ASTR264 HW Chapter 6

When your eye forms an image, the _____ plays a role analogous to the detector in a camera.

retina

Which of the following statements best describes the difference between a refracting telescope and a reflecting telescope?

A refracting telescope uses a transparent glass lens to focus light, whereas a reflecting telescope uses a mirror to focus light.

What does angular resolution measure?

The angular size of the smallest features that the telescope can see. Therefore, a smaller angular resolution is better.

Which of the following is always true about images captured with X-ray telescopes?

They are always shown with colors that are not the true colors of the objects that were photographed.

Shown following are five different colors of visible light that travel to Earth from the Sun. Rank these colors of visible light from left to right based on the altitude in the atmosphere where they are completely absorbed, from highest to lowest (Earth's surface). If two (or more) of the choices reach the surface, rank them as equal by dragging one on top of the other(s).

All are equal As your correct answer indicates, all wavelengths of visible light reach Earth's surface, which is why we can see all colors and why visible-light telescopes work fine on the ground.

Shown following are the primary mirror arrangements and total light-collecting area of five different telescopes. Notice that although the arrangements look similar to those in Part B, the light-collecting areas are not the same. Also listed is an amount of time (exposure time) that each telescope will be pointed at the same distant galaxy. Again assume that the quality of these mirrors, the detectors, and the observing conditions are identical. Rank the telescopes from left to right based on the brightness of the image each telescope will take of the galaxy in the time indicated, from brightest to dimmest. To rank two (or more) telescopes as equal, drag one on top of the other(s).

As your answer correctly indicates, it is the product of the light-collecting area and the exposure time that determines the total amount of light collected from a distant object.

Which of the following is a principal advantage of CCDs over photographic film?

CCDs capture a much higher percentage of the incoming photons than does film.

Which of the following is not one of the three main categories of observation generally used by astronomers?

Filtering to look at just a single color from an object. Astronomers often use filters, but only in support of one of the three main observational categories of imaging, spectroscopy, and time monitoring.

____ telescopes must be massive in order to intercept this high-energy light. An example is the Fermi ____ Telescope.

Gamma-ray

Which of the following best describes why radio telescopes are generally much larger in size than telescopes designed to collect visible light?

Getting an image of the same angular resolution requires a much larger telescope for radio waves than for visible light. The diffraction limit depends on wavelength, with longer wavelengths requiring larger telescopes to achieve the same diffraction limit.

Suppose you want to determine the chemical composition of a distant planet or star. Which of the following will be most useful to have?

High spectral resolution. We learn chemical composition by studying spectral lines, so higher spectral resolution makes it easier to learn chemical composition.

Shown following are the primary mirror arrangements and total light-collecting area of five different telescopes. Each mirror uses a different segmented arrangement, but assume that they are all equivalent in quality and in their ability to focus light. Also assume that the telescopes use identical detectors and have the same observing conditions. Rank the telescopes from left to right based on their ability to detect very dim objects, from greatest to least. To rank two (or more) telescopes as equal, drag one on top of the other(s).

If all else is equal (such as mirror quality, detector, and observing conditions), the ability to detect dim objects depends only on light-collecting area. Because all the telescopes shown have the same light-collecting area, they all can detect dim objects equally well. The arrangement of the mirrors does not matter, as long as they are arranged and shaped so they bring light to a perfect focus.

Sort each of the astronomical questions below into the appropriate bin based on the type of observation you would need to perform to answer it.

Imaging: How large is the Andromeda Galaxy? What are the major surface features of Mars? Are stars in the Orion Nebula surrounded by gas? Spectroscopy: What is the chemical composition of the Crab Nebula? What is the temperature of Jupiter's atmosphere? Is the star Vega moving toward us or away from us? Timing: Does the star Mira vary in brightness? Is the X-ray emission from the galactic center steady or changing? The three questions that require imaging are ones in which we are actually looking for something, while the two that require time monitoring both involve how something changes with time. We use spectroscopy to learn about chemical composition, temperature, and Doppler shifts that tell us about an object's motion.

_____ telescopes are very similar to optical telescopes, except that they use different detectors to sense the light. An example is the Spitzer ____ Space Telescope.

Infrared

Each of the following statements describes an astronomical measurement. Place each measurement into the appropriate bin based on the type of telescope you would use to make it.

Infrared telescope: Determine the surface temperature of Venus. Steady a dense cloud of cold gas in space. Visible Light telescope: Obtain a spectrum of the sunlight reflected by Mars. Measure the brightness of a star that is similar to our Sun. X-ray telescope: Observe the hot (1-million K) gas in the Sun's corona. Look for high-energy radiation from a supernova. Cool objects emit in the infrared. The Sun and similar stars emit mostly in the visible; reflected light from Mars is therefore also visible light. Very hot gas emits X rays, which have very high energy (with only gamma-ray photons having more energy for light).

What is the purpose of interferometry?

It allows two or more small telescopes to achieve the angular resolution of a much larger telescope. This has been done for decades with radio telescopes and is now being done with other types of light as well.

What does the technique of interferometry allow?

It allows two or more telescopes to obtain the angular resolution of a single telescope much larger than any of the individual telescopes.

What is meant by spectral resolution?

It is a measure of how close two spectral lines can be distinguished.

Which of the following is not an advantage of the Hubble Space Telescope over ground-based telescopes?

It is closer to the stars. Distance to the stars has absolutely nothing to do with it, which should be apparent if you consider the scale of the solar system and the distances to stars to scale (as discussed in Chapter 1).

What do we mean by the diffraction limit of a telescope?

It is the angular resolution the telescope could achieve if nothing besides the size of its light-collecting area affected the quality of its images. However, in reality, telescopes usually do not achieve their diffraction limit because of distortion caused by the Earth's atmosphere or because of distortion introduced by imperfections in the telescope's construction.

Which of the following best describes what we mean by the focal plane of a telescope?

It is the place where, if we mounted film or an electronic detector, we could get a clear (not blurry) image of an object viewed through the telescope. In other words, it is where the image is in focus.

What is the purpose of adaptive optics?

It reduces blurring caused by atmospheric turbulence for telescopes on the ground. It does this by changing the shape of a telescope mirror to compensate for the way that the atmosphere causes the light from a star to move around.

Listed following are the names and mirror diameters for six of the world's greatest reflecting telescopes used to gather visible light. Rank the telescopes from left to right based on their light-collecting area from largest to smallest. For telescopes with more than one mirror, rank based on the combined light-collecting area of the mirrors.

Larger mirrors have a larger light-collecting area. Remember that the light-collecting area is proportional to the square of the mirror diameter. For example, if Mirror A has twice the diameter of Mirror B, then Mirror A has 2x2 = 4 times the light-collecting area of Mirror B.

What do astronomers mean by light pollution?

Light pollution is light from human sources that makes it difficult to see the stars at night. Light pollution refers to light used for human activities that brightens the sky and hinders astronomical observations.

How much more light does an 8-meter telescope gather than a 2-meter telescope?

Light-collecting area depends on the square of the telescope mirror diameter. Because an 8-meter telescope has 4 times the diameter of a 2-meter telescope, its light-collecting area is 4^2 = 16 times as great.

Consider two future observatories in space. Observatory X consists of a single 50-meter telescope. Observatory Y is an interferometer consisting of five 10-meter telescopes, spread out over a region 100 meters across. Which observatory can detect dimmer stars, and which one can see more detail in its images? (Assume all else is equal, such as quality of optics, types of instruments, and so on.)

Observatory X can detect dimmer stars, and Observatory Y reveals more detail in images. This is correct because Observatory X has a larger light-collecting area, whereas Observatory Y has a better angular resolution.

____ astronomy uses dishes like satellite dishes to observe objects. In some cases, the dishes work together to form images at a much higher resolution than any one of them could achieve alone. An example is Very Large Array in New Mexico.

Radio

Listed following are distinguishing characteristics and examples of reflecting and refracting telescopes. Match these to the appropriate category.

Reflecting Telescopes: most commonly used by professional astronomers today, The Hubble Space Telescope, world's largest telescope Refracting Telescopes: The world's largest is 1-meter in diameter, Galileo's telescopes, very large telescopes become "top- heavy", incoming light passes through glass

____refers to telescopic observations in which we separate an object's light so we can measure its intensity at different wavelengths.

Spectroscopy

Which of the following studies is best suited to astronomical observations that fall into the category called time monitoring?

Studying how a star's brightness varies over a period of 3 years. Time monitoring allows us to measure changes with time.

Which of the following statements best describes the two principle advantages of telescopes over eyes?

Telescopes can collect far more light with far better angular resolution. That is, telescopes have a larger light-collecting area than the eye and a better (smaller) angular resolution.

How does the light-collecting area of an 8-meter telescope compare to that of a 2-meter telescope?

The 8-meter telescope has 16 times the light-collecting area of the 2-meter telescope. The 8-meter telescope is 4 times larger in diameter, so its light collecting area is 4^2 = 16 times greater.

Which of the following telescopes benefits most from adaptive optics?

The Keck I telescope on Mauna Kea Adaptive optics can help it achieve better angular resolution by countering the blurring effects of atmospheric turbulence.

Why does the Sun's image look distorted in shape at sunset?

The distortion arises because of the way air affects the paths of light. More specifically, light slows down when it enters air or other substances, causing light rays to bend. The lower portion of the setting is seen through more atmosphere and therefore its light is bent more than the light from the upper portion.

Which of the following statements about light focusing is not true?

The focal plane of a reflecting telescope is always located within a few inches of the primary mirror

Suppose that two stars are separated in the sky by 0.01 arcsecond, and you observe them with a telescope that has an angular resolution of 1 arcsecond. What will you see?

The light from the two stars will be blended together so that they look like one star. A telescope with an angular resolution of 1 arcsecond can only distinguish two stars if they are separated by an angle greater than 1 arcsecond in the sky. Otherwise, the light of the two stars will be blended together so they appear as one.

Suppose you have two small photographs of the Moon. Although both look the same at small size, when you blow them up to poster size one of them still looks sharp, whereas the other one becomes fuzzy (grainy) looking. Which of the following statements is true?

The one that still looks sharp at large size has better (smaller) angular resolution than the one that looks fuzzy. It reveals details that are not present in the other photograph, so it must have seen smaller angular scales on the Moon.

The angular separation of two stars is 0.1 arcseconds and you photograph them with a telescope that has an angular resolution of 1 arcsecond. What will you see?

The photo will seem to show only one star rather than two. Because the angular separation of the stars is smaller than the telescope's angular resolution, the light of the two stars will be blurred together to look like a single star.

Which of the following is not a reason why telescopes tend to be built on mountaintops that are relatively far from cities and are in regions with dry climates?

The thin air on mountaintops makes the glass in telescope mirrors less susceptible to warping.

What are the two most important properties of a telescope?

The two most important properties of a telescope are its light-collecting area, which determines how faint of objects it can detect, and its angular resolution, which determines the sharpness of its images.

Why are astronomers interested in building observatories capable of detecting neutrinos, cosmic rays, and gravitational waves?

These things are not forms of light, and therefore can provide different types of information than light about objects that emit them. Neutrinos are low mass subatomic particles that are emitted by a variety of processes. Cosmic rays are high-energy particles with origins that remain mysterious. Gravitational waves are predicted by Einstein's general theory of relativity to come from exotic objects such as orbiting pairs of neutron stars and black holes.

Shown following are six different types of light that travel to Earth from sources in space. Rank these types of light from left to right based on the altitude in the atmosphere where they are completely absorbed, from highest to lowest (Earth's surface). If two (or more) of the choices reach the same altitude or the surface, rank them as equal by dragging one on top of the other(s).

X rays > most ultraviolet light > most infrared light > most radio waves = green visible light That is why radio telescopes and visible-light telescopes work on the surface, infrared telescopes are useful on high mountaintops or airplanes, ultraviolet telescopes can be lofted up with high-altitude balloons, and X-ray telescopes are useful only in space.

Why can't X-ray and gamma-ray telescopes use the same designs as visible-light telescopes?

X rays and gamma rays have so much energy that they cannot be reflected in the same way as visible light. Trying to focus these high-energy photons is a bit like trying to focus bullets, and therefore requires different telescope technology.

The Chandra X-ray Observatory must operate in space because __________________________.

X rays do not penetrate Earth's atmosphere To detect X rays, the observatory must be above Earth's atmosphere.

____ telescopes have to gently redirect the light because it tends to go straight through things in its path. Mirrors are arranged so that the light just grazes them. An example is the Chandra Space Telescope.

X-ray

What is the primary reason that we launch X-ray telescopes into space rather than building them on the ground?

X-rays from space do not reach the ground. In fact, most forms of light do not reach the ground, with visible and radio waves as the major exceptions.

Suppose you have a camera attached to a telescope, and you want to record an image of a faint galaxy. Which of the following will help the most?

a lot of pixels and a long exposure time More pixels enable the image to be closer to the telescope's optimal resolution and longer exposure time allows the camera to collect more light.

What is the angular resolution of the human eye?

about 1 arcminute, or 1/60 of a degree

The familiar twinkling of the stars is caused by _____ , which also blurs telescopic images.

atmospheric turbulence

What causes stars to twinkle?

bending of light rays by turbulent layers in the atmosphere

If our eyes were sensitive only to X rays, the world would appear __________.

dark because X-ray light does not reach Earth's surface Because X rays from the Sun do not reach Earth's surface, eyes that were sensitive only to X rays would have nothing to see

Order the following in order of increasing efficiency of detecting photons of visible light

eye, photographic film, CCD

A camera is an example of an instrument used for____ observations.

imaging

If you had only one telescope and wanted to take both visible-light and ultraviolet pictures of stars, where should you locate your telescope?

in space While visible light can be observed from the ground, ultraviolet light can be easily observed only from space. Indeed, the capability of observing ultraviolet light is a major advantage of the Hubble Space Telescope over larger ground-based telescopes.

The James Webb Space Telescope is designed primarily to observe __________.

infrared light Its location in space allows it to observe infrared wavelengths that do not penetrate our atmosphere to the ground

Human civilization is responsible for what astronomers call _____.

light pollution

Which of the following wavelength regions can be studied with telescopes on the ground?

radio, visible, and very limited portions of the infrared and ultraviolet regions

If we want to confirm that a star's brightness alternately dims and brightens, we need ____ observations of the star.

time monitoring

The stars in our sky twinkle in brightness and color because of ______.

turbulence in the Earth's atmosphere Stars twinkle because turbulence keeps changing the precise amount by which light bends as it passes through the atmosphere.

Most of the____ wavelengths act enough like the visible wavelengths that we can use the same sorts of telescopes, with different detectors. An example is the Hubble Space Telescope.

ultraviolet

Which of the following forms of light can be observed with telescopes at sea level?

visible light, radiowaves Both visible light and radio waves pass almost freely through Earth's atmosphere, and therefore are easily observed with ground-based telescopes. The only other light that can be observed with ground-based telescopes is infrared, but it can be detected only at high altitudes (such as mountaintops) and even then only in selected portions of the infrared spectrum