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Telescopes
How do they work?
Chapter 5
1. History
2. Lenses & Hardware
3. Reflecting Telescopes
4. Refracting Telescopes
History
• Hans Lippershey
Middleburg, Holland
– invented the refractor telescope in 1608
• Galileo
– Used a small 30X scope, was the first to use a
telescope in astronomy. Galileo's designs used a
combination of convex and concave lenses.
– Observed the moon, began the “modern” age of
Astronomy where measurement more important than
philosophy.
Galileo
noticed
• moons orbiting
Jupiter
• phases of Venus
• craters on the moon
• sunspots
This was strong evidence
that Copernicus was right
although Galileo wasn’t
willing to die for it.
History
• Kepler
– improved the design to have two convex lenses,
which made the image upside-down. Kepler's design
is still the major design of refractors today, with a few
later improvements in the lenses and the glass to
make them.
Why can’t you see an object that
is far away?
The answer is simple: the object does not
take up much space on your eye’s screen
(retina).
For example, at 150 feet the writing on a
dime does not cover enough pixels on
your retinal sensor for you to read the
writing.
This can be corrected by bending the light
with lenses.
Lenses
• The lens in your eyes works like a glass lens. The light
bends as it goes through a different medium.
• Light rays are bent when they intersect glass; a curved
surface can produce an image.
• In your eye, the image is then focused at the retina.
How does this apply to
telescopes?
• If you had a bigger eye, you could collect
more light from the object. This image
could be magnified so it stretches out over
more pixels in your retina.
• In a telescope, two pieces make this
possible:
• the objective lens (refractor telescopes) or primary
mirror (reflecting telescopes)
• the eye piece
• The objective lens (in refractors) or primary mirror
(in reflectors) collects lots of light from a distant object
and brings that light, or image, to a point or focus.
• An eyepiece lens takes the bright light from the focus of
the objective lens or primary mirror and "spreads it out"
(magnifies it) to take up a large portion of the retina.
This is the same principle that a magnifying glass (lens)
uses; it takes a small image on the paper and spreads it
out over the retina of your eye so that it looks big.
How they really work
• Scopes gather light through the objective (mirror or
lens)
–bigger is better because it gathers more light
– The ability to see faint objects increases proportionally
with the square of the radius of the objective
• They focuses light
• changing the eyepiece changes the magnification
• magnification is the ratio of the focal length of the
objective to the focal length of the eyepiece
Diagram of a simple telescope. Parallel light rays enter from the left, pass
through the objective lens, come to a focus at the focal plane, and exit
through the eyepiece lens. The focal length of the objective is F, and the focal
length of the eyepiece is f.
www.ifa.hawaii.edu
When you combine an objective lens or
primary mirror with an eyepiece, you have
a telescope.
Again, the basic idea is to collect lots of light
to form a bright image inside the
telescope, and then use something like a
magnifying glass to magnify (enlarge) that
bright image so that it takes up a lot of
space on your retina.
A telescope has two general
properties
• how well it can collect the light
(the aperature)
• how much it can magnify the image
(the magnification)
The Aperture
• A telescope's ability to collect light is
directly related to the diameter of the lens
or mirror -- the aperture -- that is used to
gather light. Generally, the larger the
aperture, the more light the telescope
collects and brings to focus, and the
brighter the final image.
Magnification
• The telescope's magnification, its ability
to enlarge an image, depends on the
combination of lenses used. The eyepiece
performs the magnification. Since any
magnification can be achieved by almost
any telescope by using different
eyepieces, aperture is a more important
feature than magnification
A closer look at eyepieces
View through an eyepiece. Note that the image is upsidedown.
Eyepiece
The purposes of the eyepiece are to:
• produce and allow you to change the telescope's
magnification
• produce a sharp image
• provide comfortable eye relief (the distance between
your eye and the eyepiece when the image is in focus)
• determine the telescope's field of view:
– apparent - how much of the sky, in degrees, is seen edge-toedge through the eyepiece alone (specified on the eyepiece)
– true or real - how much of the sky can be seen when that
eyepiece is placed in the telescope (true field = apparent
field/magnification)
Filters
Filters are pieces of glass or plastic that you can place in
the barrel of an eyepiece to restrict the wavelengths of
light that come through in the image.
Set of filters for viewing, including a light pollution filter (left) and
colored filters for enhancing contrast in planetary images.
Filters can be used to:
• enhance the viewing of faint sky objects in light-polluted skies
• enhance the contrast of fine features and details on the moon and
planets
• safely view the sun
There are 2 main types of
Telescopes
• Refractor telescopes, which use glass
lenses
• Reflector telescopes, which use mirrors
instead of lenses.
Both types accomplish exactly the same thing, but
in completely different ways.
Refractor Telescopes
• Refractors are the type of telescope that
most of us are familiar with. They have the
following parts:
– a long tube, made of metal, plastic, or wood
– a glass combination lens at the front end
(objective lens)
– a second glass combination lens (eyepiece)
• Refracting telescopes focus light rays by
bending them with glass.
This is the simplest telescope design you could have. A big lens
gathers the light and directs it to a focal point and a small lens
brings the image to your eye.
Advantages and Disadvantages
• Easy to use and reliable
• Excellent for lunar,
planetary and binary star
observing especially in
larger apertures.
• High contrast images with
no secondary mirror or
diagonal obstruction.
• Sealed optical tube
reduces image degrading
air currents and protects
optics.
• More expensive per inch
of aperture
• Heavier, longer and
bulkier than equivalent
aperture Newtonians and
catadioptrics.
• Small apertures
• Less suited for viewing
small and faint deep sky
objects.
• Color aberration due to
colors of light bending
different amounts.
Refracting telescopes
are not used for
astronomical research
(anymore) because
they are large and
have heavy lenses
(i.e. expensive).
Reflecting Telescopes
• History:
– Isaac Newton developed the reflector about 1680, in
response to the chromatic aberration (rainbow halo)
problem that plagued refractors during his time. Instead of
using a lens to gather light, Newton used a curved, metal
mirror (primary mirror) to collect the light and reflect it to a
focus. Because the mirror reflected light back into the tube,
he had to use a small, flat mirror (secondary mirror) in the
focal path of the primary mirror to deflect the image out
through the side of the tube, to the eyepiece; otherwise, his
head would get in the way of incoming light.
– In 1722, John Hadley developed a design that used
parabolic mirrors, and there were various improvements in
mirror-making. The Newtonian reflector was a highly
successful design, and remains one of the most popular
telescope designs in use today.
Advantages and disadvantages
• Lowest cost per inch of
aperture
• Reasonably compact and
portable up to focal
lengths of 1000mm.
• Excellent for faint deep
sky objects such as
remote galaxies, nebulae
and star clusters.
• Reasonably good for
lunar and planetary work.
• Low in optical
aberrations.
• Open optical tube design
allows image-degrading
air currents and air
contaminants
• More fragile
• Large apertures (over 8")
are bulky, heavy and tend
to be expensive.
• Slight light loss due to
secondary mirror
obstruction when
compared with refractors.
Reflecting telescopes
focus light by bending it with mirrors
A third type
But I said there were 2 types… I lied 
Sort of.
• Catadioptrics
• Uses both mirrors and lenses to collect the
image
• Schmidt-Cassegrain
• Maksutov-Cassegrain
Catadioptric telescopes
• Best all-around, all-purpose telescope design.
– Combines the optical advantages of both lenses and
mirrors while canceling their disadvantages.
• Sharp images over a wide field.
• Excellent for deep sky observing or
astrophotography with fast films or CCD’s.
• Very good for lunar, planetary and binary star
observing or photography.
• Closed tube design reduces image degrading air
currents.
• Most are extremely compact and portable.
• Large apertures at reasonable prices and less
expensive than equivalent aperture refractors.
Schmidtt-Cassegrain
Problems with earth-based
telescopes
• Earth’s atmosphere reflects certain wavelengths
– x-rays, gamma rays and most UV light is not
transmitted by our atmosphere
• Earth’s atmosphere blurs images
– the bending of light by the atmosphere depends on
the temperature of the “air”
– “twinkling” (shimmering) effect
• “Light pollution”
• Solution? Put the telescope in space.
Hubble Space Telescope
Not everything is visible…
www.yorku.ca/eye/spectrum.gif
• Many modern day telescopes do not use visible
light to collect images.
• Radio telescopes, x-ray telescopes and infrared
(IR) telescopes have become a staple of modern
day astronomy, producing some amazing
images.
Compton Gamma
Ray Observatory
Chandra X-ray
Observatory
Disadvantages of space-based
telescopes
• Expensive to launch and maintain
• Difficult to repair
• Low lifetime
Examples of space-based
telescopes
• Hubble Space Telescope
– 3 times better resolution
– can see fainter objects
• Chandra X-ray Observatory
• Compton Gamma-Ray Observatory
• International Gamma Ray Astrophysics
Laboratory (INTEGRAL)
• Nuclear Spectroscopic Telescope Array
(NuSTAR)
• Swift Gamma Ray Burst Explorer
Famous Telescopes
• Keck Telescope
http://www.jpl.nasa.gov/events/lectures/dec04.cfm
• Kitt Peak Observatory
http://www.jpl.nasa.gov/events/lectures/dec04.cfm
Very Large Array (VLA)
radio telescopes