Question 1

Many of the telescopes sent into space have to be cooled to liquid helium temperatures. Why is this?

Accepted Answer

A) The telescope will expand to a distorted shape if it is not cooled to the temperature of its surroundings. This cooling would not be necessary if the telescope were on Earth. 
B) The telescope will radiate as a blackbody, and this radiation will obscure the astronomical signal. This blackbody radiation is reduced at cooler temperatures. 
C) The cooler temperatures reduce the wavelengths of the incoming radiation, thus increasing the resolution. 
D) The liquid helium is vaporized and used to produce a reference spectrum for comparison with the spectra of the astronomical sources. 
A) The telescope will expand to a distorted shape if it is not cooled to the temperature of its surroundings. This cooling would not be necessary if the telescope were on Earth. 
B) The telescope will radiate as a blackbody, and this radiation will obscure the astronomical signal. This blackbody radiation is reduced at cooler temperatures. 
C) The cooler temperatures reduce the wavelengths of the incoming radiation, thus increasing the resolution. 
D) The liquid helium is vaporized and used to produce a reference spectrum for comparison with the spectra of the astronomical sources.

Question 2

The largest refracting telescope in the world is the 102-cm (40-in.) diameter telescope at Yerkes Observatory, built in 1897. Refracting telescopes with larger diameter have never been built because they would:

Accepted Answer

A) sag too much under their own weight. 
B) give too much magnification. 
C) have too little chromatic aberration. 
D) be too thick, and suffer too much spherical aberration. 
A) sag too much under their own weight. 
B) give too much magnification. 
C) have too little chromatic aberration. 
D) be too thick, and suffer too much spherical aberration.

Question 3

At the present time (2018), the telescope mirror with the largest diameter in the world is in:

Accepted Answer

A) the Gran Telescopio Canarias. 
B) the European Southern Observatory. 
C) Gemini North. 
D) the Keck II telescope on Mauna Kea, Hawaii. 
A) the Gran Telescopio Canarias. 
B) the European Southern Observatory. 
C) Gemini North. 
D) the Keck II telescope on Mauna Kea, Hawaii.

Question 4

The phenomenon of refraction, the change in direction of a light beam as it enters a dense but transparent material, is caused by the:

Accepted Answer

A) slowdown of light in denser material. 
B) speedup of light as it enters denser material. 
C) change in the color or wavelength of light as it enters denser material. 
D) reflection of part of the light at the surface. 
A) slowdown of light in denser material. 
B) speedup of light as it enters denser material. 
C) change in the color or wavelength of light as it enters denser material. 
D) reflection of part of the light at the surface.

Question 5

The shortest wavelength observed by HST is 115 nm. What is its angular resolution at this wavelength?

Accepted Answer

A) 0.01 arcsec 
B) 0.1 arcsec 
C) 1.0 arcsec 
D) 100 arcsec 
A) 0.01 arcsec 
B) 0.1 arcsec 
C) 1.0 arcsec 
D) 100 arcsec

Question 6

Assuming that all effects due to variations in Earth's atmosphere could be removed (with adaptive optics) from images produced by a telescope, what diameter of a telescope would be needed to see two bright (laser) headlights of wavelength 650 nm separated by 2 m on a vehicle on the Moon? (Assume Earth-Moon distance to be 400,000 km.)

Accepted Answer

A) 0.16 m 
B) 1600 m 
C) 1.6 m 
D) 160 m 
A) 0.16 m 
B) 1600 m 
C) 1.6 m 
D) 160 m

Question 7

Which of the following systems will produce the best resolution?

Accepted Answer

A) a single radio telescope with a 100-m dish 
B) two 25-m radio telescopes separated by 200 m 
C) two 50-m radio telescopes separated by 200 m 
D) two 25-m radio telescopes separated by 380 m 
A) a single radio telescope with a 100-m dish 
B) two 25-m radio telescopes separated by 200 m 
C) two 50-m radio telescopes separated by 200 m 
D) two 25-m radio telescopes separated by 380 m

Question 8

What determines the magnification of a telescope image?

Accepted Answer

A) the size of the objective mirror or lens 
B) only the focal length of the objective 
C) only the focal length of the eyepiece 
D) the focal lengths of both the objective and the eyepiece 
A) the size of the objective mirror or lens 
B) only the focal length of the objective 
C) only the focal length of the eyepiece 
D) the focal lengths of both the objective and the eyepiece

Question 9

If part of the primary mirror of a reflecting telescope is blocked off by a small screen, which of the following characteristics of the final image is affected?

Accepted Answer

A) Although the brightness is unaffected, the size of the image will be reduced. 
B) A part of the image is missing. 
C) Only the brightness is reduced. 
D) Depending on which part of the mirror is obscured, the color of the image is affected. 
A) Although the brightness is unaffected, the size of the image will be reduced. 
B) A part of the image is missing. 
C) Only the brightness is reduced. 
D) Depending on which part of the mirror is obscured, the color of the image is affected.

Question 10

If someone says they are using an 8-inch reflecting telescope, what do they mean?

Accepted Answer

A) The length of the telescope tube is 8 inches. 
B) The radius of the mirror is 8 inches. 
C) The diameter of the mirror is 8 inches. 
D) The focal length of the mirror is 8 inches. 
A) The length of the telescope tube is 8 inches. 
B) The radius of the mirror is 8 inches. 
C) The diameter of the mirror is 8 inches. 
D) The focal length of the mirror is 8 inches.

Question 11

How are the Keck telescopes on Mauna Kea designed to work?

Accepted Answer

A) A single, thin primary mirror that has a 20-m diameter has its shape changed and controlled by a computer. 
B) The light from two 10-m telescopes, which are 85 m apart, will eventually be combined into a single image. 
C) A dish of mercury 100 m in diameter is rotated and thus forms a very large parabolic mirror. 
D) Six thin mirrors with diameters of 10 m are arranged in a circle to simulate a single mirror with a diameter of 30 m. 
A) A single, thin primary mirror that has a 20-m diameter has its shape changed and controlled by a computer. 
B) The light from two 10-m telescopes, which are 85 m apart, will eventually be combined into a single image. 
C) A dish of mercury 100 m in diameter is rotated and thus forms a very large parabolic mirror. 
D) Six thin mirrors with diameters of 10 m are arranged in a circle to simulate a single mirror with a diameter of 30 m.

Question 12

The longest wavelength observed by HST is 1 mm. What is its angular resolution at this wavelength?

Accepted Answer

A) 0.01 arcsec 
B) 0.1 arcsec 
C) 1.0 arcsec 
D) 100 arcsec 
A) 0.01 arcsec 
B) 0.1 arcsec 
C) 1.0 arcsec 
D) 100 arcsec

Question 13

Astronomers began to use radio telescopes to investigate the universe:

Accepted Answer

A) during the 1920s. 
B) during the 1980s, with the development of CCD detectors. 
C) in the early 1960s, with the development of satellites. 
D) in the late 1940s, shortly after World War II. 
A) during the 1920s. 
B) during the 1980s, with the development of CCD detectors. 
C) in the early 1960s, with the development of satellites. 
D) in the late 1940s, shortly after World War II.

Question 14

In a correctly focused refracting telescope, the objective lens and the eyepiece lens are separated by a distance equal to the

Accepted Answer

A) focal length of the eyepiece. 
B) focal length of the objective minus the focal length of the eyepiece. 
C) sum of the focal lengths of objective and eyepiece. 
D) focal length of the objective. 
A) focal length of the eyepiece. 
B) focal length of the objective minus the focal length of the eyepiece. 
C) sum of the focal lengths of objective and eyepiece. 
D) focal length of the objective.

Question 15

In the infrared view of the universe (see Figure 6-32 of Universe, 11th ed.) in the wavelength range of 10 to 100 μm, the brightest most intense radiation comes from:
​
  ​

Accepted Answer

A) individual hydrogen atoms in interstellar space. 
B) dust particles in the plane of the Milky Way. 
C) supernova remnants. 
D) distant galaxies. 
A) individual hydrogen atoms in interstellar space. 
B) dust particles in the plane of the Milky Way. 
C) supernova remnants. 
D) distant galaxies.

Question 16

What happens when a beam of white light (containing all colors) is passed through a prism of glass?

Accepted Answer

A) Different colors are refracted at different angles to produce a spectrum. 
B) Selected colors are absorbed so that the remaining light that leaves the prism is colored. 
C) The speed of the red light that leaves the prism is higher than that of the blue light, leading to a colored beam. 
D) Selected colors are reflected from the outer faces of the prism, and the light that passes through is colored. 
A) Different colors are refracted at different angles to produce a spectrum. 
B) Selected colors are absorbed so that the remaining light that leaves the prism is colored. 
C) The speed of the red light that leaves the prism is higher than that of the blue light, leading to a colored beam. 
D) Selected colors are reflected from the outer faces of the prism, and the light that passes through is colored.

Question 17

To correct for chromatic aberration in a refracting telescope, a corrective lens is mounted next to the objective lens. In this corrective lens:

Accepted Answer

A) red light bends more than blue light. 
B) blue light bends more than red light. 
C) all colors bend the same amount. 
D) no bending is experienced by any of the colors. 
A) red light bends more than blue light. 
B) blue light bends more than red light. 
C) all colors bend the same amount. 
D) no bending is experienced by any of the colors.

Question 18

What is the refraction of light?

Accepted Answer

A) the change in direction of a light ray as it reflects from a more dense material than the one in which it is traveling 
B) the absorption of light as it traverses a dense, transparent material 
C) the breaking of white light into its composite colors 
D) the change in direction of a light ray as it crosses from a less dense, transparent material to a more dense one 
A) the change in direction of a light ray as it reflects from a more dense material than the one in which it is traveling 
B) the absorption of light as it traverses a dense, transparent material 
C) the breaking of white light into its composite colors 
D) the change in direction of a light ray as it crosses from a less dense, transparent material to a more dense one

Question 19

Recently, it has become possible to equip telescope mirrors with mechanical devices called actuators, which can move a portion of the mirror small distances very rapidly. Three of the statements below describe consequences of employing this technique, and one does not. Which statement is the exception?

Accepted Answer

A) active optics - the ability to change the shape of the mirror to compensate for sagging and for expansion/contraction as the mirror temperature changes 
B) adaptive optics - the ability to change the shape of the mirror to compensate for atmospheric changes that would otherwise distort the image 
C) the capability of creating segmented mirrors much larger than any single-piece mirror 
D) the ability to build a single mirror that can detect electromagnetic radiation all across the spectrum, from gamma rays to radio waves 
A) active optics - the ability to change the shape of the mirror to compensate for sagging and for expansion/contraction as the mirror temperature changes 
B) adaptive optics - the ability to change the shape of the mirror to compensate for atmospheric changes that would otherwise distort the image 
C) the capability of creating segmented mirrors much larger than any single-piece mirror 
D) the ability to build a single mirror that can detect electromagnetic radiation all across the spectrum, from gamma rays to radio waves

Question 20

In telescopes, the resolution is worse for:

Accepted Answer

A) larger diameter lenses or mirrors and shorter wavelength light (or other electromagnetic radiation). 
B) smaller diameter lenses or mirrors and longer wavelength light (or other electromagnetic radiation). 
C) smaller diameter lenses or mirrors and shorter wavelength light (or other electromagnetic radiation). 
D) larger diameter lenses or mirrors and longer wavelength light (or other electromagnetic radiation). 
A) larger diameter lenses or mirrors and shorter wavelength light (or other electromagnetic radiation). 
B) smaller diameter lenses or mirrors and longer wavelength light (or other electromagnetic radiation). 
C) smaller diameter lenses or mirrors and shorter wavelength light (or other electromagnetic radiation). 
D) larger diameter lenses or mirrors and longer wavelength light (or other electromagnetic radiation).

Exam 6: Optics and Telescopes

Many of the telescopes sent into space have to be cooled to liquid helium temperatures. Why is this?

The largest refracting telescope in the world is the 102-cm (40-in.) diameter telescope at Yerkes Observatory, built in 1897. Refracting telescopes with larger diameter have never been built because they would:

At the present time (2018), the telescope mirror with the largest diameter in the world is in:

The phenomenon of refraction, the change in direction of a light beam as it enters a dense but transparent material, is caused by the:

The shortest wavelength observed by HST is 115 nm. What is its angular resolution at this wavelength?

Which of the following systems will produce the best resolution?

What determines the magnification of a telescope image?

If part of the primary mirror of a reflecting telescope is blocked off by a small screen, which of the following characteristics of the final image is affected?

If someone says they are using an 8-inch reflecting telescope, what do they mean?

How are the Keck telescopes on Mauna Kea designed to work?

The longest wavelength observed by HST is 1 mm. What is its angular resolution at this wavelength?

Astronomers began to use radio telescopes to investigate the universe:

In a correctly focused refracting telescope, the objective lens and the eyepiece lens are separated by a distance equal to the

In the infrared view of the universe (see Figure 6-32 of Universe, 11th ed.) in the wavelength range of 10 to 100 μm, the brightest most intense radiation comes from: ​ ​

What happens when a beam of white light (containing all colors) is passed through a prism of glass?

To correct for chromatic aberration in a refracting telescope, a corrective lens is mounted next to the objective lens. In this corrective lens:

What is the refraction of light?

In telescopes, the resolution is worse for:

Astronomy and the Universe

Knowing the Heavens

Eclipses and the Motion of the Moon

Gravitation and the Waltz of the Planets

The Nature of Light

Comparative Planetology I: Our Solar System

Comparative Planetology II: the Origin of Our Solar System

The Living Earth

Our Barren Moon

Mercury, Venus, and Mars: Terrestrial, yet Unique

Jupiter and Saturn: Lords of the Planets

Jupiter and Saturns Satellites of Fire and Ice

Uranus, Neptune, Pluto, and the Kuiper Belt: Remote Worlds

Asteroids, Comets, and Impacts

Our Star, the Sun

The Nature of the Stars

The Birth of Stars

Stellar Evolution: on and After the Main Sequence

Stellar Evolution: The Death of Stars

Stellar Remnants: Neutron Stars and Black Holes

Our Galaxy

Galaxies

Quasars and Active Galaxies

Cosmology: The Origin and Evolution of the Universe

Exploring the Early Universe

The Search for Extraterrestrial Life

Filters

In the infrared view of the universe (see Figure 6-32 of Universe, 11th ed.) in the wavelength range of 10 to 100 μm, the brightest most intense radiation comes from: