Spitzer Images. These images from the Spitzer Space Telescope, now in orbit around the Sun, clearly show its camera’s capabilities. (a) This unnamed star-forming region displays much dust (red-orange) amid myriad stars (blue-white). (b) The much larger spiral galaxy M100 also radiates heat from its embedded dust. (JPL)

Discovery 5-1 The ALMA Array The ALMA Array is a set of 66 radiotelescopes located in northern Chile, which detect radiation with wavelengths between 0.3 mm and 10 mm. The antennas are mobile, allowing excellent resolution.

5.7 Space-Based Astronomy Ultraviolet observing must be done in space, as the atmosphere absorbs almost all ultraviolet rays. Figure 5-32 Ultraviolet Images. (a) A camera on board the Extreme Ultraviolet Explorer satellite captured this image of the Cygnus loop supernova remnant, the result of a massive star that blew itself virtually to smithereens. The release of energy was enormous, and the afterglow has lingered for centuries. The glowing field of debris shown here within the telescope’s circular field of view lies some 1500 light-years from Earth. (b) This false-color image of the spiral galaxy M81 and its companion M82, made by the Galaxy Evolution Explorer satellite, reveals stars forming in the blue arms well away from the galaxy’s center. (NASA; GALEX)

5.7 Space-Based Astronomy X rays and gamma rays will not reflect off mirrors as other wavelengths do; need new techniques X rays will reflect at a very shallow angle and can therefore be focused Figure 5-33 X-Ray Telescope. (a) The arrangement of nested mirrors in an X-ray telescope allows the rays to be reflected at grazing angles and focused to form an image. (b) A cutaway 3-D rendition of the mirrors, showing their shape more clearly.

5.7 Space-Based Astronomy X-ray image of supernova remnant Figure 5-35 X-Ray Image. This is a false-color Chandra X-ray image of the supernova remnant Cassiopeia A, a debris field of scattered, hot gases that were once part of a massive star. Roughly 10,000 light-years from Earth and barely visible in the optical part of the spectrum, Cas A is awash in brilliantly glowing X-rays spread across some 10 light-years. (CXC/SAO)

5.7 Space-Based Astronomy Gamma rays cannot be focused at all; images are therefore coarse Figure 5-36 Gamma-Ray Astronomy. (a) The Fermi Gamma-Ray Space Telescope, shown here in an artist’s conception, was named after Enrico Fermi, an Italian-American scientist who did pioneering work in high-energy physics. The wide arrays are solar panels to power the spacecraft; the box amidship contains layers of tungsten that detect the gamma rays. (b) A typical false-color gamma-ray image—this one showing the remains of a violent event (a supernova) in a region named W44. The gamma rays are shown mainly in magenta. (NASA)

5.8 Full-Spectrum Coverage Much can be learned from observing the same astronomical object at many wavelengths. Here is the Milky Way. Figure 5-37 Multiple Wavelengths. The Milky Way Galaxy as it appears at (a) radio, (b) infrared, (c) visible, (d) X-ray, and (e) gamma-ray wavelengths. Each frame is a panoramic view covering the entire sky. The center of our Galaxy, which lies in the direction of the constellation Sagittarius, is at the center of each map. (ESA; UMass/Caltech; A. Mellinger; MPI; NASA)

Summary of Chapter 5 Refracting telescopes make images with a lens Reflecting telescopes make images with a mirror Modern research telescopes are all reflectors CCDs are used for data collection Data can be formed into image, analyzed spectroscopically, or used to measure intensity Large telescopes gather much more light, allowing study of very faint sources Large telescopes also have better resolution

Summary of Chapter 5 (cont.) Resolution of ground-based optical telescopes is limited by atmospheric effects Resolution of radio or space-based telescopes is limited by diffraction Active and adaptive optics can minimize atmospheric effects Radio telescopes need large collection area; diffraction limited Interferometry can greatly improve resolution

Summary of Chapter 5 (cont.) Infrared and ultraviolet telescopes are similar to optical Ultraviolet telescopes must be above atmosphere X rays can be focused, but very differently than visible light Gamma rays can be detected but not imaged