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Advances in Modern Telescope Design (1)
Modern computer technology has made
significant advances in telescope design
possible:
Segmented mirror
1. Lighter mirrors
with lighter
support
structures, to be
controlled
dynamically by
computers
Floppy mirror
Advances in Modern Telescope Design (2)
2. Simpler, stronger mountings (“Alt-azimuth mountings”)
to be controlled by computers
Adaptive Optics
Computer-controlled mirror support adjusts the mirror
surface (many times per second) to compensate for
distortions by atmospheric turbulence
CCD Imaging
CCD = Charge-coupled device
• More sensitive than
photographic plates
• Data can be read
directly into computer
memory, allowing easy
electronic manipulations
Negative image to
enhance contrasts
False-color image to visualize
brightness contours
Examples of Modern
Telescope Design (1)
Design of the
Large Binocular
Telescope (LBT)
Examples of Modern Telescope
Design (2)
The Very Large Telescope (VLT)
8.1-m mirror of the Gemini Telescopes
Interferometry
Recall: Resolving power of a telescope depends on
diameter
This holds true even
if not the entire
surface is filled out.
• Combine the signals
from several smaller
telescopes to simulate
one big mirror 
Interferometry
Radio Telescopes
Large dish focuses
the energy of radio
waves onto a small
receiver (antenna)
Amplified signals are
stored in computers
and converted into
images, spectra, etc.
Radio Maps
Radio maps are
often color coded:
Like different colors in
a seating chart of a
baseball stadium may
indicate different seat
prices, …
colors in a radio
map can
indicate different
intensities of the
radio emission
from different
locations on the
sky.
Radio Telescopes Advantages
1. Can reveal
clouds of cool
hydrogen because
the emit a radio
signal.
2.They can penetrate
dust clouds that
obscure visible light.
3.They can detect the most distant objects in
the universe.
Radio Interferometry
For radio telescopes, resolving power is a big
problem since Radio waves are much longer than
visible light.
 Use
interferometry to improve resolution!
Radio Interferometry (2)
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)!
The Largest Radio Telescopes
The 300-m telescope in
Arecibo, Puerto Rico
The 100-m Green Bank Telescope in
Green Bank, WVa.
NASA’s Space Infrared
Telescope Facility (SIRTF)
Infrared light with wavelengths much longer
than visible light (“Far Infrared”) can only be
observed from space.
The Hubble Space Telescope
• Launched in 1990; maintained and
upgraded by several space shuttle
service missions throughout the
1990s and early 2000’s
• Avoids turbulence in the Earth’s atmosphere
• Extends imaging and spectroscopy to (invisible)
infrared and ultraviolet
Optical Telescopes
Astronomers use
telescopes to gather
more light from
astronomical objects.
The larger the
telescope, the more
light it gathers.
The Powers of a Telescope:
Size Does Matter
1. Light-gathering
power:
Depends on the
surface area A
of the primary
lens / mirror.
Area depends on
diameter.
More light collected
means you can
see fainter
objects.
D
The Powers of a Telescope (2)
2. Resolving power: the ability to
make out fine detail.
The larger the telescope the more
resolving power.
Seeing
Weather
conditions
and
turbulence in
the
atmosphere
set further
limits to the
quality of
astronomical
images.
Bad seeing
Good seeing
The Best Location for a
Telescope
Far away from civilization – to avoid light pollution
The Best Location for a
Telescope (2)
Paranal Observatory (ESO), Chile
On high mountain-tops – to avoid atmospheric
turbulence ( seeing) and other weather effects
The Powers of a Telescope (3)
3. Magnifying Power = ability of the
telescope to make the image appear
bigger.
The magnification depends on the ratio of focal
lengths of the primary mirror/lens (Fo) and the
eyepiece (Fe):
A larger magnification does not improve the
resolving power of the telescope!
Refracting/Reflecting Telescopes
Focal length
Focal length
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.
Secondary Optics
In reflecting
telescopes:
Secondary
mirror, to redirect the light
path towards
the back or side
of the incoming
light path.
Eyepiece: To
view and
enlarge the
small image
produced in
the focal
plane of the
primary
optics.
Limitations of Refractors
•
Lens must be made of high-quality
glass with no imperfections
• Larger lenses weigh a lot; lenses can
be supported only around their rims
• The lens will sag under its own weight
So, the largest refractor
telescope anyone has built is
about 1.1 m.
Usefulness of Reflectors
•
Objective (mirror) can be made of
many things, even plastic or metal
• Only one side of the mirror is polished
• Mirror can be supported from its
entire back, not the rim only
So, all large modern telescopes
are reflectors because they are
easier to make and maintain.
A Prime Focus
Reflector
(some light blocked)
A Newtonian
Reflector
(secondary mirror)
A Schmidt Cassegrain
Reflector
(hole in primary mirror)
Example: Hubble Space Telescope
Traditional Telescopes (2)
The 4-m
Mayall
Telescope at
Kitt Peak
National
Observatory
(Arizona)