Download reflecting telescope

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Lecture 22
Telescopes
January 10b, 2014
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The Hubble Space Telescope
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Why Use a Telescope?
• All astronomical objects are distant so a
telescope is needed to
– Gather light -- telescopes sometimes referred to
as “light buckets”
– Resolve detail (angular resolution)
– Magnify an image (least important of the three)
• Uses combination of lenses and/or mirrors
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Refracting Telescope
• Refraction = as light passes from one medium
to another (e.g. air to glass) it is bent
• Light is gathered and focused by a curved lens.
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Refracting Telescope
• First telescopes were of this type
• Not made for astronomical use any more
– Very difficult to make large, defect-free lenses
– Weight of large lenses makes them deform over
time.
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Reflecting Telescope
• A curved mirror is used to collect and focus
the light.
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Reflecting Telescope
• Used in modern telescopes
– Mirror can be supported from the back.
– Do not need large, defect free glass since surface
is coated with reflective material.
– Can be shaped to minimize aberrations
– Can be rapidly deformed to compensate for
atmospheric turbulence (adaptive optics)
– Can be manufactured at much larger scales than
refracting telescopes
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Focal Length
• Distance from lens or
mirror to focus =
focal length.
• Rarely will put an
eye or detector at the
prime focus (image
is too small)
• An eyepiece (usually
a lens) will be used
to magnify the
image.
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Eyepiece and Magnification
focal length of objective
Magnificat ion 
focal length of eyepiece
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What should the eyepiece focal length be for a
telescope with a 5.0-m focal length objective if a
magnification of 250× is desired?
A. 2.0 cm
B. 5.0 m
C. 50 m
D. 1,250 m
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What should the eyepiece focal length be for a
telescope with a 5.0-m focal length objective if a
magnification of 250× is desired?
A. 2.0 cm
f objective
M
B. 5.0 m
f eyepiece
C. 50 m
f
5.0 m
objective
D. 1,250 m
f


eyepiece
M
250
 0.020 m  2.0 cm
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Telescope Design
• For a reflecting telescope, a secondary mirror is used to
reflect the image to a detector outside of the telescope.
Prime Focus
Newtonian
Cassegrain
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Observing the Image
• On research telescopes astronomers rarely look through
the telescope
• Detector is put at the focus to record a digital image.
• Data are processed on a computer
CCD Chip
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Telescope Properties: Gathering Light
• The larger the area of
the primary mirror, the
more light can be
collected and the fainter
the object we can
detect.
Light Gathering Power  Diameter
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Gathering Light
Sometimes many smaller mirrors are
combined to make one large-area mirror
Keck telescopes: primary mirrors made of
36 hexagonal mirrors
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How much more light can a 12-inch reflecting
telescope gather compared with an 8-inch
reflecting telescope?
A. 1.5×
B. 2.25×
C. 8×
D. 144×
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How much more light can a 12-inch reflecting
telescope gather compared with an 8-inch
reflecting telescope?
 new 4 D
 
 old
4 D

A. 1.5×
B. 2.25×
C. 8×
D. 144×
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new
2
old
2
 12 in 

  2.25
 8 in 
 new  2.25   old
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Telescope Properties:
Angular Resolution
• The smallest
separation in angle
which can be
distinguished by the
telescope
• Angular resolution is
limited by
atmospheric blurring
and light diffraction
by the primary lens or
mirror
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Angular Resolution
due to diffraction
• Absolute limit of angular resolution:
  2.5 10
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
D
 = best angular resolution in
arcseconds
 = wavelength (meters)
D = diameter of mirror (meters)
• We want  to be small
• Shorter wavelengths = better resolution
• Larger mirror = better resolution
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Angular Resolution
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What is the maximum angular resolution of the
UWSP 16 inch (0.406 m) diameter telescope?
Assume a wavelength of 500 nm.
A.
B.
C.
D.
50 arcsec
25 arcsec
4.1 arcsec
0.31 arcsec
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What is the maximum angular resolution of the
UWSP 16 inch (0.406 m) diameter telescope?
Assume a wavelength of 500 nm.
A.
B.
C.
D.

  2.5  10
50 arcsec
D
25 arcsec
9
500  10 m 
5 
4.1 arcsec
 2.5  10
0.406 m 

0.31 arcsec
  0.308 arcsec
5
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Angular Resolution
due to atmospheric blurring
• Resolution often limited by motions in the
atmosphere (“twinkling”)
– Need site with calm, dry weather, little atmosphere
above the telescope to reduce twinkling effect.
– Adaptive optics: sensors monitor distortions due to
atmosphere and correct the shape of the mirror 10
to 100 times per second
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Adaptive Optics
Object viewed through
typical telescope
Object viewed with
adaptive optics
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Spectroscopy
• Light coming though telescope is separated
by prism or diffraction grating to produce a
spectrum.
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Observing at Other Wavelengths
• Wavelengths other than visible are very useful
since they are often produced by different objects
or processes than optical light
• Atmosphere blocks some types of light
– Low opacity: Optical and Radio
– Medium opacity: Infrared and UV
– High opacity: Gamma Rays, X-rays & some UV
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Infrared Telescopes
• Telescope design much like optical
telescope, but with different detector.
• Infrared light can pass through dust
• Used to observe star formation, center of
galaxies, low T objects (i.e. planets)
• Best if telescope is placed above much of
the atmosphere.
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IRAS
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Radio Telescopes
•
•
•
•
Can observe day or night
Not affected by Earth’s atmosphere
Radio light can pass though dust in space
Because wavelength is long, we need large
telescope to get good resolution
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64 m telescope at Parkes Obs. in
Austrailia
305 m telescope at Arecibo
Observatory in Puerto Rico
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Interferometers
• More than one radio telescope is used to increase
resolution.
– Creates a large effective diameter
– Image made only after much computer processing
Very Large
Array in New
Mexico
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Ultraviolet, X-rays and Gamma Rays
• All blocked by atmosphere
• Telescopes must be above atmosphere
Compton Gamma-ray Observatory
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Hubble Space
Telescope
Chandra X-ray Telescope
Spitzer Infrared
Telescope
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Sky at Many
Wavelengths