Chapter 5 Telescopes Chapter 5 opener. This composite photograph shows two of the premier optical telescopes available to astronomers today. At the top, overflying Earth is HST—the Hubble Space Telescope—in orbit some 600 kilometers above the surface of our planet; operating well above Earth’s atmosphere, it has superb angular resolution. At the bottom, in the Atacama Desert high in the Chilean Andes is the new VLT—the Very Large Telescope—which is actually an array of four large telescopes working together to view the universe with equally superb resolution. (NASA; ESO)

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

Optical Telescopes Refracting lens: Light is bent Figure 5-2. (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. A light ray traveling along the axis of a lens is undeflected as it passes through the lens. Parallel rays arriving at progressively greater distances from the axis are refracted by increasing amounts in such a way that they all pass through a single point—the focus.

Optical Telescopes Images can be formed through reflection or refraction Reflecting mirror: light is bounced back to a central point Figure 5-3. (a) Reflection of light from a flat mirror occurs when light is deflected, depending on its angle of incidence. (b) A curved mirror can be used to focus to a single point all rays of light arriving parallel to the mirror axis. Light rays traveling along the axis are reflected back along the axis, as indicated by the arrowheads pointing in both directions. Off-axis rays are reflected through greater and greater angles the farther they are from the axis, so that they all pass through the focus.

Optical Telescopes Reflecting and refracting telescopes: Figure 5.5 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.

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 Lens needs two optically acceptable surfaces; mirror needs only one

Optical Telescopes Types of reflecting telescopes: Figure 5.7 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.

Optical Telescopes Keck telescope Figure 5-8. (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)

Hubble Space Telescope Main mirror - 2.4 m in diameter Designed for visible, infrared, and ultraviolet radiation

Hubble Space Telescope Comparison best ground-based image of M100, on the left, with the Hubble image on the right

Telescope Size Light-gathering power: Improves detail Brightness proportional to square of radius of mirror Below: (b) was taken with a telescope twice the size of (a) Figure 5-10. Effect of increasing telescope size on an image of 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. (Adapted from AURA)

Telescope Size Resolution is proportional to wavelength and inversely proportional to telescope size—bigger is better! Figure 5-13. 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.

Telescope Size Effect of improving resolution: (a) 10′; (b) 1′; (c) 5″; (d) 1″ Figure 5-14. Detail becomes clearer in the Andromeda galaxy as the angular resolution is improved some 600 times, from (a) 10’, to (b) 1’, (c) 5”, and (d) 1”. (Adapted from AURA)

Diffraction and Telescope Resolution Diffraction is an intrinsic property of waves, and limits telescope resolution depending on wavelength and size

Images and Detectors Image processing by computers can sharpen images Figure 5-16. 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 bluer wavelength. (AURA/NASA)

High-Resolution Astronomy Atmospheric blurring: Due to air movements Figure 5-18. 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.

High-Resolution Astronomy Solutions: Put telescopes on mountaintops, especially in deserts Put telescopes in space Figure 5-19. 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 state-of-the-art active-optics device. (ESO)

High-Resolution Astronomy Adaptive optics: Track atmospheric changes with laser; adjust mirrors in real time Figure 5-21. 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)

High-Resolution Astronomy These images show the improvements possible with adaptive optics: Figure 5-22. Adaptive Optics in Action (a) The improvement in image quality produced by such systems can be seen in these images acquired by the 8-m Gemini telescope atop Mauna Kea in Hawaii. The uncorrected visible-light image (left) of the star cluster NGC 6934 is resolved to a little less than 1”. With adaptive optics applied (right), the resolution in the infrared is improved by nearly a factor of 10, allowing more stars to be seen more clearly. (b) These visible-light images were acquired at a military observatory atop Mount Haleakala in Maui, Hawaii. The uncorrected image (left) of the double star Castor is a blur spread over several arc seconds, giving only a hint of its binary nature. With adaptive compensation applied (right), the resolution is improved to a mere 0.1”, and the two stars are clearly separated. (NOAO; MIT Lincoln Laboratory)

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 5-23. 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, showing the path taken by an incoming beam of radio radiation. (NRAO)

Radio Astronomy Largest radio telescope: 300-m dish at Puerto Rico Figure 5-24. Arecibo Observatory An aerial photograph of the 300-m-diameter dish at the National Astronomy and Ionospheric Center near Arecibo, Puerto Rico. The receivers that detect the focused radiation are suspended nearly 150 m (about 45 stories) above the center of the dish. 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)

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 5-26. 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)

Interferometry Combine information from several widely spread radio telescopes as if they came from a single dish Resolution will be that of dish whose diameter = largest separation between dishes Figure 5-27. 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)

Interferometry Involves combining signals from two receivers; the amount of interference depends on the direction of the signal Figure 5-28. 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.

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 5-29. Radio–Optical Comparison (a) VLA radio “image” (or radiograph) of the spiral galaxy M51, observed at radio frequencies with an angular resolution of a few arc seconds. (b) Visible-light photograph of that same galaxy, made with the 4-m Kitt Peak optical telescope and displayed on the same scale as (a). (NRAO/AURA)

Space-Based Astronomy Infrared radiation can image where visible radiation is blocked Generally can use optical telescope mirrors and lenses Figure 5-31. 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)

Space-Based Astronomy Infrared telescopes can also be in space; the image on the left is from the Infrared Astronomy Satellite Figure 5-32. Infrared Image (a) This infrared image of the Orion region was made by the Infrared Astronomy Satellite. In this false-color image, colors denote different temperatures, descending from white to orange to black. The resolution is about 1’. (b) The same region photographed in visible light with 1” resolution. The labels α and β refer, respectively, to Betelgeuse and Rigel, the two brightest stars in the constellation. Note how the red star Betelgeuse is easily seen in the infrared (part a), whereas the blue star Rigel is very faint. (NASA; J. Sanford)

Space-Based Astronomy Spitzer Space Telescope – orbiting the sun Figure 5-33. Spitzer Images Sample images from the Spitzer Space Telescope, now in orbit around the Sun, clearly show its camera’s capabilities. (a) The magnificent spiral galaxy, M81, about 12 million light-years away. (b) Its companion, M82, is not so serene, rather resembling a “smoking hot cigar.” ( JPL)

5.7 Space-Based Astronomy Ultraviolet observations Must be done in space Atmosphere absorbs almost all ultraviolet rays Figure 5-34. 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. Based on the velocity of the outflowing debris, astronomers estimate that the explosion itself must have occurred about 12,000 years ago. (b) This false-color image of the spiral galaxy M81 and its companion M82, (see also Figure 5.34) made by the Galaxy Evolution Explorer satellite, reveals stars forming in the blue arms well away from the galaxy’s center. (NASA; GALEX)

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 5-35. 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.

Space-Based Astronomy X-ray image of supernova remnant Figure 5-37. X-Ray Image 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. Here, color represents the intensity of the X-rays observed, from white (brightest) through red (faintest). Roughly 10,000 light-years from Earth and barely visible in the optical part of the spectrum, Cas A is now awash in brilliantly glowing X-rays spread across some 10 light-years. (NASA)

Space-Based Astronomy Gamma rays cannot be focused at all; images are therefore coarse Figure 5-38. Gamma-Ray Astronomy (a) This photograph of the 17-ton Compton Gamma-Ray Observatory (CGRO; named after an American pioneer in gamma-ray telescopy) was taken by an astronaut during the satellite’s deployment from the space shuttle Atlantis over the Pacific Coast of the United States. (b) A typical false-color gamma-ray image—this one showing a violent event in the distant galaxy 3C279, also known as a “gamma-ray blazar.” (NASA)

Full-Spectrum Coverage Much can be learned from observing the same astronomical object at many wavelengths. Here, the Milky Way: Figure 5-39. 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. (NRAO; NASA; Lund Observatory; MPI; NASA)

Summary of Chapter 5 Refracting telescopes make images with a lens Reflecting telescopes 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

Ferdinand the Bull