Download Telescope - St John Brebeuf

Telescopes
&
Light
The Powers of a Telescope
Light Gathering Power: Astronomers prefer *large*
telescopes. A large telescope can intercept and focus more starlight
than does a small telescope. A larger telescope will produce
brighter images and will be able to detect fainter objects.
Resolving Power: A large telescope also increases the sharpness
of the image and the extent to which fine details can be
distinguished.
Magnification: The magnifying power is the ability of the
telescope to make the image appear large in the field of view.
Diameter makes a difference
Light-Gathering Power:
Depends on the surface
area (A) of the primary
lens and is proportional to
the telescopes diameter.
D
The Powers of a Telescope
Resolving Power:
Wave nature of light: The telescope aperture
produces fringe rings that set a limit to the resolution
of the telescope.
The Powers of a Telescope
Magnifying Power
The ability of the telescope
to make the object’s optical
image appear bigger while
being observed
A larger magnification
does not improve the
resolving power of the
telescope!
Types of Telescopes
Refracting Telescopes: Use lenses as the
optics to focus and bend light.
Galileo used a refracting telescope.
The human-eye is partly a refracting
telescope.
Refracting Telescope
Objective
Lens
Eyepiece Lens
Focal Length
Objective
Focal Length
of Eyepiece
Refracting Telescope:
Lens focuses light onto
the focal plane
Focal length
Disadvantages
Refracting telescopes suffer from Chromatic Aberration. As light
passes through a lens, just as a prism will disperse light, the lens
will focus bluer wavelengths differently than the redder
wavelengths.
Blue Focus
Red Focus
Disadvantages
• Cannot be made very large for a multitude of reasons.
• Get to be very expensive to maintain.
• Lenses can grow “cloudy” over time.
• Lenses can distort over time.
140-ft Hevelius telescope 1673
Types of Telescopes
Reflecting Telescopes: Use mirrors as the
optics to focus and bounce light.
The rear view mirror on your car is a simple
reflecting telescope.
Reflecting Telescope:
Concave Mirror focuses
light onto the focal plane
Focal length
Most modern telescopes are reflecting telescopes.
Reflecting Telescope
Advantages
Reflecting telescopes do not suffer from Chromatic
Aberration. All wavelengths will reflect off the mirror in the
same way.
Reflecting telescopes can be made very large because the
mirrored surfaces have plenty of support. Thus, reflecting
telescopes can greatly increase in light gathering and
resolving power.
Reflecting telescopes are often cheaper ($$$) to make than
similarly sized refracting telescopes.
Newton’s Telescope:
The first reflecting telescope
Hey! Where are you going?
The Doppler Effect
The wavelength of light is effected by the relative motion between the
source and the observer. Christian Doppler (1803-1853), a professor
of mathematics in Prague, pointed out in 1842 that the observed
wavelength of light is affected by motion.
The frequency increases when the source and observer are
approaching each other, and decreases when they are moving away
from each other.
The Doppler Effect
Observer
Stationary
Towards
Blueshift
Away
Redshift
Observer
Observer
Strength of the Light with this
Wavelength
How does this help?
Star moving
Stationary Star
towards from us
Wavelength
Star moving
away from us
Telescopes
Where to put a Telescope?
Far away from civilization – to avoid light pollution
Where to put a Telescope?
On high mountain-tops – to avoid atmospheric turbulence and other weather effects
On high mountain-tops – to avoid atmospheric turbulence and other weather effects
“Seeing”
Weather conditions
and turbulence in the
atmosphere set
further limits to the
quality of
astronomical images.
Bad seeing
Good seeing
Hubble
Space
Telescope
X-Ray Astronomy
X-rays are completely absorbed in the atmosphere.
X-ray astronomy has to be done from satellites.
NASA’s
Chandra X-ray
Observatory
Gamma-Ray Astronomy
Gamma-rays: most energetic electromagnetic radiation;
traces the most violent processes in the Universe
The Compton
Gamma-Ray
Observatory
Radio Astronomy
Radio Interferometry
Radio Interferometry
The Very Large Array
(VLA): 27 dishes are
combined to simulate
a large dish of 36 km
in diameter.
Even larger arrays consist of dishes spread out over the entire U.S.
(VLBA = Very Long Baseline Array) or even the whole Earth (VLBI = Very
Long Baseline Interferometry)!