Telescopes: Astronomers ”Eyes”n00006757/astronomylectures/ECP4e/First Lectur… · 1 © 2010...

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© 2010 Pearson Education, Inc.

Telescopes: Astronomers ”Eyes”

Dr. Brian UzpenGuest Speaker: Discovering Astronomy


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.


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


How do light and matter interact?

• Emission• Absorption• Transmission

– Transparent objects transmit light.– Opaque objects block (absorb) light.

• Reflection/scattering


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


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


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.


How does your eye form an image?


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.


Focusing Light

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


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 shellB) A coconutC) A bucket (from your sunken ship)D) Your hydroflask (which washed ashore)


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.


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 = (D/2)2

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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 = (D/2)2

how many times more light does the 10 m diameter Keck telescope gather than the 5m diameter Hale Telescope?

A. 2B. 50C. 4D. 0.5


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)

110 155 240 330 420 610 116 204 303 40


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.


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 HubbleSpace Telescope

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The Powers of a TelescopeMagnifying Power = ability of the telescope to

make the image appear bigger.

A larger magnification does not improve the resolving power of the telescope!


What are the two basic designs of telescopes?

• Refracting telescope: focuses light with lenses

• Reflecting telescope: focuses light with mirrors


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.


Reflecting Telescope

• Reflecting telescopes can have much greater diameters.

• Most modern telescopes are reflectors.


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.)


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.


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 smallerB) 36 times greaterC) 18 times greaterD) 324 times greater


What astronomers see


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.


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


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.


Spectroscopy

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


Spectroscopy

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

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What astronomers see


Timing

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


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.


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


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


Why do we put telescopes into space?


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





If you wanted to look at the furthest galaxies, which telescope would you choose? And

why?A) 8-inch Celestron telescope





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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Telescopes: Astronomers ”Eyes”n00006757/astronomylectures/ECP4e/First Lectur… · 1 © 2010...

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Transcript of Telescopes: Astronomers ”Eyes”n00006757/astronomylectures/ECP4e/First Lectur… · 1 © 2010...