Mastering Astronomy Chapter 5: Telescopes

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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

Angular resolution

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

Reflecting telescopes

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

Spectrographs

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

interferometry

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

grazing incidence

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

light-collecting area

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

refracting telescopes

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

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

Reflecting telescopes usually suffer from chromatic aberration. T/F

False.
Chromatic aberration is associated with refracting telescopes

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

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

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

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

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

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

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

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

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

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

Radio observations may be made in the daytime as well as at night. T/F

True.
Yes. Although the Sun is a radio source, it does not interfere unduly with daytime use of radio telescopes.

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

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.

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

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

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

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

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

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

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

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

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

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.

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

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.

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

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.

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

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

Radio telescopes using very long baseline interferometry have produced images with resolutions similar to those of images produced with optical telescopes. T/F

True.
As far as resolving power is concerned, the effective diameter of an interferometer is the distance between its outermost dishes.

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

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.

The primary purpose of a telescope is to provide a magnified image of the stars. T/F

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.

As telescopes become larger and larger, astronomers favor

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

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?

16 times

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

The effects of atmospheric turbulence

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

Earth's atmosphere absorbs most ultraviolet wavelengths

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

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.

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

The best possible angular resolution that can be obtained by a telescope of a given size if all other sources of blurring are eliminated

Radio telescopes would not represent a good choice for astronomical study of which of the following types of object?

ordinary stars

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

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

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

The point at which the incoming light is focused to produce an image, where analysis instruments and imaging cameras can be placed

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

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.

What type of telescope focus is behind a central hole in the main mirror?

Cassegrain

Which of the following is an advantage the Hubble Space Telescope has over ground-based telescopes?

There is no blurring due to the atmosphere

What is a CCD?

An electronic imaging detector

What is image processing?

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

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

Optical

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

False

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

False

The Keck telescopes contain the largest single mirrors ever constructed.

False

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

False

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.

True

As a rule, larger telescopes can detect fainter objects.

True

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

True

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

False

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

large mirrors are easier to build than large lenses.

The thickest lenses deflect and bend light

The most

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

infrared light

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

Earth is a heat source and the telescope must be kept very cool.

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

the distance between telescopes

Compared with radio telescopes, optical telescopes can

resolve finer detail

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

reduce atmospheric blurring

If telescope mirrors could be made of odd sizes, the one with the most light-gathering power would be

rectangle with two 1- sides and two 2- sides.

The image shown in Figure 5.10 in the textbook ("Resolution") is sharpest when the ratio of wavelength to telescope size is

small

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