Astro- Chp. 5: Telescopes

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.

Largest> Large . Binocular Telescope (Two 8.4-m mirrors)

Keck I (One 10-m mirror)

Hobby-Ebberly (One 9.2-m mirror)

Subaru (One 8.3-m mirror)

Gemini North (One 8-m mirror)

Magellan II (One 6.5-m mirror)

>Smallest

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.

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

Part complete

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 todayThe Hubble Space Telescope world's largest telescope

Refracting telescopes

The world's largest is 1-meter in diameter very large telescopes become "top-heavy"incoming light passes through glassGalileo's telescopes

The BLANK 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 BLANK BLANK.

reflecting telescopes

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

Spectographs

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

interferometry

The Chandra X-ray observatory focuses X rays with BLANK BLANK mirrors.

grazing incidence

A 10-meter telescope has a larger BLANK-BLANK BLANK than a 4-meter telescope

light-collecting area

Galileo's telescope designs using lenses were examples of BLANK BLANKS.

refracting telescopes

Astronomical sources emit electromagnetic radiation at various wavelengths. Some sources might emit just visible and infrared radiation. Other sources might emit gamma, UV, visible, and infrared radiation. Some of that radiation travels in the direction of Earth and can be detected with the right telescopes placed in the right locations. Some wavelengths can be read in the atmosphere. However, the majority of wavelengths are either read from space or Earth's surface. Which observations would require you to launch a telescope above the Earth's atmosphere? Please drag each observing mode below into either the ground-based (Surface) zone or space-borne (Space) zone, depending on the respective observing requirements.

Space zone

Far-infrared observationsGamma-ray observationsX-ray observationsFar-ultraviolet observations

Surface zone

Visible (optical) observationsRadio observations

t these wavelengths, it is necessary to build telescopes with very large light-collecting BLANK to obtain maps of sufficient sensitivity to faint cosmic signals.

areas

At these wavelengths, it is necessary to build telescopes with very large signal-sensing BLANK to obtain maps of sufficient detail.

baselines

Radio astronomers have pioneered the use of multiple telescopes working in concert that can produce maps of radio emission as detailed as optical images. These arrays of multiple telescopes are known as BLANKS.

Interferometers

Astronomical observations at these wavelengths can be obtained even during BLANKS.

Storms

The value of X-ray and gamma-ray astronomy is based on the fact that some of the most energetic sources in the cosmos radiate at these wavelengths. Magnetic storms in stellar atmospheres (including storms on the Sun), exploding stars and their nebular remnants, the regions surrounding neutron stars and black holes, and the innermost regions associated with active galactic nuclei characterize the highest energy, shortest wavelength regime. What would you need to obtain a successful X-ray image of a high-energy source?

A telescope designed and built to be launched into space.

A cosmic source of high temperature.

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

Reflecting telescopes

The Hubble Space TelescopeMost commonly used by professional astronomers today world's largest telescope

Refracting telescopes

very large telescopes become "top-heavy"incoming light passes through glass Galileo's telescopesThe world's largest is 1-meter in diameter

Part complete

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 dusty disks of gas?

Spectroscopy

What is the temperature of Jupiter's atmosphere?Is the star Vega moving toward us or away from us?What is the chemical composition of the Crab Nebula?

Is the star Vega moving toward us or away from us?What is the chemical composition of the Crab Nebula?

Time Monitoring

Does the star Mira vary in brightness?Is the X-ray emission from the galactic center steady or changing?

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

Study a dense cloud of cold gas in space.Determine the surface temperature of Venus.

Visible light telescope

Measure the brightness of a star that is similar to our Sun.Obtain a spectrum of the sunlight reflected by Mars.

X-ray telescope

Observe the hot (1-million K) gas in the Sun's corona.Look for high-energy radiation from a supernova.

Part complete

One advantage of the Hubble Space telescope over ground based ones is that:

in orbit, it can operate close to its diffraction limit at visible wavelengths.

What is image processing?

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

What is "seeing"?

a measurement of the image quality due to air stability

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.

Which of the following is NOT a difference between the operation of a radio telescope and that of an optical reflecting telescope?

Optical reflectors can have their detector placed at the prime focus, whereas radio telescopes never do.

Therefor their differences are...

Optical reflectors can detect the entire visible range of wavelengths simultaneously, whereas radio telescopes can detect only a small band of wavelengths at one time.

Optical reflecting telescopes can only observe interstellar objects during the night, whereas radio telescopes can also observe them during the day.

Optical reflectors must have smooth surfaces for reflecting visible light, whereas radio telescope dishes don't.

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

Earth's atmosphere absorbs most ultraviolet wavelengths

Why are most large telescopes reflectors, not refractors?

Large, very clear lenses are harder to cast than more tolerant mirror blanks.

Reflectors do not suffer from chromatic aberration like refractors do.

Large lenses deform under their own weight, but mirrors can be supported.

Large mirrors need only one optical surface, achromats four surfaces to grind

The image shown in Figure 5.12 in the textbook ("Resolution") is sharpest when the ratio of wavelength to telescope size is

small

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

Reduce atmospheric blurring

Compared with radio telescopes, optical telescopes can

Resolve finer detail

Part complete

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

The distance between telescopes

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

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

The best frequency range in which to study the hot (million-kelvin) gas found among the galaxies in the Virgo galaxy cluster would be in the following region of the electromagnetic spectrum:

X-Ray

The best way to study warm (1000 K) young stars forming behind an interstellar dust cloud would be to use

Infrared

Galaxies look the same whether viewed in visible or X-ray wavelengths.

FALSE

A 1 m telescope can collect a given amount of light in 2 hour

Under the same observing conditions, how much time would be required for a 8 m telescope to perform the same task?

1.9 min

A 1 m telescope can collect a given amount of light in 2 hour

A 12 m telescope?

.83 mins

All large modern telescopes are reflecting telescopes, not refracting telescopes.

TRUE

The Hubble Space Telescope is designed to detect ultraviolet, visible, and infrared radiation.

TRUE

The reason stars "twinkle" is that stars are constantly varying in their intrinsic brightness. Thus, stars would also appear to "twinkle" if viewed from space.

FALSE

It is possible to compensate for atmospheric blurring by adjusting the telescope mirror in real time, in a process known as ________ ________ (2 words).

Active Optics

______ telescopes can observe 24 hours a day, and can even observe through clouds, rain, and snow.

Radio

Using multiple small telescopes together in unison to serve as one large telescope is referred to as...

interferometry

Infrared radiation is useful for detecting which of the following? (SELECT ALL THAT APPLY.)

star forming regions

cool dust

Ultraviolet observing must be done from space.

TRUE

Two giant "bubbles" were detected coming out of the center of the Milky Way galaxy. What type of radiation were these bubbles detected in?

Gamma Rays

Because radio telescopes are so much larger than other types of telescopes, they produce radio "images" with much better resolution than normal visible images.

FALSE

Images can be formed through reflection or

refraction

FACT

• Light traveling through a lens is refracted

differently depending on wavelength

FACT

• Some light traveling through lens is absorbed

FACT

Large lens can be very heavy, and can only be

supported at edge

FACT

• A lens needs two optically acceptable

surfaces; mirror needs only one

FACT

Resolving power: When

better, can distinguish

objects that are closer

together

FACT

Resolution is proportional

to wavelength and

inversely proportional to

telescope size—bigger is

better!

FACT

This diffraction of light sets the best-case limit

of angular resolution, based on wavelength

(shorter=better) and telescope size

(larger=better).

FACT

Image acquisition: Charge-coupled devices

(CCDs) are electronic devices, which can be

quickly read out and reset

FACT

How CCDs work, in a nutshell...

• Millions of cells, pixels

• Each one is light sensitive even to a single

photon à each photon builds up electrical

charge in the pixel

• Computer reads out the results

• Reconstruct image

FACT

Modern telescopes

use mirrors rather

than lenses for all

of these reasons

EXCEPT

Reflecting telescopes arent affected by

the atmosphere as much.

Resolution is

improved by

using

c) larger telescopes & shorter wavelengths.

Diffraction is the

tendency of light

to

a) bend around corners and edges.

Atmospheric blurring is due to

air movements

Adaptive optics: Track atmospheric changes

with laser guide; adjust mirrors in real time

FACT

Gamma rays are from the most energetic

reactions known, such as supernovas or black

holes.

FACT