question

The _______ ________ of the Hubble Space Telescope is better for shorter (bluer) wavelengths of light than for longer (redder) wavelengths of light

answer

Angular resolution

question

The large research observatories on Mauna Kea use giant ________ __________.

answer

Reflecting telescopes

question

_____________ separate the various colors of light, allowing astronomers to determine stellar composition and may other stellar properties.

answer

Spectrographs

question

The twin 10-m Keck telescopes can work together to obtain better angular resolution through a technique known as __________.

answer

interferometry

question

The Chandra X-Ray observatory focuses X rays with _________ _________ mirrors.

answer

grazing incidence

question

A 10-meter telescope has a larger __________-_________ _____ than a 4-meter telescope.

answer

light-collecting area

question

Galileo's telescope designs using lenses were examples of _________ ________.

answer

refracting telescopes

question

Gamma-Ray telescopes are hard to make because they require large lenses or mirrors for focusing in order to make images. T/F

answer

False. Gamma-ray telescopes work by absorbing high-energy photons, not by reflecting or refracting them.

question

Reflecting telescopes usually suffer from chromatic aberration. T/F

answer

False. Chromatic aberration is associated with refracting telescopes

question

An optical telescope of a given size would have better resolution in orange light than in blue light. T/F

answer

False. The diffraction limit to resolution would be larger, producing worse resolution, in orange light than in blue light for a given telescope.

question

It is impossible to overcome the atmospheric blurring experienced by ground-based optical telescopes. T/F

answer

False. Active and/or adaptive optics can be used to compensate for much of the distortion of light due to atmospheric turbulence.

question

Radio telescopes often position their detectors at prime focus. T/F

answer

True. Section 5.5 of the text and the online image archive for Chapter 5 contain many pictures showing such arrangements.

question

The amount of light collected by a telescope is proportional to its area. T/F

answer

True. This is the reason that astronomers need larger telescopes to study fainter objects.

question

Cassegrain-style telescopes are characterized by a hole in the objective mirror. T/F

answer

True. The resulting folded optical path is used in many commercial amateur and professional telescope designs.

question

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.

question

Ultraviolet and X-ray telescopes can be used only above Earth's atmosphere. T/F

answer

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.

question

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

answer

False. The diffraction limit on resolution would actually be better (smaller) for the larger telescope.

question

The biggest telescopes in use today are reflectors, not refractors. T/F

answer

True. The largest telescopes in all wavelength regions from radio to X-rays are all reflectors.

question

The Hubble Space Telescope is the largest optical telescope ever built. T/F

answer

False. The HST owes its image quality (after correction for an error in the primary mirror) to the lack of atmospheric blurring.

question

CCD cameras and photographic film have roughly the same efficiency for collecting light. T/F

answer

False. Although CCD cameras have replaced photogenic film in most applications, film is still used or some specialized applications.

question

Radio waves can penetrate dusty regions of interstellar space because they are relatively unaffected by this intervening matter. T/F

answer

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.

question

Both the Keck and VLT observatories are designed to permit interferometry to be used. T/F

answer

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.

question

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.

question

One challenge in building large radio telescopes is getting the large surfaces smooth enough to act as mirrors. T/F

answer

False. Since radio waves have fairly long wavelengths, centimeters to meters, the reflector surfaces do not have to be extremely smooth.

question

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.

question

Many observatories are placed on top of mountains so they will be closer to the objects they are observing. T/F

answer

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.

question

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.

question

As telescopes become larger and larger, astronomers favor

answer

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

question

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

question

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

question

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.

question

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?

answer

There is no blurring due to the atmosphere

question

What is a CCD?

answer

An electronic imaging detector

question

What is image processing?

answer

Computer processing of electronic images to correct for defects and remove noise

question

What type of single telescope currently in operation has the highest angular resolution?

answer

Optical

question

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

answer

False

question

The main advantage to using the HST is the increased amount of "night time" viewing it affords.

answer

False

question

The Keck telescopes contain the largest single mirrors ever constructed.

answer

False

question

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

question

The Spitzer Space Telescope (SST) is stationed far from Earth because

answer

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

question

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

answer

reduce atmospheric blurring

question

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.

question

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

Infrared astronomy can only be done from space.

answer

False

question

One of the primary advantages of CCDs over photographic plates is the former's high efficiency in detecting light.

answer

True

question

Because gamma rays have very short wavelengths, gamma-ray telescopes can achieve extremely high angular resolution.

answer

False

question

What problem do refractor telescopes have that reflectors don't?

answer

chromatic aberration

question

The most important advantage of CCDs over film is that

answer

they record much more light in a given exposure time.

question

The primary purpose of a telescope is to

answer

collect a large amount of light and bring it into focus.

question

The amount of diffraction and thus the resolution of the scope depends upon

answer

the wavelength used and the size of the main telescope objective lens or mirror.

question

What is the resolution of a telescope?

answer

its ability to distinguish two adjacent objects close together in the sky

question

What problem does adaptive optics correct?

answer

turbulence in the earth's atmosphere which creates twinkling

question

What is true of radio telescopes?

answer

They have poorer angular resolution than a refractor.

question

Compared to optical telescopes, radio telescopes are built large because

answer

radio photons don't carry much energy.

question

One advantage of the Hubble Space telescope over ground based ones is that

answer

in orbit, it can operate close to its diffraction limit at visible wavelengths.

question

CCD detectors gather light 10-20 times faster than the most sensitive photographic film.

answer

True

question

A telescope design that uses a lens and no mirrors is a refractor.

answer

True

question

Chromatic aberration affects reflector telescopes.

answer

False

question

The Hubble Space Telescope gives us its best resolution with X-rays.

answer

False

question

Photography with film is still the preferred way of capturing fine detail in the faintest, most distant galaxies.

answer

False

question

Radio telescopes have poorer angular resolution than optical telescopes because radio waves have a much longer wavelength than optical waves.

answer

True

question

All modern large optical telescopes are refractors.

answer

False

question

Stars do not twinkle; the instability of the atmosphere causes this effect.

answer

True

question

Optical telescopes are usually used only at night, but radio telescopes can be used day or night.

answer

True

question

The Earth's atmosphere is the major factor limiting the use of ground-based radio telescopes.

answer

False

question

Radio astronomy can only be done from up in space, due to our ionosphere.

answer

False

question

Like radio and optical astronomy, infrared astronomy is easily done with ground-based telescopes.

answer

False

question

The mirrors for X-ray telescopes are the same shape as those of optical reflectors.

answer

False

question

characteristics of telescopes

answer

chapter 3 sorting task

question

A major advantage of a Newtonian reflector over a refractor is

answer

the elimination of chromatic aberration.

question

The Arecibo radio telescope is laid out like which optical telescope design?

answer

prime focus reflector

question

Galileo is credited with designing the first reflector telescope.

answer

False

question

All wavelengths of light travel at the same speed in a vacuum, and carry the same energy per photon.

answer

false

question

The opacity of the atmosphere is partially corrected via adaptive optics.

answer

false

question

Which type of telescope did Galileo turn skyward in 1610?

answer

single lens refractor

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