Astronomy Ch 5 - Telescope Fundamentals

The mirror in the Hubble telescope has a diameter of about 250 centimeters. If it is observing light with a wavelength of 500 nanometers the resolving power is about ___ of 1arc seconds. (Enter as a number.)

0.04

What is the smallest separation angle resolvable by a 50-centimeter telescope observing blue light of 500 nanometers?

0.25 arc seconds - (α > 0.025 λ / D )

Rank these telescopes according to their ability to collect light, from best (top) to worst (bottom).

1. 5.0 2. 3.5 3. 2.5 4. 1.5 5. 0.75

To display data collected by a radio telescope, specific visible ___ are assigned to represent the original intensity range of the data in an image.

colors

Infrared telescopes, such as the Spitzer Space Telescope, must be placed in space. These telescopes have a limited lifetime because they ____. (Select all that apply.)

constantly use coolant may be too far away for a repair mission

The most common designs for small reflector telescopes are Newtonian and Cassegrain. A Cassegrain telescope has a ____.

curved secondary mirror hole in the primary mirror

Earth's atmosphere is a concern to astronomers because it ___ the image in a telescope.

degrades, blurs, or distorts

A telescope with a higher ratio of aperture diameter to observed wavelength of light would ____.

detect finer detail in objects - (The ability to detect fine detail is the resolution. Resolution is determined by the ratio of aperture size to wavelength)

The size of a telescope's ___ determines its light-gathering power.

diameter

The ring and streak patterns seen around this image of a star are caused by the wave phenomenon of ____.

diffraction

Arrange the statements to explain how adaptive optics are used to improve ground-based telescope observations

1. A laser beam is projected into the atmosphere near the target of observation 2. The artificial star's image is observed by the telescope 3. A computer compares the image to what is expected 4. The telescope primary and/or secondary mirrors are adjusted by tiny motors to correct for the atmospheric distortions 5. The process is repeated continuously as quickly as possible

Rank the choices from the most efficient (top) to least efficient (bottom) method of collecting light.

1. CCD 2. Photographic film 3. Human eye

If your new telescope collects nine times more light than your old one, it is ___ times larger in diameter than your old telescope.

3

You solve the equation and calculate the answer Dcm = 300 centimeters. This means the telescope diameter must be ____ to achieve the desired resolving power.

300 centimeters or larger

How much more light will a 10-meter telescope collect than a 5-meter telescope?

4 times as much - (Since the area depends on the square of the diameter, twice the diameter captures four times as much light)

To record a visible image of a dim galaxy in the shortest time, which of these telescopes would be the best choice?

5.0-meter reflector on Earth's surface

Match the reflecting telescope design to where the focus is placed.

A - In front of the mirror (Prime focus) B - To the side of the light path (Newtonian) C - Behind the mirror (Cassegrain)

Match the telescope type to its diagram.

A - Refractor B - Reflector

Which of these is the best choice for observing a dim galaxy?

A CCD - (A CCD absorbs most of the photons that fall on it)

Select all of the properties that describe reflecting telescopes.

A mirror is the primary collector. Often the secondary mirror blocks some of the primary.

When two telescopes are linked together as an interferometer, they are ____.

Acting as opposite edges of a large telescope Using interference to improve resolving power

The image shows an electronic detector called a ___ that is the basis of most digital cameras and widely used in astronomy.

CCD, charge-coupled device, charged coupled device, or charge coupled device

___ occurs when light waves pass through a small opening, creating patterns of smaller waves.

Diffraction

True or false: The primary purpose of a telescope is to make images of astronomical objects larger (magnify the view).

False - (Actually, the primary purpose of most telescopes is to collect more light from celestial objects, allowing dim objects to be seen in more detail)

Select the choices that describe information an astronomer might include in a proposal to use a telescope

Information about the specific cameras to be used An argument why the observation is useful Analysis of previous research on the target object An explanation of what scientific results will be obtained

Advantages of reflecting (mirror) telescopes over refracting (lens) telescopes include the property that ____. (Select all that apply.)

Mirrors do not absorb short wavelengths of light Mirrors focus all colors equally well You can support a mirror from behind

Which statements accurately describe how CCD works?

Photons interact with the semiconductor to free electrons in a pixel. Brightness is determined by the number of electrons in each pixel.

The largest optical telescopes in the world are reflecting telescopes, because it is difficult to make a refractor (lens) telescope larger than about 1 meter in diameter. It is difficult because ____. (Select all that apply.)

Refracting telescopes have long focal lengths and are very large The heavy lens distorts under its own weight It is very difficult to support large pieces of glass only by the edge

Match the telescope property to its definition.

Resolving power - The ability to see fine detail Magnification - The ability to enlarge objects Sensitivity - The ability to detect low surface-brightness objects Field of view - The angular size of the sky that can be observed

Two radio telescopes observe the same frequency of light. Telescope A has a diameter six times the diameter of telescope B. Select all the statements that are true of A and B.

Telescope A has 36 times the sensitivity of B. Telescope A can resolve six times finer detail than B.

The resolving power of your new telescope is 2 arc seconds. This means that 2 arc seconds is ____.

The minimum angular separation between two stars so they are seen as two stars and not one

A ruler placed in a glass of water appears bent because ____.

The speed of light changes when light passes from water to air - (The difference in the speed of light in the two materials causes a slight change in the direction of the light waves)

Select all that are common purposes of telescopes.

To allow astronomers to see objects in more detail To collect nonvisible wavelengths of light from celestial objects To collect more light than the eye collects

To maintain a large telescope's primary mirror shape as it is pointed in different directions, ____

a segmented mirror is used - (Each segment can be individually adjusted so the entire surface maintains its shape)

In addition to directly obtaining their own observations of a specific target from a ground-based observatory, astronomers today might ____

apply for time using a space telescope such as the Hubble Space Telescope or the Chandra X-ray Observatory create theoretical models and simulations of galaxy formation with clusters of computers and compare with online archives use data from an online archive of a survey, such as observations of thousands of galaxies in the Sloan Digital Sky Survey

Light pollution refers to ____

artificial light from inhabited areas being reflected and scattered in the night sky

The number of photons striking a "charge-coupled device" (CCD) is determined by collecting the ___ freed when photons are absorbed by atoms in the semiconductor (usually silicon) layer of the CCD.

electrons

To maximize the quality of observations, an observatory with a very large telescope would best be built ____

in a dry area

Two widely separated telescopes might be linked together as an interferometer to ____

increase the resolution of observations - (An interferometer has a much bigger effective diameter, so it has a high resolution.)

To study a cool dust cloud, an astronomer would have to observe at radio or slightly shorter ___ wavelengths.

infrared

The telescopes in this image are set up to work as a(n)

interferometer

The effect shown in this figure is called ___ pollution. It can make observing very difficult but can be mitigated by using downward-pointing fixtures (which is also more energy efficient).

light

A mirror designed for visible light would not be suitable to observe X ray wavelengths because the ____

light would pass through the mirror - (X rays would not reflect off the surface; they would penetrate the mirror and either be absorbed or pass straight through)

To minimize atmospheric effects, major observatories are built ____

on mountaintops in deserts

In this diagram, the small squares are called

pixels

Compared to telescope A, telescope B with the same eyepiece will

produce brighter images

A telescope of this design is used to observe at ___ wavelengths.

radio

An astronomer is using a telescope that consists of a metal antenna mounted on struts above a 10-meter curved metal dish, mounted outside (no dome). The whole telescope is steerable and can be used during the day or at night. What kind of astronomy is being done?

radio - (The description is of a radio telescope. Some optical telescopes are this big, but they would be in domes, and would use a camera rather than an antenna)

In contrast to film, CCDs, such as those used in digital cameras, ____

record more of the photons that strike them can be reused

A telescope that focuses light by bending light rays through a lens is a ___ telescope

refracting

False-color images are useful in astronomy to ____

represent the brightness of non-visible radiation overlay different kinds of information on a single image represent the energies or wavelengths of non-visible photons

A telescope's ability to separate two closely spaced stars is called ___ power.

resolving

Atmospheric irregularities slightly disperse light, making the star's color dance. The dancing image of a star or planet distorts its picture when recorded by a camera or other device. This distortion is called ___ by astronomers

seeing

The largest ground-based telescopes use ____

segmented mirrors - (This allows large-aperture mirrors to be produced more easily)

Astronomers might create a false-color image to ____

show relative brightness of radio photons show surface elevation of a moon show photon energies of an X-ray object

While looking at a star, you notice it twinkles a lot. This is caused by ____

small pockets of warmer and cooler air distorting the image - (This effect is called scintillation and results in poor "seeing")

Telescopes are placed in orbit ____

so that local weather is not a problem to observe wavelengths blocked by the atmosphere to eliminate atmospheric effects that degrade the image

An example of a false color image is shown. A false color image assigns ___

specific visible colors to photons of nonvisible wavelengths

Light is refracted (bent) as it passes through a telescope lens because its ___ changes

speed

The first step an astronomer takes to observe with an observatory's telescope is ____

submitting a proposal

During an observing session, you find the sky to be crystal clear but the stars are twinkling and the image in your telescope dances around. You might conclude that ____

the seeing will make your images look blurry

In an interferometer, the image resolution is set by ____.

the separation of the individual telescopes - (The separation between individual telescopes is what sets the resolution of an image)

To correct for the effects of small pockets of warmer and cooler air on light passing through the atmosphere (scintillation), astronomers ____

use adaptive optics

A refracting telescope ___

uses a lens to bend light