What we call

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What we call light is only one type of
Electromagnetic Radiation a way in which
energy moves through space. Do not confuse EM
radiation with Particle radiation. Well start
by discussing the visible light spectrum. When
white light is passed through a prism, a
rainbow-like band of color is seen. ROY G. BIV
Ever met him? ? Red Orange Yellow Green Blue In
digo Violet This is the order from longest to
shortest wavelength. This order of colors never
changes!
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What is wavelength? Wavelength (?) length
over which the wave repeats. Or Wavelength (?)
Distance between two crests of a wave.  
  Light wavelengths are very short! ? measured in
Angstroms (Å). Named after 19th century Swedish
physicist A.J. Angstrom. 1 Å 1x10-10
m Approximate sensitivity limit of the human
eye. 4000 Å - 7000 Å (or 400 nm - 700 nm) ? gt
(longer than) 7000 Å - Infrared ? lt (shorter
than) 4000 Å - Ultraviolet
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Electromagnetic Radiation a way in which
energy moves through space. Speed of light in a
vacuum c 3 x 108 m/s 186,000
miles/sec 7 times around the Earth in 1
second!! Electromagnetic Radiation gets its
name from the fact that it is a manifestation of
rapidly varying Electric Magnetic fields.
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There are seven different regions of the E-M
Spectrum They are, in order of increasing
wavelength Gamma Rays X-Rays Ultraviolet
Visible Infrared Microwave Radio wave (Be able
to describe applications of each region, or where
we encounter them in life.)
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All of these regions can be used in astronomy to
measure objects in the universe. Different
methods are needed to detect different
wavelengths of radiation. Only certain parts of
the E-M spectrum can penetrate the Earths
atmosphere.
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• The parts that get through are called windows of
  transparency.
• Earths atmosphere has two main windows in
  visible and radio.

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TELESCOPES
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• For this discussion, we will use the word
  telescope to mean Optical Telescope
• These are the types of telescopes you (the
  student) are most likely to use.
• What are the most important things telescopes do
  in astronomy?
• 1. Gathering light (Our eyes pupil is only
  about 8 mm wide)
• Magnification
• 3. Increasing light-gathering time
• Using CCDs or film
• Our brain refreshes about 30 times/sec.
• No such thing as a long-exposure brain images!
• 4. Increase resolution or resolving power.
• Resolution ? 1/(diameter of optical element)
  (see Dawes Limit)
• Resolution inversely proportional to diameter
  The larger the optical element, the smaller the
  features that can be resolved.

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So What Does a Telescope Do?

• Gathers light its most
  important job!
• Allows us to see faint, distant objects.
• Doubling the diameter quadruples the light
  gathered.
• Since A ?r²
• ?(1)² / ?(2)² ¼
• Thus, the bigger one gathers 4x the light!

Radius 1
Radius 2
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What a Telescope Does

• Magnifies
• Important when
• viewing planets,
• nebulae, and
• other objects in space.
• Magnification equals the focal length of the
  telescope divided by the focal length of the
  eyepiece. Or, in equation form
• Mag FLT FLE
• The longer the eyepiece focal length,
• the less the magnification.

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What a telescope does

• Resolves
• Separates two objects at a distance so they
  appear separate.

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• Resolution
• In the figure right, two lights are imaged by
  telescopes of different apertures. Apertures are
  decreased by a factor of two each step to the
  right.
• Notice the decrease in the ability to resolve the
  two lights as separate objects.
• Bigger aperture Better resolution
• Note also the wavelength dependence.
• Bluer light Better resolution.
• This is due to the Dawes Limit
• r (arcsec) 2 x 10-3 ?/d
• ? - wavelength in Angstroms
• d - diameter of the aperture in cm.

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Important Because of air turbulence, the best
resolution from ground-based telescopes is
limited to ½ arcsec. 1degree 60 arcmin 1
arcmin 60 arcsec ½ arcsec the width of a
human hair across two football fields ( 200
yards). The limiting factor of air turbulence in
Earths atmosphere is why the Hubble Space
Telescope (and other orbiting observatories) are
placed into space.
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Refractors vs Reflectors

• Primary/Objective mirror
• Secondary mirror
• Eyepiece

• Primary/Objective lens
• Secondary lens or eyepiece

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RefractingTelescope Refraction the bending of
light, as when it travels from one medium to
another.
Focal Length Distance at which the light rays
converge.
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Chromatic Aberration Different colors are
focused at different points. This causes blurry
images.
Chromatic Aberration can be solved using compound
lenses, but this costs light and makes images
dimmer. It also makes telescopes heavier and more
expensive.
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Reflectors RuleBut Why??????

• Large diameter refractors are expensive to make.
• Mirrors are easier to mount. Large mirrors can be
  supported from behind.
• Refractors suffer from chromatic aberration.
• Also, large lenses sag under gravity.

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However, reflectors can suffer from Spherical
Aberration
Parallel light rays do not image at the same
point.
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Parabolic mirrors solveSpherical Aberration
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Types of Reflectors

• Newtonian Eyepiece on the side of the tube near
  the front aperture or opening.

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Types of Reflectors

• Cassegrain Eyepiece at the rear of telescope.
  Light travels through a hole in the primary
  mirror.

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Types of Reflectors

• Schmidt-Cassegrain Similar to a Cassegrain, but
  with a spherical mirror and a lens on the front
  called a corrector plate. Maksutov telescopes are
  also similar.

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Telescope Mounts

• Altitude-azimuth mountsSimple up-down,
  side-to-side motion

• Equatorial mountsOriented to track the stars

German Equatorial
Dobsonian
Classic Alt-Az
Fork Equatorial
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Manual Telescopes

• More mirror for the .
• Suitable for all ages.
• You must learn your way around the night sky.

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Automated Telescopes

• Ease in finding objects.
• A necessity for astrophotography.
• Expensive.
• Not suitable for children.

GPS system alignments
Manual star alignments