Chapter 5:
Telescopes
“Nature composes some of her loveliest poems for the telescope”
- Theodore Roszah
Invention of the Telescope
Galileo did not invent the
telescope! The first idea was
credited to the Dutch optician
Hans Lippershey in 1608.
Early Telescopes
Galileo with his early
telescope 1609
The first telescope to use a
mirror to gather light
invented by Newton
in 1670
Two Main Types
1) Refracting Telescope2) Reflecting TelescopeUses lenses to bring light
Uses mirrors to bring light to
to a focus and form an image
a focus and form an image
Refracting Telescopes
The ideal distance between the two lenses
is just the sum of the focal lenses!!
Building Your Own Telescope is Easy!
Materials
• two magnifying glasses: perhaps 1-1.5 inches
in diameter (it works best if one is larger than the
other)
• a cardboard tube: paper towel roll or gift
wrapping paper roll (it helps if it is long)
• duct tape
• scissors
• a ruler, yard stick, or tape measure
• sheet of printed paper: e.g. newspaper or
magazine
Building Your Own Telescope is Easy!
Assembling
1.
2.
3.
4.
5.
6.
7.
8.
9.
Hold one magnifying glass (the bigger one) between you and the paper. The
image of the print will be blurry.
Get the two magnifying glasses and a sheet of printed paper.
Place the second magnifying glass between your eye and the first magnifying
glass.
Move the second magnifying glass forward or backward until the print comes into
sharp focus. You will notice that the print appears larger and upside down.
Have a friend measure the distance between the two magnifying glass and write
the distance down.
Cut a slot in the tube the same distance from the first slot as your friend wrote
down. This is where the second magnifying glass will go.
Place the two magnifying glasses in their slots (big one at front, little one at back
and tape them in with duct tape)
Leave about 0.5-1 inch (1-2 cm) of tube behind the small magnifying glass and cut
off any excess tube remaining.
Check to see that it works by looking at the printed page; you may have to play
slightly to get the exact distances between the two glasses right so that the image
comes into focus.
Refracting Telescopes
Chromatic Aberration:
Different colors are focused at
different distances from the lens
Need to add extra lenses to correct
the problem, but the correction is
NOT 100 %!
Refracting telescopes have
disadvantages
• Chromatic aberration
• More expensive!
• Can’t be built too
large
• Sagging due to
gravity distorting the
lens
Reflecting telescopes use mirrors to
concentrate incoming starlight
Newtonian Focus
Prime Focus
Cassegrain focus
coude’ focus
Inside the Cage!
Radio Telescopes
Arecibo Observatory, Puerto Rico
Very Large Array
Three Main Functions of a
Telescope
most important
• brighten
(called light gathering power)
• see fine detail
(called resolving power)
and least important,
• magnify
Bigger is Better!
the functions that depend upon the diameter of
the telescope are:
• Light gathering power
• Resolving power (or resolution)
The function that depends upon the focal
length of the telescope is,
• Magnification: M= fobj/feye
Figure 4.1
Light Gathering Power
Light
Gathering
Power:
A measure
of how
much light
a telescope
collects
The bigger the telescope, the more light it will collect,
and the brighter the image will be!
A larger
diameter
provides a
brighter (not
bigger) image
Resolving Power of a Telescope
Resolving Power: The ability to detect fine detail
Telescope1
Unresolved
D
Telescope2
Barely resolved
2XD
Telescope3
Fully resolved
4XD
D = Diameter of Telescope1
The bigger the telescope, the higher the resolving power, but
the maximum resolution is limited by the Earth’s atmosphere
Group Activity
1. What is the difference between a reflecting and
a refracting telescope?
2. Which type of telescope do professional
astronomers prefer to build and why?
3. What are the two most important functions of a
telescope?
4. If the focal length of the objective is 20 cm and
the focal length of the eyepiece is 5 cm, what is
the magnification of the telescope?
Concept Question
What aspects of a telescope depends upon
the diameter of the telescope?
A) light gathering power and resolution
B) light gathering power and magnification
C) resolution and magnification
D) light gathering power, magnification,
resolution
Telescopes you might buy
Refracting Telescope
Newtonian
Reflecting
Telescope
Cassegrain
Reflecting
Telescope
Major Obstacles in
Observing the Stars
• Light Pollution from Cities
• Scattering of light by Earth’s atmosphere
Light pollution as viewed
from space
Earth’s Atmosphere Hinders
Astronomical Research
Image of stars taken with a
telescope on the Earth’s surface
Same picture taken with Hubble
Space Telescope high above
Earth’s blurring atmosphere
Best places to build
observatories
• air has to be very dry!
• little to no light pollution!
• calm air currents!
- on mountain tops!
- on mountain tops in deserts!
- on mountain tops on islands!
- in space!
Fremont Peak Observatory,
San Juan Bautista, CA
Lick Observatory
San Jose, CA
Kitt Peak National Observatory (KPNO)
Tucson Arizona
Cerro-Telolo Inter-American Observatory
(CTIO) La Serena, Chile
Keck Observatory
Mauna Kea Hawaii
Keck Observatory
Gran Telescopia Canarias (GTC),
Canary Islands
Not all radiation can penetrate
Earth’s atmosphere
Space Telescopes
Hubble Space Telescope
2.4 meter diameter reflector
Launched in 1990 and still
operational
Hubble Ultra Deep Field
The James Webb Space Telescope
will replace HST in 2014
Innovations
and
Advancements
in Technology
A Charge-Coupled Device (CCD)
Ordinary Photographs
vs. CCDs
Adaptive Optics uses a deformable mirror to correct for
the loss of resolution due to Earth’s atmosphere
Adaptive Optics:
Laser guide star
Adaptive Optics
Thirty Meter
Telescope
(TMT)
with
Adaptive
Optics
Interferometry
Building Your Own
Observatory
1.
2.
3.
4.
5.
6.
What kind of telescope (reflecting or refracting) and
why?
What is the difference between a reflecting and
refracting telescope?
Diameter of objective (express in meters)?
Where would you build it and why?
What would you observe with it?
Draw and name your telescope or observatory.