Telescopes
Functions of Telescopes
• Magnification (make things look bigger)
easy to make a telescope with good magnification
• Collection of large amounts of light (see fainter things)
most important feature of a telescope
• Sharp images (see more detail and structure)
• Map large areas of sky (search for rare objects)
• Detect light across the electromagnetic spectrum (see
new phenomena)
• Sophisticated analysis of light (e.g., spectroscopy)
• Record images (e.g., photographs, digital pictures)
Types of Telescopes
A telescope collects light over a large area and focuses it in a
small area where it can be viewed with an eye or a camera.
This can be done in 2 ways:
• By using a lens to bend, or refract the light
• By using a mirror to reflect the light
The size of a telescope is characterized by the diameter of its
lens or mirror: a “36-inch telescope” has a lens or mirror
that is 36 inches in diameter.
Refracting Telescopes
Refracting Telescopes
The first telescopes were refractors, but they have limitations:
• A larger lens collects more light and allows one to see fainter
objects. However, it is difficult to physically support a big lens
because it must be held at the edges.
• Lenses focus different wavelengths of light at different
locations. As a result, images cannot appear in focus at all
wavelengths simultaneously.
Reflecting Telescopes
It is much easier to build and support a large mirror than a large
lens. Mirrors also bring all wavelengths to a common focus,
unlike refractors. As a result, all modern telescopes used by
professional astronomers are reflectors.
Comparison of refractors and reflectors
The pupil of the eye expands at night in order to collect more
light. Similarly, astronomers build telescopes with larger mirrors
so that they can gather more light and see fainter objects in the
sky.
Day
Night
The largest telescopes can collect more than a million times as
much light as the human eye.
The mirrors in the largest optical/infrared telescopes have
diameters of 10 meters. Rather than use single mirrors that
are even larger, future telescopes will contain several mirrors.
current
future
Smaller mirrors are easier and cheaper to build, so
large telescopes also use segmented mirrors.
The next generation of telescopes will have multiple
mirrors that are equivalent to a single 30-50 meter mirror.
In addition to collecting more light and seeing fainter
objects, larger mirrors also provide sharper images.
Twinkle, twinkle, little star
Although large mirrors have the potential to produce sharp
images, turbulence in the Earth’s atmosphere causes stars
to appear blurry.
Sharper images from space
The mirror of the Hubble Space Telescope is 2.4 meters in
diameter, smaller than the largest ground-based
telescopes. But it is above the atmosphere, so it produces
very sharp images.
ground-based telescope
Hubble
ground-based telescope
Hubble
ground-based telescope
Hubble
Some telescopes on the ground now have the ability to
partially correct for the blurring by the atmosphere.
normal blurry
image of a star
corrected for blurring
normal blurry image
of Neptune
corrected for blurring
Mapping the Sky
Hubble provides very sharp images of tiny patches of sky. In
comparison, some telescopes are designed to photograph the
entire sky, making it possible to search for new rare objects.
Telescopes allow us to collect light across the entire
electromagnetic spectrum
We see different phenomena at different wavelengths
Atmospheric Windows
Gamma-ray and x-ray telescopes must be in space since those
wavelengths don’t penetrate the atmosphere. Optical,
infrared, and radio telescopes can operate from the ground,
although the atmosphere makes optical and infrared images
blurry.
Where to Put a Telescope on the Ground
For an optical observatory, you need
• Dark skies (avoid city lights)
• Clear skies/few clouds
• Stable air for sharper images
• High altitude to get above as much of the atmosphere
as possible
For a radio observatory, you need
• An area far from cell phones and TV/radio stations
Optical (= Visible) & Infrared Observatories
Hawaii
Chile
Texas
Arizona
Chile
Radio Observatories
Aricebo
Very Large Array (VLA)
Square Kilometer Array
The Square Kilometer Array will be an array of thousands of radio
dishes with a total collecting area of 1 km2 and a baseline of 3000
km. It will be built in Australia and Africa and will be 100 times more
sensitive than the VLA.
Space Observatories
Hubble (visible)
Compton
(gamma ray)
Galex (UV)
Chandra (x-ray)
Spitzer (infrared)
WMAP (radio)
Space Observatories
Chandra X-ray Observatory
• X-ray wavelengths
• Can see things at very hot temperatures
Space Observatories
Spitzer Space Telescope
• Infrared wavelengths
• Can see things at very low temperatures
James Webb
Space Telescope