PHYS1404 Chapter 6

How much greater is the light-collecting area of a 6-meter telescope than a 3-meter telescope?

four times

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

light-collecting area

how much total light it can collect at one time

interferometry

in the 1950s, radio astronomers developed an ingenious technique for improving the angular resolution of radio telescopes: they learned to link two or more individual telescopes to achieve the angular resolution of a much larger telescope, which works by taking advantage of the wavelike properties of light that cause interference

exposure time

light can reach the detector only when the shutter is open. we can use the shutter to control the exposure time of an image, the amount of time during which light collects on the detector

light curves

results of time monitoring are often shown as light curves: graphs that show how an object's intensity varies with time

time-monitoring

tracks how an object changes with time

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

turbulence in the Earth's atmosphere

Which of the following effects is caused by atmospheric turbulence?

twinkling of stars

The Hubble Space Telescope obtains higher-resolution images than most ground-based telescopes because it is:

above earth's atmosphere

What do astronomers mean by light pollution?

Light pollution is light from human sources that makes it difficult to see the stars at night.

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.

Choose the correct statement describing, what will you see if you look at them with a telescope that has an angular resolution of 0.5 arcsecond.

One point of light that is the blurred image of both stars.

diffraction limit

The angular resolution that a telescope could achieve if it were limited only by the interference of light waves (diffraction is a technical term for the effects of interference that limit telescope resolution). the diffraction limit depends on both the diameter of the telescope's primary mirror and the wavelength of the light being observed

What does angular resolution measure?

The angular size of the smallest features that the telescope can see.

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.

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, while 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.

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.

image

a copy of an object formed by reflected or refracted rays of light

focus (focal point)

a place in a lens or mirror where all of the parallel light rays meet

pixels

modern detectors use electronic chips that are physically divided into grids of picture elements, or pixels for short

refracting telescope

operates much like an eye, using transparent glass lenses to collect and focus light

light pollution

out atmosphere scattering bright lights from cities at night

detector

the camera lens bends the light, bringing it to a focus on a detector that makes a permanent record of the image

spectral resolution

the information we can glean from a spectrum

focal plane

the place where the image appears in focus

chromatic abberation

the property of a lens whereby light of different colors is focused at different places

angular resolution

the smallest angle over which we can tell that two dots—or two stars—are distinct. human eye has an angular resolution of about one arcminute (1/60 degrees)

spectrographs

use diffraction gratings (or other devices) to separate the various colors of light into spectra, which are then recorded with a detector

reflecting telescope

uses a precisely curved primary mirror to gather light. this mirror reflects the gathered light to a secondary mirror that lies in front of it. the secondary mirror then reflects the light to a focus at a place where the eye or instruments can observe it—sometimes through a hole in the primary mirror and sometimes through the side of the telescope (often with the aid of additional small mirrors).

The only wavelengths that make it all the way to the surface are _____ and _____ wavelengths, along with small parts of the _____ and _____ spectrum. Most of other wavelengths, including _____ and _____, can be observed only from very high altitudes or space. As a result, without space-based observatories we could not study light from much of the electromagnetic spectrum.

visible light, radio infrared, ultraviolet gamma ray, x-ray

spectroscopy

which astronomers obtain and study spectra

imaging

yields photographs (images) of astronomical objects