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Telescopes: Astronomers ”Eyes”

Dr. Brian Uzpen

Guest Speaker: Discovering Astronomy

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How do we experience light?

• The warmth of sunlight tells us that light is

a form of energy.

• We can measure the flow of energy in light

in units of watts: 1 watt = 1 joule/s.

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Colors of Light

• White light is made up of many different colors.

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Wavelengths and Colors

Different colors of visible light correspond to

different wavelengths.Source: Seeds

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How do light and matter interact?

• Emission

• Absorption

• Transmission

– Transparent objects transmit light.

– Opaque objects block (absorb) light.

• Reflection/scattering

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The Electromagnetic Spectrum

Need satellites

to observe

Wavelength

Frequency

High

flying air

planes or

satellites

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If you wanted to look at the Moon, which

telescope would you choose? And why?

• A) 8-inch Celestron telescope

• B) 2.4m Hubble Space Telescope

• C) 10m Keck Telescope

• D) 2.3m Wyoming InfraRed Observatory

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If you wanted to look at the furthest galaxies,

which telescope would you choose? And

why?

A) 8-inch Celestron telescope

B) 2.4m Hubble Space Telescope

C) 10m Keck Telescope

D) 2.3m Wyoming InfraRed Observatory

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Each image was taken from a telescope on the same

mountain top during the same evening. Which image

was taken from the largest telescope.

A B

C

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The left shows what a star would look like through a small

telescope. Take a few minutes and draw what it would look

like with higher magnification.

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How does your eye form an

image?

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Refraction

• Refraction is the

bending of light

when it passes

from one

substance into

another.

• Your eye uses

refraction to

focus light.

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Example: Refraction at Sunset

• Sun appears distorted at sunset because of how light bends in Earth’s atmosphere.

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Focusing Light

• Refraction can cause parallel light rays to converge to a focus.

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Image Formation

• The focal plane is where light from different directions comes into focus.

• The image behind a single (convex) lens is actually upside-down!

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You are stranded on a desert

island…..

Which object do you want to lay out to collect

the rain?

A) an oyster shell

B) A coconut

C) A bucket (from your sunken ship)

D) Your hydroflask (which washed ashore)

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What are the two most important

properties of a telescope?

1. Light-collecting area: Telescopes with a

larger collecting area can gather a greater

amount of light in a shorter time.

2. Angular resolution: Telescopes that are

larger are capable of taking images with

greater detail.

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Light-Collecting Area

• A telescope’s diameter tells us its light-collecting area:

• The largest optical telescopes currently in use have a diameter of about 10 meters.

• Construction is underway of telescopes as large as 30 meters.

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The Powers of a Telescope:

Bigger is better!

1. Light-gathering power:

Depends on the

surface area A of the

primary lens / mirror,

proportional to

diameter squared:

A = p (D/2)2

D

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If the light gathering power of a telescope is described

by the area, A, of the primary lens or mirror, with D

the diameter

A = p (D/2)2

how many times more light does the 10 m diameter

Keck telescope gather than the 5m diameter Hale

Telescope?

A. 2

B. 50

C. 4

D. 0.5

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Angular Resolution

• The minimumangular separation that the telescope

can distinguish

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Who wears glasses?

• The two dots are 0.5 cm

in diameter and

separated by 1 cm.

• Humans with perfect

vision have a resolving

power of 1 or 2 arc

minutes.

• Where do the lines blur?

Distance (ft) Resolving

power

(arcminutes)

110 1

55 2

40 3

30 4

20 6

10 11

6 20

4 30

3 40

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Angular Resolution

• Ultimate limit to resolution comes from interference of light waves within a telescope.

• Larger telescopes are capable of greater resolution because there’s less

interference.

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Angular Resolution

• The rings in this image of a star come from interference of light wave.

• This limit on angular resolution is known as the

diffraction limit.

Close-up of a star from the Hubble

Space Telescope

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The Powers of a Telescope

Magnifying Power = ability of the telescope to

make the image appear bigger.

A larger magnification does not improve the

resolving power of the telescope!

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What are the two basic designs of

telescopes?

• Refracting telescope: focuses light with

lenses

• Reflecting telescope: focuses light with

mirrors

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Refracting / Reflecting Telescopes

Refracting

Telescope: Lens

focuses light onto

the focal plane

Reflecting

Telescope:

Concave Mirror

focuses light onto

the focal plane

Almost all modern telescopes are reflecting telescopes.

Focal length

Focal length

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Refracting Telescope

• Refracting

telescopes

need to be

very long,

with large,

heavy lenses.

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Reflecting Telescope

• Reflecting telescopes can have much greater

diameters.

• Most modern telescopes are reflectors.

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Designs for Reflecting Telescopes

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Types of reflectors

• Classical Cassegrain: the secondary reflects the light through a

hole in the primary so that light can be observed with the

instrument beneath the telescope. Telescope on the right is

WIRO (Wyoming InfRared Observatory on Jelm Mtn.)

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Types of reflectors

• Nasmyth Focus: the secondary reflects the light to a tertiary

(third) mirror and that light can be observed with an instrument

of the side of the telescope.

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Mirrors in Reflecting Telescopes

Twin Keck telescopes on

Mauna Kea in Hawaii

Segmented 10-meter mirror

of a Keck telescope

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Segmented MirrorsThe Keck telescope (previously shown) and the

James Webb Telescope that is currently under

construction are two (of many) telescopes with

segmented mirrors. The James Webb telescope will

combine 18 hexagonal into the pattern shown below.

Compared to a single segment, the composite mirror

has a light gathering power of…..

A) 18 times smaller

B) 36 times greater

C) 18 times greater

D) 324 times greater

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What astronomers see

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What do astronomers do with

telescopes?

• Imaging: taking pictures of the sky

• Spectroscopy: breaking light into spectra

• Timing: measuring how light output varies

with time

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What astronomers see

• With the photography came photographic plates in astronomy.

• They evolved over time but were made of particles suspended in emulsion. (Emulsion plates).

• Eventually let to color plates.

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CCD ImagingCCD = Charge-coupled device

• More sensitive than

photographic plates

• Data can be read directly into

computer memory, allowing

easy electronic manipulations

False-color image to visualize

brightness contours

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What astronomers see

• Astronomers use CCD in different filters and combine them to see the “full” color image.

• Top left- the Horsehead nebulae first discovered with photographic plates.

• Bottom left-Andromeda galaxy.

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What astronomers see

• Astronomer’s combine

the different filters to

make “color” images.

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Spectroscopy

• A spectrograph separates the different wavelengths of light before they hit the detector.

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Spectroscopy

• Graphing relative brightness of light at each wavelength shows the details in a spectrum.

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What astronomers see

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Timing

• A light curve represents a series of brightness measurements made over a period of time.

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How does Earth’s atmosphere

affect ground-based observations?

• The best ground-based sites for

astronomical observing are:

– calm (not too windy)

– high (less atmosphere to see through)

– dark (far from city lights)

– dry (few cloudy nights)

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Light Pollution

• Scattering of human-made light in the atmosphere is a growing problem for astronomy.

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Twinkling and Turbulence

Turbulent air flow in Earth’s atmosphere distorts our view, causing stars to appear to twinkle.

Bright star viewed with

ground-based telescope

Same star viewed with

Hubble Space Telescope

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Adaptive Optics

Rapidly changing the shape of a telescope’s mirror compensates for some of the effects of turbulence.

Without adaptive optics With adaptive optics

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Calm, High, Dark, Dry

• The best observing sites are atop remote mountains. Primary reason is to avoid the clouds. Secondary is atmospheric turbulence.

Summit of Mauna Kea, Hawaii

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Why do we put telescopes into

space?

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Transmission in Atmosphere

• Only radio and visible light pass easily through Earth’s atmosphere.

• We need telescopes in space to observe other forms.

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If you wanted to look at the Moon, which

telescope would you choose? And why?

A) 8-inch Celestron telescope

B) 2.4m Hubble Space Telescope

C) 10m Keck Telescope

D) 2.3m Wyoming InfraRed Observatory

© 2010 Pearson Education, Inc.

If you wanted to look at the furthest galaxies,

which telescope would you choose? And

why?

A) 8-inch Celestron telescope

B) 2.4m Hubble Space Telescope

C) 10m Keck Telescope

D) 2.3m Wyoming InfraRed Observatory

© 2010 Pearson Education, Inc.

Each image was taken from a telescope on the same

mountain top during the same evening. Which image

was taken from the largest telescope.

A B

C

© 2010 Pearson Education, Inc.

The left shows what a star would look like through a small

telescope. Take a few minutes and draw what it would look

like with higher magnification.