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Ch. 3 Review
Ch. 3: Why do we need telescopes?
• Humans can only see 6000 stars at night and
cannot resolve (discern) the detail of objects
in the sky if they are too close together.
• Humans also cannot see anything but visible
light.
• We also need some way to record images,
like in photography.
A Reflecting Mirror brings light to a focus
A Refracting Lens
can form an image
by using the
principle of the
refraction of light
Image Formation can also occur with a concave mirror
Two types of Telescope: Reflectors and Refractors
Types of Reflecting Telescopes
The Palomar Telescope was the largest for many years
Palomar Telescope (notice the person near the focus)
Telescopes should be large to
give lots of sensitivity to dim light
by collecting a large area of light.
Longer exposures also
produce better images.
But you have to quit
before dawn, so the
exposure time of the photo
is obviously limited.
Mauna Kea Observatory in Hawaii
Resolution is critical for
seeing image detail
Diffraction is an effect that limits resolution in images.
Large openings (apertures) reduce this spreading effect.
Atmospheric Turbulence is reduced at high altitudes
The Hubble Space Telescope is above the atmosphere.
Hubble Space Telescope images are much better
than from ground-based telescopes of the same size.
Telescopes are available for
most of the Electromagnetic Spectrum
Radio (and microwave), infrared, visible, ultraviolet, X-ray, Gamma ray
The radio telescope at Green Bank,
West Virginia can operate day and night.
The Arecibo Observatory (Puerto Rico) is the largest.
A radio “image” of a Radio Galaxy, along with the visible image
The Very Large Array Interferometer near Socorro,
New Mexico, uses an array connected by electronics
Comparison of radio image (left) and optical image (right)
Infrared Telescopes are hoisted by balloons or
put in orbit to get above the water vapor in the atmosphere.
Infrared Image of the constellation Orion,
compared to the visible image on the right
The Hubble Space Telescope can obtain ultraviolet pictures.
Ultraviolet images are shown using false color.
X-ray Telescopes are made of mirrors of special layered metal
and are put in orbit because X-rays do not penetrate far in air.
Chandra Observatory,
mounted in
the shuttle Columbia
The Compton Gamma Ray Observatory,
during deployment from the space shuttle
A Gamma-ray image does not show the detail of
visible images, but indicates very energetic phenomena.
Multiple Wavelengths give us a fuller picture of the cosmos