question

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

answer

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

answer

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

answer

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

answer

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

question

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?

answer

16 times

question

Astronomers use the term "seeing" to describe which of the following?

answer

The effects of atmospheric turbulence

question

What is the main reason that ultraviolet astronomy must be done in space?

answer

Earth's atmosphere absorbs most ultraviolet wavelengths

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What is meant in astronomy by the phrase "active optics"?

answer

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.

question

What is meant by the "diffraction limited resolution" of a telescope?

answer

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?

answer

ordinary stars

question

Which of the following statements about the use of infrared telescopes is true?

answer

The detector and telescope must be cooled to very low temperatures.

question

What is meant by the "prime focus" of a telescope?

answer

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

answer

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?

answer

Cassegrain

question

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?

answer

An electronic imaging detector

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What is image processing?

answer

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?

answer

Optical

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The primary purpose of any telescope is to produce an enormously magnified image of the field of view.

answer

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

question

The Keck telescopes contain the largest single mirrors ever constructed.

answer

False

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The term seeing is used to describe how faint an object can be detected by a telescope.

answer

False

question

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.

answer

True

question

As a rule, larger telescopes can detect fainter objects.

answer

True

question

An object having a temperature of 300 would be best observed with an infrared telescope.

answer

True

question

Gamma-ray telescopes employ the same basic design that optical instruments use.

answer

False

question

The main reason that most professional research telescopes are reflectors is that

answer

large mirrors are easier to build than large lenses.

question

The thickest lenses deflect and bend light

answer

The most

question

The best way to study young stars hidden behind interstellar dust clouds would be to use

answer

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.

question

When multiple radio telescopes are used for interferometry, resolving power is most improved by increasing

answer

the distance between telescopes

question

Compared with radio telescopes, optical telescopes can

answer

resolve finer detail

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The primary reason professional observatories are built on the highest mountaintops is to

answer

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

answer

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

answer

small

question

Telescope

answer

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

answer

A telescope that uses a lens to gather and focus light from a distant object

question

Reflecting Telescope

answer

A telescope that uses a mirror to gather and focus light from a distant object

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Newtonian Telescope

answer

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

question

Cassegrain Telescope

answer

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

answer

The total area of a telescope capable of capturing incoming radiation

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Angular Resolution

answer

The ability of a telescope to distinguish between adjacent objects in the sky

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Charge-Coupled Device "CCD"

answer

An electronic device used for data acquisition

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Pixel

answer

One of many tiny picture elements, organized into a two-dimensional array, making up a digital image

question

Image

answer

The optical representation of a object produced when from the object is reflected or refracted by a mirror or lens

question

Photometry

answer

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

answer

Instrument used to produce detailed spectra of stars

question

Seeing

answer

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

question

Seeing Disks

answer

Roughly circular region on a detector over which a star's point-like images is spread, due to atmospheric turbulence

question

Active Optics

answer

Collection of techniques used to increase the resolution of ground-based telescopes

question

Adaptive Optics

answer

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

answer

Large instrument designed to detect radiation from space at radio wavelengths

question

Interferometer

answer

Collection of two or more telescopes working together as a team, observing the same object at the same time and at the same wavelength

question

Interferometry

answer

Technique in widespread use to dramatically improve the resolution of radio and infrared maps

question

Infrared Telescope

answer

Telescope designed to detect infrared radiation

question

Ultraviolet Telescope

answer

A telescope that is designed to collect radiation in the ultraviolet part of the spectrum

question

High-Energy Telescope

answer

Telescope designed to detect x- and gamma radiation

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