Download Ordinary Photography Versus CCDs REFRACTING TELESCOPES

New technology has created chargecoupled devices (CCDs) to gather light
more efficiently than photographic
plates.
These devices
produce images
with astounding
detail. Shown in this
picture are the
intricate details of
the Rosetta Nebula
5000ly away.
Ordinary Photography Versus CCDs
ORDINARY
PHOTOGRAPH
NEGATIVE
NEGATIVE
USING CCDs
COMBINED CCD
IMAGES USING
COLORED FILTERS
REFRACTING TELESCOPES
Light rays traveling into a transparent medium such as glass
bend at the surface. The bending of light rays between two
transparent media is called refraction.
If the lens is curved, parallel rays will converge at a focal point,
just like the rays in a reflecting telescope.
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Like reflecting telescopes, extended objects
produce images in a focal plane.
Refracting telescopes use an objective lens
to gather light and an eyepiece through
which the image is viewed.
Limitations of Refracting Telescopes
•Different colors of light are refracted differently
and have different focal points. Thus, all the
colors of the image will not be focused at once.
This is called chromatic abberation.
•It is difficult to grind a lens into the proper
shape to have all parallel rays converge at a
single focal point.
•The weight of a large lens can cause the lens
to sag and distort the image.
•Air bubbles in the glass cause unwanted
refractions, distorting the image.
THE LARGEST
REFRACTING
•Glass is opaque to certain wavelengths of light, TELESCOPE AT YERKES
OBSERVATORY
meaning they do not go through the glass.
These issues do not affect
reflecting telescopes
because the light from the
stars does not travel
through a glass lens before
being focused. However,
reflecting telescopes do
have some problems.
One problem is that the
secondary mirror used to
deflect the light out the side
partially blocks the light
from the star.
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Another problem with
reflecting telescopes is
called spherical
aberration.
When a sphericallyshaped mirror is used,
the light rays hitting far
from the center will not
converge at the same
point. One solution is to
grind the mirror into a
parabolic shape.
Another solution is to
use a correcting lens to
make all the light rays
converge at a single
point.
Making a Large Parabolic Mirror
40,000 pounds of
glass are loaded into a
rotated furnace and
heated to 1500K.
After melting, spinning
and cooling, the
surface is ready to be
coated with a highly
reflective material.
Ground Based Telescopes are Limited
Our atmosphere causes the stars to appear to shift in color and
brightness. This cuases the star to appear to flicker. We call this
twinkling.
GROUND BASED TELESCOPES
BLURRED BY TWINKLING
TELESCOPES IN ORBIT ABOVE
OUR ATMOSPHERE AVOID THIS
PROBLEM
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Light from cities scatters in our atmosphere reducing the
visibility of celestial objects. This is called light pollution, and
has been an increasing problem in recent years.
The view from Kitt
Peak National
Observatory of the
Tuscon,
Tuscon, Arizona
skyline in 1959
The same skyline in 1972
To avoid adverse effects of our atmosphere, we launch
telescopes on satellites that circle the Earth above the
atmosphere.
THE HUBBLE
SPACE
TELESCOPE (HST)
Using adaptive optics to calculate the amount of twinkling of
our atmosphere, as well as changes to the shape of the
mirror, we can receive better images from ground based
telescopes.
IMAGES OF SATURN
GROUND BASED
WITH NO
ADAPTIVE OPTICS
GROUND BASED
WITH ADAPTIVE
OPTICS
HUBBLE IMAGE
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