Transcript telescopes - hrsbstaff.ednet.ns.ca

TELESCOPIC ASTRONOMY
What is a telescope?
•
What are telescopes used for?
• View distant objects
• Collect light
First Telescope
• 1608- Hans Lippershey
• Hans Lippershey was a Dutch lens maker.
• 1609- Galileo. Built his own telescope, used it
for scientific study.
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Mountains and valleys on Moon
Moons of Jupiter
Phases of Venus
Saturn’s rings
Galileo’s Telescope
Optical Telescopes
Optical telescopes use a lens
and light.
One type: Refracting telescope
uses two large
lenses
to gather and focus light.
Primary lens:
the main lens
in a
refracting
telescope. It
is also called
an
objective
lens.
Eyepiece:
A small lens
to magnify
the image
produced
by the
objective
(primary)
lens
Focal length –
the distance from the lens or
mirror to the image formed of a
distant light source
Pros
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Simple design
Minimal maintenance
Good for planets and moons
Good for photography
High contrast
Cons
• Costly
• Bulky and large (large focal length)
• Chromatic aberration
Refraction Limitation
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.
Refraction Limitation
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.
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.
Do we still use these types of
telescopes??
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
Reflecting Telescope
• 1666- Newton found that a prism breaks
up white light into a rainbow of colours
• Telescope lenses do the same
– Creates haloes of coloured light around
objects being viewed
Newton’s Telescope
Primary mirror:
the main mirror in a
reflecting telescope.
It is larger than the
secondary mirror.
It is also called an
objective mirror.
• NOTE: The
primary mirror is in
a different spot
than the primary
lens of a refracting
telescope.
Secondary
mirror:
the smaller
mirror in a
reflecting
telescope. It
directs the
light through a
small hole in
the primary
mirror to the
eyepiece.
Benefits of Reflecting
Telescopes
Less expensive. Only the front surface of
the mirror must be ground.
The mirror can be supported to reduce
sagging.
They do not suffer from chromatic aberration
because the light is reflected toward the
focus before it can enter the glass.
Cons of Reflecting Telescopes
• More maintenance
William Hershel (1757-1822)
• By late 1770s, he was making the best
metallic mirrors and telescopes in the
world.
• 1781- Discovered Uranus
• His telescope had a 125cm mirror and
was 40ft in length
Hershel’s Telescope
Large Telescopes
• Up until mid-1800s telescopes were
generally small and not very powerful
• 1838- Earl of Rosse, Ireland, taught
himself art of 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
Rosse’s Telescope
Rosse is important because he built the largest
telescope the world had ever seen and he saw further
into space than anyone had before him.
Cassegrain Telescope
• Specific type of reflecting
telescope
• a wide-angle reflecting
telescope
• the eyepiece or camera is
mounted at the back end
of the tube
• developed in 1672
Schmidt-Cassegrain Telescope
• Specific type of reflecting
telescope
• a wide-angle reflecting
telescope (Cassegrain
telescope) with a
correcting lens that
minimizes spherical
aberration
• The correcting plate (a
lens) was added in 1930
by the Estonian
astronomer and lensmaker Bernard Schmidt
(1879-1935).
New Generation Telescopes
• Refraction and Reflection telescopes are
OPTICAL telescopes (uses light)
• New generation telescopes are RADIO
telescopes (uses waves)
• Radio telescopes use dishes to pick up
radio waves. Sensors on the dishes
collect the waves and turn them into a
picture.
Radio Telescopes
New Generation Telescopes
• 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
Keck Telescope
Handicaps to Radio Telescopes
1) Poor resolution: To improve resolution,
two or more radio telescopes can be
combined to improve the resolving power
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
• Interference:
This occurs
because of
poorly designed
transmitters in
Earth satellites
to automobiles
with faulty
ignition systems.
Powers of a Telescope
1. Lightgathering
powerThis is the ability of
a telescope to
collect
light.
The larger the
telescope’s
lens, the more
light it can
gather.
Powers of a Telescope
2. Resolving Power –
the ability of a telescope
to reveal fine detail.
•
The larger the
telescope, the better
its resolving power.
However, optical
quality of the lens
and atmospheric
conditions limit the
detail we can see.
Powers of a Telescope
Magnifying power –
the ability to make the
image bigger.
Magnification of a
telescope can be
changed by changing
the eyepiece.
Astronomers identify telescopes by
diameter because that determines
both light-gathering power and
resolving power.
Telescopes and Observatories
• An observatory is a place for viewing
the sky through telescopes.
• Observatories can be small with just a
single telescope outside or they can be
large buildings housing a number of
telescopes, each in their own room.
Hale Observatory, California
• The traditional image of an observatory is
probably that of a large building with
domed rooms housing telescopes. But an
observatory can be just a single room with
a telescope, or nothing more than an open
area with telescopes.
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
Telescopes and the Scientific
Method
The Scientific Method
The Scientific Method’s 4 Steps
1)
Observation and description of a phenomenon.
The observations are made visually or with the aid of scientific
equipment (like a telescope).
2) Formulation of a hypothesis to explain the phenomenon (usually in
the form of math or a causal mechanism or a mathematical
relation.
•
Test the hypothesis by analyzing the results of observations or by
predicting and observing the existence of new phenomena that
follow from the hypothesis. If experiments do not confirm the
hypothesis, the hypothesis must be rejected or modified (Go back
to Step 2).
•
Establish a theory based on repeated verification of the results.