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Astronomical Tools
Optics
Telescope Design
Optical Telescopes
Radio Telescopes
Infrared Telescopes
X Ray Telescopes
Gamma Ray Telescopes
Laws of Refraction and Reflection
Law of Refraction
n1 sin θ1 = n2 sin θ2
where
n = c/v
Law of Reflection
θ1 = θ2
Lenses and Mirrors
A focusing lens can be
designed using the law of
refraction
A focusing mirror can be
designed using the law of
reflection
Refracting / Reflecting Lenses
Focal length
Focal length
A lens can
focus an image
on a plane. A
source at
infinity focuses
on the focal
plane.
A concave
mirror can
focus an image
on a plane. A
source at
infinity focuses
on the focal
plane.
The Focal Length
Focal length = distance from the center of the lens to
the plane onto which parallel light is focused.
Telescope Design
Reflecting and Refracting Telescopes
Galilean
Newtonian
Secondary Optics
Galilean
Cassegrainian
In reflecting
telescopes:
Secondary
mirror, to redirect light
path towards
back or side of
incoming light
path.
Eyepiece: To
view and
enlarge the
small image
produced in
the focal
plane of the
primary
optics.
Disadvantages of
Refracting Telescopes
 Chromatic aberration:
Different wavelengths are
focused at different focal
lengths (prism effect).
Can be improved, but not
eliminated by a second
lens out of different
material.
 Difficult and expensive to
produce: All surfaces
must be perfectly shaped;
glass must be flawless;
lens can only be
supported at the edges.
Reflectors
Most research telescopes are reflectors.
Types of reflecting telescopes
The Powers of a Telescope:
Bigger is better
1. Light-gathering
power: Depends on
the surface area A of
the primary
lens/mirror, which is
proportional to the
diameter squared:
A = π (D/2)2
D
The Powers of a Telescope
2. Resolving power: Wave nature of light
 the telescope aperture produces
fringe rings that set a limit to the
resolution of the telescope.
Astronomers cannot eliminate these
diffraction fringes, but the larger the
telescope diameter, the diffraction
fringes are smaller. Thus the larger
the telescope, the better its resolving
power.
αmin = 1.22 (λ/D)
For optical wavelengths, this gives
αmin ≈ 11.6 arcsec / D [cm]
amin
Resolving Power
Effect of improving resolution:
(a) 10′; (b) 1′; (c) 5″; (d) 1″
Seeing
Weather conditions and turbulence in the atmosphere
set further limits to the quality of astronomical images.
Atmospheric motion blurs the image.
Bad seeing
Good seeing
The Powers of a Telescope
3. Magnifying Power: ability of the telescope to
make the image appear bigger. Magnification is
usually changed by changing the focal length of
the eyepiece.
A larger magnification does not improve
the resolving power of the telescope!
Higher magnification is useful for extended bodies
such as the Sun, the Moon and planets—not stars,
which are seen as points of light.
The Best Location for a Telescope
Far away from civilization – to avoid light pollution
The Best Location for a Telescope
Paranal Observatory (ESO), Chile
On high mountain-tops—to avoid atmospheric turbulence (i.e.
improve seeing) and other weather effects
Traditional Telescopes
Secondary mirror
Traditional primary
mirror: sturdy, heavy
to avoid distortions.
Traditional
Telescopes
Mount Wilson Observatory
Hooker 100 inch reflector
Mount Palomar Observatory
Hale 200 inch (5.1 m) reflector
Traditional
Telescopes
Mount Palomar Observatory Kitt Peak National Observatory
Mayall (4 m) Telescope
Schmidt Camera (48 inch)
Traditional Telescopes
Hubble (2.4 m) Space Telescope
Advances in Modern
Telescope Design
Modern computer technology has made possible
significant advances in telescope design:
1. Simpler, stronger mountings (―alt-azimuth
mountings‖) to be controlled by computers
Advances in Modern Telescope Design
2. Lighter mirrors with lighter support structures,
to be controlled dynamically by computers
Prime Focus Cage
Floppy mirror
Segmented mirror
High-Resolution Astronomy
Adaptive optics: track atmospheric changes with
a laser, adjust mirrors in real time
Adaptive Optics
Computer-controlled mirror supports adjust the mirror
surface (many times per second) to compensate for
distortions by atmospheric turbulence
Interferometry
Recall: Resolving power of a telescope depends on diameter D.
Combine the signals
from several smaller
telescopes to
simulate one big
mirror
 Interferometry
The amount of
radiation collected is
smaller, but the
improved resolution
is important.
Images and Detectors
Image acquisition: Photographic plates are being
replaced by charge-coupled devices (CCDs), which are
electronic devices that can be read out and reset quickly.
Smaller CCDs are used in digital cameras.
Radio Astronomy
Recall: Radio waves of l ≈ 1 cm – 1 m also penetrate the
Earth’s atmosphere and can be observed from the ground.
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
In radio maps, the intensity of the radiation is color-coded:
For example:
Red = high intensity
going to
Black = low intensity
Analogy: Seat prices in a baseball
stadium: Red = expensive going to
Purple = cheap.
Radio Astronomy
Largest radio telescope: 300-m dish at Arecibo
Radio Astronomy
Disadvantage: Longer wavelength means poor
angular resolution—hence astronomical
interferometry began in radio astronomy.
Advantages of radio astronomy:
• Can observe 24 hours
a day. Clouds, rain, and
snow don’t interfere
• Observations at a
different frequency give
different information
Radio Interferometry
Just as for optical
telescopes, the
resolving power of a
radio telescope
depends on the
diameter of the
objective lens or mirror
amin = 1.22 l/D.
For radio telescopes,
this is a big problem:
Radio waves are much
longer than visible light
 Use interferometry to
improve resolution!
The Very Large Array (VLA): 27 dish
antennae are combined to simulate a
large dish of as much as 36 km in
diameter.
Science of Radio Astronomy
Radio astronomy reveals several features,
not visible at other wavelengths:
• Neutral hydrogen clouds (which don’t
emit any visible light), containing ~90 %
of all the atoms in the universe.
• Molecules (often located in dense
clouds, where visible light is
completely absorbed).
• Radio waves penetrate gas and
dust clouds, so we can observe
regions from which visible light is
heavily absorbed.
Infrared Astronomy
Most infrared radiation is absorbed in the lower atmosphere.
However, from high
mountain tops or
high-flying aircraft,
infrared radiation
can be observed at
some wavelengths.
Infrared astronomy
is best done from
spacecraft.
NASA infrared telescope on Mauna Kea, Hawaii
Infrared Astronomy
The Spitzer infrared telescope is in space
Infrared Astronomy
Infrared observations of M81 at different wavelengths. The images
a, b, and c are colored blue, green and red respectively and
combined to give an artificial color image in d.
4 μm
24 μm
8 μm
Ultraviolet Astronomy
• Ultraviolet radiation with l <
290 nm is completely absorbed
in the ozone layer of the
atmosphere.
• Ultraviolet astronomy must be
done from spacecraft.
• Several successful ultraviolet
astronomy satellites: IRAS,
IUE, EUVE, FUSE
• Ultraviolet radiation traces hot
(tens of thousands of degrees),
moderately ionized gas in the
universe.
X Ray Astronomy
X rays and gamma rays cannot reflect off mirrors as other
wavelengths do.
X rays can undergo Bragg reflection at very shallow angles and
they can be focused in special telescopes.
X Ray Astronomy
X-ray image of a supernova remnant
Gamma Ray Astronomy
Gamma rays cannot be focused at all; therefore
images are coarse.
Compton Gamma Ray Observatory (1991-2000) and an image
made by it.
Full-Spectrum Coverage
Much can be
learned from
observing the
same
astronomical
object at many
wavelengths.
Here is the
Milky Way.