ASTRONOMY 161 Introduction to Solar System Astronomy

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ASTRONOMY 161Introduction to Solar System
Astronomy
Class 11
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• If a light source is moving toward you, the
  wavelength is shorter (called a blueshift).
• If a light source is moving away from you, the
  wavelength is longer (called a redshift).

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Size of Doppler shift is
proportional to radial velocity.
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Example
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Few closing questions

• 1) What kind of spectrum will be produced by very
  hot, but also very dense hydrogen gas?
• 2) If you have hot gas in front of a star, what
  kind of spectrum will you see?
• 3) Which spectrum is more complex that of
  hydrogen or that of helium?
• 4) What is the maximum blueshift possible?
• 5) What is the maximum redshift possible?
  (tricky)

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TelescopesMonday, October 19
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Telescopes Key Concepts

• (1) Telescopes use either a lens or a mirror to
  gather light.
• (2) The main purposes of a telescope are to
  gather light and resolve detail.
• (3) Radio and microwave telescopes use a
  reflecting dish to focus waves.
• (4) Telescopes in orbit avoid the distorting
  effects of the atmosphere.
• (5) Ultraviolet, X-ray, gamma ray, and most
  infrared light is visible only from orbit.

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(1) Telescopes use either a lens or
a mirror to gather light.

• Refracting telescopes use a lens to bend (or
  refract) the path of light.

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• Light with a short wavelength is bent through a
  larger angle than light with a long wavelength.
• (This is why prisms spread light into a spectrum.)

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• A convex lens (thicker in the middle) focuses
  light to a point
• Light from a large area is funneled into a small
  area.

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• Reflecting telescopes use a mirror to change the
  path of light.
• Angle of incidence (I) equals angle of reflection
  (R), regardless of wavelength.

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• A mirror shaped like a parabola focuses light to
  a point
• Light from a large area is funneled into a small
  area.
• Lenses and mirrors (if shaped correctly) produce
  an accurate image of an object.

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• Reflecting telescopes use a number of designs

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(2) The main purposes of a telescope are
to gather light and resolve detail.

• A telescope is sometimes called a light bucket.
• Number of photons collected per second is
  proportional to the area of the lens/mirror
• Area p/4 x D2
• where D diameter of the lens/mirror.

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• A bigger telescope is able to resolve finer
  detail.
• Two stars are resolved if they are seen as two
  separate points.
• Smallest angle resolved is proportional to 1/D.

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• BIGGER IS
  BETTER
• Larger diameter for your lens or mirror
  means more light, higher resolution.

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The worlds biggest telescopes are reflectors
(mirrors), not refractors (lenses).

• The problem with lenses
• 1) Lenses absorb light.
• 2) Lenses sag.
• 3) Lenses have chromatic aberration colors
  do not focus at same point.

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The worlds largest refracting telescope

• Yerkes Observatory, Wisconsin
• 1 meter diameter
• Completed 1897

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Some of the smallest professional refracting
telescopes

• HATNet, Arizona, Hawaii 0.1 meter diameters.
• Established 2003

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Data from HATNet
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One of the worlds
largest reflecting telescopes (2x8.4 m)
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LBT First Light ImageNGC 891
NGC 891 LBT First Light 2005 October 12 LBC-B
camera
LBC Blue Image
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NGC 6946
LBC-B color composite
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Crab Nebula
LBC-B color composite
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NGC 2419
LBC-B color composite
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r 25.13?0.06
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GRB 070311
LBC-B Data
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(3) Radio and microwave telescopes use a
reflecting dish to focus waves.

• Parabolic dish of a radio telescope acts as a
  mirror, reflecting radio waves to the focus.

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• Radio telescopes can be huge, because they do not
  have to very smooth.
• Mirrors must be smooth compared to the wavelength
  of light observed
• Visible light l 0.0005 mm
• Microwaves l gt 1 mm
• Radio waves l gt 100 mm.
• Resolution of a telescope q l/D

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(4) Telescopes in orbit avoid the distorting
effects of the atmosphere.

• Turbulence in the air makes stars twinkle and
  limits resolution (so called seeing).
• City lights drown out faint stars.
• Great idea place a telescope in
  orbit, above the atmosphere.

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Earth at Night
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• Hubble Space Telescope (launched 1990)
• Diameter of mirror 2.4 meters
• Angular resolution 0.05 arcseconds

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(5) UV, X-ray, g ray, and most IR is visible only
from above the atmosphere.

• Infrared (IR) light is absorbed by water vapor in
  the Earths atmosphere.
• The Spitzer Space Telescope, launched in 2003,
  uses a 0.85 meter mirror.

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• Ultraviolet (UV) light is absorbed by ozone in
  the Earths ozone layer, 25 km up.
• The Hubble Space Telescope could detect UV light
  with l 100 nm.

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• X-rays are absorbed by most molecules in the
  Earths atmosphere.
• Chandra X-ray Observatory, launched in 1999, uses
  grazing incidence mirrors to focus X-ray light.

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• Gamma rays are absorbed by just about everything.
• Making gamma ray mirrors is beyond current
  technology.
• Compton Gamma Ray Observatory, launched in 1991
  and deorbited in 2000, had several instruments.

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Sky Radio
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Sky Microwaves
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Sky Infrared
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Sky Optical
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Sky Optical Again
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Sky X-ray
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Sky gamma-ray
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Telescopes Few closing questions

• 1) If you double the diameter of a telescope, how
  much more light will it collect?
• 2) If you double the diameter of a telescope, how
  will its resolution change?
• 3) Which has better resolution 1 meter diameter
  optical telescope, or 30 meter radio dish?
• 4) What is the largest X-ray telescope on Earth?
• 5) How much () is a 10 meter optical
  telescope?