The Tools of Astronomy Answer Key Part 2

35) What is "seeing"? A) a measure of the quality of the telescope's optics B) a measure of the transparency of the scope's objective lens C) a measurement of the sharpness of vision of the astronomer's eyes D) a measurement of the image quality due to air stability E) a measurement of clarity and absence of clouds Answer: D 36) A mountain top is an especially good site for infrared telescopes since: A) there you are closer to celestial objects. B) you are above most of the carbon dioxide and water vapor in the atmosphere. C) the cold weather helps the sensitivity of infrared detectors. D) less air above means better seeing in many cases. E) All of the above are factors. Answer: E 37) What problem does adaptive optics correct? A) defects in the optics of the telescope, such as the original Hubble mirror B) the opacity of the Earth's atmosphere to some wavelengths of light C) the light pollution of urban areas D) turbulence in the Earth's atmosphere which creates twinkling E) chromatic aberration due to use of only a single lens objective Answer: D 38) What problem is adaptive optics designed to correct? A) the opacity of the Earth's atmosphere B) defects in the optical figuring, such as with the adaptive optics on HST C) slight tracking errors in trying to compensate for our unsteady rotation D) the effects of atmospheric turbulence E) the absorption of ultraviolet by the ozone layer Answer: D 39) The Arecibo radio telescope is laid out like which optical telescope design? A) prime focus reflector B) Newtonian reflector C) Cassegrain reflector D) Coude reflector E) grazing incidence reflector Answer: A 40) What is true of radio telescopes? A) They have poorer angular resolution than a refractor. B) They have better angular resolution than a reflector. C) They are the smallest, most compact telescopes. D) They can only be used above the atmosphere. E) They are most sensitive to the opacity of the ozone layer. Answer: A 41) Which of the following is a problem inherent in all large radio telescopes? A) They are badly affected by poor seeing and atmospheric turbulence. B) The lightest breeze shakes them, making the observations blurry. C) Their waves are blocked by water vapor, so they must be located in deserts. D) Radio waves have long wavelengths, so radio telescopes have poor resolution. E) The dust clouds in the Milky Way block almost all wavelengths except light. Answer: D 42) Radio dishes are large in order to: A) attract funding from NASA and the NSF. B) give greater magnification. C) increase their angular resolution and collect the very weak radio photons. D) increase the range of waves they can collect. E) detect shorter waves than optical telescopes for superior resolution. Answer: C 43) Compared to optical telescopes, radio telescopes are built large because: A) they're less expensive to make than optical telescopes. B) radio photons don't carry much energy. C) atmospheric turbulence is more of a problem. D) radio sources are harder to find. E) radio waves are absorbed by the atmosphere. Answer: B 44) Radio telescopes have poorer angular resolution than optical telescopes because: A) the reflecting surfaces in radio telescopes have holes in them. B) radio telescopes often use multiple telescopes to make observations. C) radio is distorted more by Earth's atmosphere than visible light is. D) the wavelength of radio waves is longer than that of visible light. E) Radio telescopes actually have better resolution than optical telescopes. Answer: D 45) In astronomy, an interferometer can be used to: A) yield better seeing conditions with optical telescopes. B) decrease the effects of light pollution in getting darker sky backgrounds. C) improve the angular resolution of radio telescopes. D) increase the sensitivity of infrared telescopes to longer wavelengths. E) speed up the processing of CCD images. Answer: C 46) This design combines the radiation from two different telescopes to greatly enhance resolution via computer synthesis. A) Cassegrain reflector B) Newtonian reflector C) prime focus reflector D) refractor E) interferometer Answer: E 47) Which of the following greatly improves the angular resolution of radio maps? A) Schmidt corrector plates B) more sensitive spectrometers C) switching from photographic plates to CCD imaging D) use of interferometers E) chilling the infrared detectors Answer: D 48) The Atacama Large Millimeter Array achieves exceptional resolution by using exceptionally: A) large radio antennas in a fixed array. B) large radio antennas in a mobile array. C) many radio antennas in a fixed array. D) many radio antennas in a mobile array. E) many portions of the electromagnetic spectrum. Answer: D 49) In which part of the electromagnetic spectrum have astronomers been unable to get any information? A) gamma rays B) X-rays C) ultraviolet D) microwaves E) We now can access information in all spectral lengths. Answer: E 50) Which of the following is currently supplying high resolution X-ray images from space? A) ROSAT B) Chandra C) Einstein D) HEAO-2 E) COBE Answer: B 51) The design of modern X-ray telescopes depends on: A) lenses made of germanium. B) the prime focus design, with mirrors made of iron. C) grazing incidence optics. D) achromatic lenses to keep the X-rays in focus. E) the Cassegrain design, with mirrors made of lead. Answer: C 52) The name of the Infrared Orbiting Observatory launched in 2003 is the: A) Hubble Space Telescope. B) Compton Observatory. C) Spitzer Space Telescope. D) Chandra Orbiting Telescope. E) Newton Imaging System. Answer: C 53) The detectors on the Spitzer Space Telescope were cooled to near absolute zero in order to: A) allow the electronics in these instruments to operate most efficiently. B) keep the telescope more nearly stationary. C) prevent the infrared radiation from the instruments interfering with the infrared signals from space. D) improve the angular resolution of the telescope. Answer: C 54) The Chandra X-ray Observatory must use: A) a lens of fluorite. B) grazing incidence optics to focus the short wavelengths. C) a Newtonian reflecting design to avoid chromatic aberration. D) X-ray photographic plates instead of CCDs for imaging. E) a Schmidt corrector plate to avoid spherical aberration. Answer: B 55) Which of the following telescopes observes the highest energy photons? A) Hubble B) Chandra C) Fermi D) James Webb E) ALMA Answer: C 56) The James Webb Space Telescope is different than other space telescopes because it A) is launched on a rocket. B) uses a segmented mirror. C) observes infrared light. D) is more compact. E) is closer to the Sun. Answer: B 57) Which of the following telescopes does (did) NOT observe infrared light? A) Hubble B) Herschel C) Spitzer D) Chandra E) James Webb Answer: D 5.3 Fill-in-the-Blank Questions 1) The light-gathering power of a telescope varies with the ________ of the diameter of the lens or mirror. Answer: square 2) The ability of a telescope to separate two closely spaced stars is called ________. Answer: resolution 3) Because of ________, images in small telescopes are more blurred than with larger objectives. Answer: diffraction 4) Very accurate measurements of stellar brightnesses and sudden changes are made with a(n) ________. Answer: photometer. 5) If the ________ is good, the atmosphere is stable, and image quality is sharp. Answer: seeing 6) The twinkling of starlight and the focusing of images by lenses are due to ________ of light. Answer: refraction 7) The ________ is a measure of the stability of the atmosphere, usually degraded by turbulent currents in the air. Answer: seeing 8) Radio telescopes image a universe much ________ in temperature than with visible light. Answer: colder 9) If used in a single instrument to greatly enhance its resolution, the four separate mirrors of the VLT will be acting as a(n) ________. Answer: interferometer 10) The Fermi Space Telescope observes ________ radiation. Answer: gamma-ray 11) The James Webb Space Telescope is optimized to observe ________ radiation. Answer: infrared 12) The Chandra Space Telescope observes ________ radiation. Answer: X-ray 5.4 Short Answer Questions 1) Contrast image formation in reflectors and refractors. Answer: Reflectors use a curved mirror to reflect the light back to a focus, while refractors use a convex lens to bend the transmitted light to a focus. 2) Which reflector telescope would be the easiest to construct, and why? Answer: The prime focus reflector needs only a single optical surface, on the front of the main mirror. 3) Explain why the image seen in astronomical telescopes is inverted. Answer: In both reflectors and refractors, the eyepiece must be located behind the prime focus, so the light reaching it has "flipped over" in passing through the focus. 4) You are a novice telescope builder. Should you attempt a 6" Newtonian or Cassegrain reflector first? Why? Answer: The Newtonian uses only a curved main mirror and a small flat secondary. The primary for the Cassegrain also needs a hole to be drilled in its center, and its small secondary is convex in shape, harder to make than the flat for the Newtonian. 5) Why is it easier to travel to a remote viewing site with an 8" Cassegrain than an 8" Newtonian reflector? Answer: The Cassegrain design is much more compact, and thus easier to carry, transport, and set up. 6) How does the placement of the eyepiece with Newtonian and Cassegrain reflectors differ? Answer: In the Newtonian, the eyepiece is located on the front side of the tube, but with the Cassegrain, it lies behind the hole in the main mirror. 7) Why doesn't the Hubble Space Telescope need adaptive optics? Answer: Adaptive optics correct poor seeing in our atmosphere, but above the atmosphere, Hubble always has perfect seeing and sharp images all the time. 8) Why do stars appear to twinkle? Answer: In space, the light of the stars appears steady and images are sharp. But in passing through our atmosphere, the narrow shafts of star light are shifted constantly by turbulence, hence the images dance around in the eyepiece for us on the ground. 9) Why is the angular resolution of radio telescopes much worse than that of smaller optical telescopes? Answer: Radio waves are much longer in wavelength than light waves, so they carry little energy, and a lot of them are needed for sharp images. 10) Why is UV astronomy difficult to do from the ground? Answer: The atmosphere blocks most of the UV waves before they reach the ground. In order to see a wide range of UV wavelengths, you have to get very high in the atmosphere, such as with a balloon observatory, or above the atmosphere. 5.5 Essay Questions 1) Why is a complete image formed even if half of a lens or mirror is blocked? Answer: While the image formed is only half as bright, each part of the objective "sees" and images the entire object. 2) Why is the image seen in astronomical telescope eyepieces inverted? Answer: As the eyepiece must lie behind the objective's focus, the light reaching it has already been flipped over once it passed the focal point. 3) Based on their optical design, would an airline rather you bring your 8" Newtonian or Cassegrain to the next eclipse? Why? Answer: Considering the restrictions on size and weight now, you would find your Cassegrain, with its folded optical design and compact size, more welcome. 4) Compare how doubling the diameter of a mirror will affect both resolution and light grasp. Answer: The resolution would now be twice as sharp, but the increase in surface area, with the squaring for the area, would now detect stars four times fainter than before. 5) What is a CCD, and how does it work? Why has it replaced photographic plates? Answer: A charge coupled device consists of a grid of thousands of light-sensitive pixels that build up a digital image that can be stored on computer as light strikes it. It is digital, so it can be placed directly on magnetic tape or disks, or sent over a computer network. CCDs are also much more sensitive to fainter objects than are the fastest photo plates. 6) Explain how adaptive optics can help produce a sharper image. Answer: Adaptive optics distorts the mirror to counter the distortions created by the atmosphere. This can be done by computer controlled deformations of the mirror based on observations of a bright star in the telescopes field of view, or by use of an artificial star created by reflecting a laser off of material in the high atmosphere. 7) You are going to construct a new optical observatory, and money is no object. Other than putting it into space, what factors will affect your decision for its location on Earth? Answer: Closer to the equator will give you a view of more of the entire celestial sphere. The higher your location, the less turbulence you will experience, and less water to block the infrared. You want a large number of clear nights, very stable atmospheric conditions, and isolation from manmade light pollution. Such sites are often located on mountain tops in deserts, isolated coastal mountains in the Americas, and on mountaintops on islands like Hawaii and the Canary Islands. 8) Why must radio dishes be built so large? Answer: Radio waves carry little energy, so a lot of them must be collected to give resolution and image quality comparable to light telescopes. 9) What are some advantages of radio telescopes over optical scopes? Answer: Radio telescopes can be used day or night, they are much less affected by cloudy skies, and they open a new window to observe the Universe. They allow us to observe astronomical objects at a different wavelength than an optical telescope, thus giving an opportunity to compare and contrast the images. 10) How do radio interferometers greatly enhance resolution in radio wavelengths? Answer: Hooking two or more radio dishes together with a computer lets us get resolution not equal to the diameters of the dishes, but to the separations between the scopes.