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TELESCOPIC ASTRONOMY
First Telescope
• 1608- Hans Lippershey. Dutch lens
grinder.
• 1609- Galileo. Built his own telescope,
used it for scientific study.
– Mountains and valleys on Moon
– Moons of Jupiter
– Phases of Venus
– Saturn’s rings
– Sunspots
Galileo’s Telescope
Powers of a Telescope
1. Lightgathering
powerThis is the
ability of a
telescope
to collect
light.
Powers of a Telescope
2. Resolving
Power –
the ability of a
telescope to
reveal fine
detail.
When light is focused into an image, a blurred
fringe surrounds the image (diffraction fringe).
We can never see any detail smaller than the
fringe.
Large diameter telescopes have small fringes and
we can see smaller details. Therefore the larger
the telescope, the better its resolving power.
Optical quality and atmospheric conditions limit the
detail we can see.
Powers of a Telescope
3. Magnifying
power –
the ability to
make the
image
bigger
Magnification of a telescope can be
changed by changing the
eyepiece. We cannot alter the
telescope’s light-gathering Or
resolving power.
Astronomers identify telescopes by
diameter because that determines
both light-gathering power and
resolving power.
Optical Telescopes
Refracting telescope uses a
large lens to gather and focus
light.
Reflecting telescope uses a
large mirror
Focal length –
the distance from the lens or
mirror to the image formed of a
distant light source
Primary lens:
the main lens
in a
refracting
telescope. It
is also called
an
objective
lens.
Primary
mirror:
the main mirror
in a
reflecting
telescope. It is
also called an
objective
mirror.
Eyepiece:
A small lens
to magnify
the image
produced
by the
objective
(primary)
lens
Chromatic Aberration
When light is refracted through glass,
shorter wavelengths bend more than
longer wavelengths, and blue light
comes to a focus closer to the lens than
does red light.
Chromatic Aberration
If we focus on the blue image, the red
image is out of focus and we see a
red blur around the image. This color
separation is called chromatic
aberration.
Achromatic Lens
An achromatic lens is made of two
components made of different kinds of
glass and brings the two different
wavelengths to the same focus. Other
wavelengths are still out of focus.
Yerkes Refracting Telescope
• Largest refracting
telescope in the
world is at Yerkes
Observatory in
Wisconsin
• Lens is 1m in
diameter
• ½ tonne
• The glass sags under
its own weight
Newton’s Reflecting Telescope
• 1666- Newton found that a
prism breaks up white light
into a rainbow of colours
– Telescope lenses do the
same
• 1663- James Gregory
designed a telescope with
a large concave primary
mirror and a smaller
concave secondary mirror
• 1672- Newton modified the
design, and it won huge
acclaim
Benefits of Reflecting
Telescopes
Less expensive. Only the front surface of
the mirror must be ground.
The glass doesn’t need to be perfectly
Transparent
The mirror can be supported over its back
surface to reduce sagging.
They do not suffer from chromatic aberration
because the light is reflected toward the
focus before it can enter the glass.
Four ways to look through
reflecting telescopes
Prime Focus
Newtonian Focus
Cassegrain Focus
Schmidt-Cassegrain
Hershel’s Telescope
• Late 1770s, William
Hershel was making the
best metallic mirrors and
telescopes in the world.
• 1781- Discovered Uranus
• 1789 - Built a giant
telescope which he used
with his sister Caroline
– His telescope had a 125cm
mirror
– 40ft in length
Rosse’s Telescope
• 1838- Earl of Rosse,
Ireland, taught himself
mirror-making and
built a 91cm telescope
• 1842- attempted to
build a 181cm
telescope but it broke
when moved
– built another one that
couldn’t be moved
Observatories
• 1874- 91cm telescope and
observatory at University of
California (James Lick)
• 1880- 76cm telescope in France
• 1897- 102cm telescope at Yerkes
Observatory in Wisconsin
• 1908- 153cm telescope on Mount
Wilson California (George Ellery
Hale)
• 1917- 254cm telescope also built
on Mount Wilson (John D.
Hooker)
• 1948- 500cm mirror. Hale
Observatory, Mount Palomar,
California.
Observatories are built on top of
mountains because:
1) air is thin and more transparent
2) the sky is darker
3) stars are brighter
4) wind blows smoothly over some
mountaintops
5) there is less pollution
New Generation Telescopes
Keck Telescope
• 1993 – Keck telescope
1000cm mirror, made of
smaller segments
• Photographic plates
were more sensitive and
permitted a permanent
record of observations
– Photographic plates have
since been replaced by
electronic imaging devices
A large mirror sags in
the middle. To prevent
this:
1. Mirrors can
be made very
thick but they
are very heavy
and very costly.
A large mirror sags in
the middle. To prevent
this:
2. Spincasting – an oven
turns and molten glass
flows outward in a mold to
form a concave upper
surface.
A large mirror sags in
the middle. To prevent
this:
3. A mirror can
be made in
segments.
A large mirror sags in
the middle. To prevent
this:
4. Thin mirrors (floppy
mirrors) can have
their shape
controlled by a
computer – called
active optics. They
cool quickly to
adjust to
surrounding
temperatures.
Radio Telescopes
• Objects in space emit
light waves of many
different wavelengths.
• Radio Telescopes receive
very long wavelengths
(radio waves).
• 1937 – first Radio
telescope; picks up long
wave radio emissions
from deep space
Handicaps to Radio Telescopes
1. Poor resolution
To improve resolution,
two or more radio
telescopes can be
combined to improve
the resolving power
(called a radio
interferometer).
Resolving power
equals the separation
of the telescopes.
Handicaps to Radio Telescopes
2. Low intensity
In order to get strong
signals focused on
the antenna, the
radio astronomer
must build large
collecting dishes.
The largest dish is
the 300 m dish at
Arecibo, Puerto
Rico.
Handicaps to Radio Telescopes
3. Interference
This occurs
because of
poorly designed
transmitters in
Earth satellites
to automobiles
with faulty
ignition systems.
Space Telescopes
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Hubble
Spitzer
Kepler
Webb
Chandra
Hershel
Planck
Fermi
XMM-Newton