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Draw the
four types
of reflectors.
Light from different
directions focuses at
different points, and
an image is formed
near the prime focus.
Light entering at great
angles cannot be
accurately focused.
Off-center star
images acquire
“tails” (coma).
Coma worsens as
the picture moves
farther from the
center of view.
A Schmidt telescope
uses a correcting
lens, which sharpens
the final image of the
entire field of view.
This image is focused
on a specially shaped
piece of photographic
film (often called a
“Schmidt Camera”)
Light can also be
focused on a
photometer. This
measures the total
light received on all
or part of the image.
This light information
can be split into
its spectrum on a
photographic plate
or be stored
on a computer.
A larger telescope
can gather more
light, so it is said
to have a large
collecting area.
Angular resolution
is the ability to
distinguish between
two adjacent objects
in the sky (with or
without a telescope).
When the angular
resolution is greater
than the separation
of the objects, all that
can be seen is a fuzzy
Even large
telescopes have
angular resolution
problems because
of “atmospheric
Atmospheric blurring
is the result of the
different densities of
the air deflecting the
incoming light slightly.
This causes the
image to move
around on the
(or on our retina).
This continual
movement of the
image causes
stars to “twinkle”.
After a few minutes
of exposure time,
the image has been
smeared out over a
circular region. This is
called the seeing disk.
The Hubble Space
Telescope avoids these
problems by placing the
telescope above the
It was launched into
Earth orbit in 1990.
Hubble gives images
that are up to 20
times sharper than
those of groundbased telescopes.
Most newer
telescopes use
devices (CCD’s)
rather than
photographic film.
The tiny picture
elements on
CCD’s are
called pixels.
CCD’s have great benefits:
• 1) gather more light,
• 2) gather light info faster,
• 3) images can be stored
• 4) “noise” can be eliminated.
Radio telescopes detect
cosmic radio waves.
They can be used
24 hours a day and
weather has no effect
on the waves.
Radio astronomy opens
up a new picture of the
universe. Objects that
produce a great deal of
visible light are not
necessarily very high
in radio wave emission.
Objects that emit
detectable levels of
radio waves may be
completely undetectable
in the visible region of
the spectrum.
Emissions can also be
detected in the other
regions of the spectrum.
Ultraviolet, infrared,
microwaves, gamma
rays, x-rays, etc.
Why do stars
Why don’t
planets twinkle?