Telescopes
How do they work?
1. History
2. Lenses & Hardware
3. Reflecting Telescopes
4. Refracting Telescopes
History
• Hans Lippershey
Middleburg, Holland
– invented the refractor telescope in 1608
• Galileo
– the first to use a telescope in astronomy. Galileo's
designs used a combination of convex and concave
lenses.
• 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).
Using a digital camera analogy, 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.
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 the objective lens or
primary mirror with the 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.
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.
Reflecting telescopes focus light by bending them
with mirrors
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.
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