Telescopes: Astronomers ”Eyes”n00006757/astronomylectures... · telescope would you choose? And...
Transcript of Telescopes: Astronomers ”Eyes”n00006757/astronomylectures... · telescope would you choose? And...
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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.