Combo with "Mastering Astronomy Chapter 5: Telescopes" and 1 other – Flashcards

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The _______ ________ of the Hubble Space Telescope is better for shorter (bluer) wavelengths of light than for longer (redder) wavelengths of light
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Angular resolution
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The large research observatories on Mauna Kea use giant ________ __________.
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Reflecting telescopes
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_____________ separate the various colors of light, allowing astronomers to determine stellar composition and may other stellar properties.
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Spectrographs
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The twin 10-m Keck telescopes can work together to obtain better angular resolution through a technique known as __________.
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interferometry
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The Chandra X-Ray observatory focuses X rays with _________ _________ mirrors.
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grazing incidence
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A 10-meter telescope has a larger __________-_________ _____ than a 4-meter telescope.
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light-collecting area
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Galileo's telescope designs using lenses were examples of _________ ________.
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refracting telescopes
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Gamma-Ray telescopes are hard to make because they require large lenses or mirrors for focusing in order to make images. T/F
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False. Gamma-ray telescopes work by absorbing high-energy photons, not by reflecting or refracting them.
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Reflecting telescopes usually suffer from chromatic aberration. T/F
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False. Chromatic aberration is associated with refracting telescopes
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An optical telescope of a given size would have better resolution in orange light than in blue light. T/F
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False. The diffraction limit to resolution would be larger, producing worse resolution, in orange light than in blue light for a given telescope.
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It is impossible to overcome the atmospheric blurring experienced by ground-based optical telescopes. T/F
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False. Active and/or adaptive optics can be used to compensate for much of the distortion of light due to atmospheric turbulence.
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Radio telescopes often position their detectors at prime focus. T/F
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True. Section 5.5 of the text and the online image archive for Chapter 5 contain many pictures showing such arrangements.
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The amount of light collected by a telescope is proportional to its area. T/F
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True. This is the reason that astronomers need larger telescopes to study fainter objects.
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Cassegrain-style telescopes are characterized by a hole in the objective mirror. T/F
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True. The resulting folded optical path is used in many commercial amateur and professional telescope designs.
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Radio observations may be made in the daytime as well as at night. T/F
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True. Yes. Although the Sun is a radio source, it does not interfere unduly with daytime use of radio telescopes.
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Ultraviolet and X-ray telescopes can be used only above Earth's atmosphere. T/F
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True. The atmosphere is opaque to electromagnetic radiation higher in frequency (i.e., shorter in wavelength) than the near ultraviolet, so UV and X-ray astronomy must be conducted from space. Gamma rays, however, can be observed from the ground—not directly, but by observing the results of their interactions with the atmosphere.
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A telescope with a 2-m diameter primary mirror would be more affected by diffraction than one with a 1-m-diameter mirror. T/F
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False. The diffraction limit on resolution would actually be better (smaller) for the larger telescope.
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The biggest telescopes in use today are reflectors, not refractors. T/F
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True. The largest telescopes in all wavelength regions from radio to X-rays are all reflectors.
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The Hubble Space Telescope is the largest optical telescope ever built. T/F
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False. The HST owes its image quality (after correction for an error in the primary mirror) to the lack of atmospheric blurring.
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CCD cameras and photographic film have roughly the same efficiency for collecting light. T/F
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False. Although CCD cameras have replaced photogenic film in most applications, film is still used or some specialized applications.
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Radio waves can penetrate dusty regions of interstellar space because they are relatively unaffected by this intervening matter. T/F
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True. Many parts of interstellar space (including the center of our Galaxy) cannot be seen at optical wavelengths, but are easily detectable in the radio region.
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Both the Keck and VLT observatories are designed to permit interferometry to be used. T/F
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True. Although the technique of interferometry was pioneered using radio telescopes, it is now being extended to the infrared, and perhaps eventually optical regions. See Section 5.6 in your textbook.
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One scheme for improving telescopic images involves collecting data from a part of the sky where there are no known sources of radiation. T/F
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True. The data collected would consist of background noise. This data can be used in computer processing of images taken with the telescope to improve the quality of the image.
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One challenge in building large radio telescopes is getting the large surfaces smooth enough to act as mirrors. T/F
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False. Since radio waves have fairly long wavelengths, centimeters to meters, the reflector surfaces do not have to be extremely smooth.
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Radio telescopes using very long baseline interferometry have produced images with resolutions similar to those of images produced with optical telescopes. T/F
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True. As far as resolving power is concerned, the effective diameter of an interferometer is the distance between its outermost dishes.
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Many observatories are placed on top of mountains so they will be closer to the objects they are observing. T/F
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False. An observatory on a tall mountain is above much of the atmosphere. Atmospheric effects are a primary reason why Earth-based telescopes cannot achieve their diffraction-limited resolution.
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The primary purpose of a telescope is to provide a magnified image of the stars. T/F
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False. Magnification is secondary. Almost all stars appear as only points of light through even the most powerful telescopes. Light-gathering power and resolving power are the most important properties of astronomical telescopes.
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As telescopes become larger and larger, astronomers favor
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Reflecting telescopes, in part because large lenses and mirrors sag under their own weight, and it is easier to support a mirror along its entire back side than it is to support a lens only around its edge
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How much more light would a 2-m-diameter telescope gather in a certain amount of time at a given wavelength compared with one that is 50 cm in diameter?
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16 times
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Astronomers use the term "seeing" to describe which of the following?
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The effects of atmospheric turbulence
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What is the main reason that ultraviolet astronomy must be done in space?
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Earth's atmosphere absorbs most ultraviolet wavelengths
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What is meant in astronomy by the phrase "active optics"?
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Rapid modifications are made to the tilt and location of the elements of a telescope to correct for the effects of atmospheric and instrumental distortion.
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What is meant by the "diffraction limited resolution" of a telescope?
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The best possible angular resolution that can be obtained by a telescope of a given size if all other sources of blurring are eliminated
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Radio telescopes would not represent a good choice for astronomical study of which of the following types of object?
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ordinary stars
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Which of the following statements about the use of infrared telescopes is true?
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The detector and telescope must be cooled to very low temperatures.
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What is meant by the "prime focus" of a telescope?
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The point at which the incoming light is focused to produce an image, where analysis instruments and imaging cameras can be placed
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Large arrays of widely separated radio telescopes can achieve very good resolution. What is the primary reason that such good resolutions cannot be achieved by individual radio telescopes operating independently
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Radio dishes larger than a few tens of meters across are difficult to build, limiting their individual resolutions to several times worse than the largest optical mirrors.
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What type of telescope focus is behind a central hole in the main mirror?
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Cassegrain
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Which of the following is an advantage the Hubble Space Telescope has over ground-based telescopes?
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There is no blurring due to the atmosphere
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What is a CCD?
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An electronic imaging detector
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What is image processing?
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Computer processing of electronic images to correct for defects and remove noise
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What type of single telescope currently in operation has the highest angular resolution?
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Optical
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The primary purpose of any telescope is to produce an enormously magnified image of the field of view.
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False
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The main advantage to using the HST is the increased amount of "night time" viewing it affords.
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False
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The Keck telescopes contain the largest single mirrors ever constructed.
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False
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The term seeing is used to describe how faint an object can be detected by a telescope.
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False
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Radio telescopes are large in part to improve their angular resolution, which is poor because of the long wavelengths at which they are used to observe the skies.
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True
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As a rule, larger telescopes can detect fainter objects.
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True
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An object having a temperature of 300 would be best observed with an infrared telescope.
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True
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Gamma-ray telescopes employ the same basic design that optical instruments use.
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False
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The main reason that most professional research telescopes are reflectors is that
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large mirrors are easier to build than large lenses.
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The thickest lenses deflect and bend light
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The most
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The best way to study young stars hidden behind interstellar dust clouds would be to use
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infrared light
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The Spitzer Space Telescope (SST) is stationed far from Earth because
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Earth is a heat source and the telescope must be kept very cool.
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When multiple radio telescopes are used for interferometry, resolving power is most improved by increasing
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the distance between telescopes
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Compared with radio telescopes, optical telescopes can
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resolve finer detail
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The primary reason professional observatories are built on the highest mountaintops is to
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reduce atmospheric blurring
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If telescope mirrors could be made of odd sizes, the one with the most light-gathering power would be
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rectangle with two 1- sides and two 2- sides.
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The image shown in Figure 5.10 in the textbook ("Resolution") is sharpest when the ratio of wavelength to telescope size is
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small
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Telescope
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Instrument used to capture as many photons as possible from a given region of the sky and concentrate them into a focused bean for analysis
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Refracting Telescope
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A telescope that uses a lens to gather and focus light from a distant object
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Reflecting Telescope
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A telescope that uses a mirror to gather and focus light from a distant object
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Newtonian Telescope
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A reflecting telescope in which incoming light is intercepted before it reaches the prime focus and is deflected into an eyepiece at the side of the instrument
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Cassegrain Telescope
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A type of reflecting telescope in which incoming light hits the hits the primary mirror and is then reflected upward toward the prime focus, where a secondary mirror reflects the light back down through a small hole in the main mirror into a detector or eyepiece
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Collecting Area
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The total area of a telescope capable of capturing incoming radiation
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Angular Resolution
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The ability of a telescope to distinguish between adjacent objects in the sky
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Charge-Coupled Device "CCD"
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An electronic device used for data acquisition
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Pixel
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One of many tiny picture elements, organized into a two-dimensional array, making up a digital image
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Image
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The optical representation of a object produced when from the object is reflected or refracted by a mirror or lens
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Photometry
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Branch of observational astronomy in which the brightness of a source is measured through each of a set of standard filters
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Spectrometer
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Instrument used to produce detailed spectra of stars
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Seeing
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Term used to describe the ease with which good telescopic observations can be made from Earth's surface, given the blurring effects of of atmospheric turbulence
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Seeing Disks
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Roughly circular region on a detector over which a star's point-like images is spread, due to atmospheric turbulence
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Active Optics
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Collection of techniques used to increase the resolution of ground-based telescopes
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Adaptive Optics
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Technique used to increase the resolution of a telescope by deforming the shape of the mirror's surface under computer control while a measurement is being taken
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Radio Telescopes
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Large instrument designed to detect radiation from space at radio wavelengths
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Interferometer
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Collection of two or more telescopes working together as a team, observing the same object at the same time and at the same wavelength
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Interferometry
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Technique in widespread use to dramatically improve the resolution of radio and infrared maps
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Infrared Telescope
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Telescope designed to detect infrared radiation
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Ultraviolet Telescope
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A telescope that is designed to collect radiation in the ultraviolet part of the spectrum
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High-Energy Telescope
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Telescope designed to detect x- and gamma radiation
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