Transcript Telescopes.

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
Reflecting Telescope
• 1663- James Gregory
designed a telescope with a
large concave primary mirror
and a smaller concave
secondary mirror
• 1666- Newton found that a
prism breaks up white light
into a rainbow of colours
– Telescope lenses do the same
• 1672- Newton modified the
design, and it won huge
acclaim
Newton’s Telescope
Optical Telescopes
Refracting telescope uses a
large lens to gather and focus
light.
Reflecting telescope uses a
concave mirror.
Lenses vs Mirrors
• Lenses allow the
light to pass
through.
• As light passes
through the lens it
slows down and
bends
(REFRACTION).
• The bend depends
on the shape and
material of the lens.
Lenses vs Mirrors
• Mirrors do not allow
the light to pass
through.
• The light passes
through the glass
but is then
REFLECTED by the
silver backing.
• The shape of the
mirror directs the
light.
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
Lens and Mirror Activity
• STATION 1: Making Rainbows
Use the overhead as a light source
Use the objects to try to create rainbows on the
white paper
Observation:
What direction do you need to hold the prisms to
make the rainbow?
What happens to the edge of the image created by
the lens?
Lens and Mirror Activity
• STATION 2: Looking Through Lenses
Look at the writing on the worksheet with the
various lenses
Observation:
What happens to the size and orientation of
the writing?
Lens and Mirror Activity
• STATION 3: Focusing the Flame
Line up the paper with the flame. Move the lens
back and forth until you are able to focus an
image of the flame on the paper.
Use the different lenses.
Observation:
Measure the focal length of each lens.
What happened to the orientation of the flame?
Lens and Mirror Activity
• STATION 4: Mirror Mirror on the Wall
Attach the mirror to the wall at eye level.
Move your positions until you can see each other
and not yourself.
Observation:
Draw a diagram that shows your angles relative to
the mirrow.
What do you notice about these angles?
Lens and Mirror Activity
• STATION 5: Reflecting the Image
Stand by the window
Hold a mirror in each hand. Move the two arms
until you are able to focus a distance image
outside.
Observation:
Measure the distance between your hands
What happens to the orientation of the image?
Powers of a Telescope
1. Light-gathering
power:
The ability of a
telescope to
collect light.
Powers of a Telescope
2. Resolving
Power –
the ability of a
telescope to
reveal fine
detail.
Resolving Power (con’t)
• 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.
Resolving Power (con’t)
• Optical quality and
atmospheric conditions
limit the detail we can
see.
• Seeing is the blurred
image caused by
unsteady, turbulent
atmosphere. A star near
the horizon will twinkle
more than an overhead
star. Most telescopes
are built on high
mountains.
Powers of a Telescope
3. Magnifying
power –
the ability to
make the
image bigger
Magnifying power con’t
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.
Observatories are built on top of
mountaintops because:
1. Air is thin and more transparent
2. The sky is dark
3. Stars are brighter
4. Wind blows smoothly over some
mountain tops
5. There is less pollution
• Refracting Telescopes have
limitations…
1. 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.
1. Chromatic Aberration con’t
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.
• SOLUTION…
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.
2. Size
• Largest
refracting
telescope in the
world is at
Yerkes
Observatory in
Wisconsin (1m).
• It weighs ½ ton
and the glass
sags under its
own weight.
• SOLUTION…
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.
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
•
•
•
•
•
•
•
•
•
Hubble
Spitzer
Kepler
Webb
Chandra
Hershel
Planck
Fermi
XMM-Newton