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Of course, those first telescopes weren't perfect. While distant objects did
appear closer, they weren't very clear. It took several hundreds of years and a
lot of experimentation to get really sharp images through telescopes. And people
were always trying to see farther and farther into space, so the telescope was
constantly being improved and sometimes even reinvented.
The kind of telescope first constructed by Lippershey and Galileo was a
refracting telescope, which worked by allowing light to pass through two lenses,
one convex or curving outward and the other concave or curving inward. As light
from the sun, moon, or stars travels through these two lenses to the eye of the
observer, it becomes refracted or bent by the lenses. However, when light is
bent through a glass in that way, the different colors that make up light bend
at different angles. Just like when light passes through a prism and makes
rainbow spots on the wall, or when light passes through a rain shower and makes
a rainbow, light through a refracting telescope lens breaks an image into
several different colored images. All these images bunch close together, but
they do not align perfectly one on top of the other to produce a unified image.
When this happens, the image seems to have a fuzzy appearance. The yellow image
is brightest, for example, but its blue and red counterparts are there too,
except slightly to one side or the other, just like the separations of a
rainbow. So the brightest image looks as if it has a colored haze around it.
This "color aberration" or color problem was a real annoyance to the first
telescope owners, and this problem, along with a related problem called
"spherical aberration," were both due to the curve of the convex lenses and
remained unsolved for many decades. [Diagram of Galileo/Lippershey refractor]
But before the color and spherical problems were even identified, other
refinements in telescopes took place. Johannes Kepler (1571-1630) was the first
to make significant improvements to the telescope. Instead of a concave and
convex lens combination, he proposed two convex lenses, which would increase the
astronomer's field of vision. Kepler got this idea by studying the structure of
the human eye. With this arrangement, the resulting image in the telescope would
be upside-down, but that was a small price to pay for a better view. Another
lens could flip the image right-side up, but the final image would not be as
bright. Modern refractors still produce an upside-down image, but astronomers
who use them to take pictures just publish the photos upside down. [Diagram of
Kepler refractor]
Kepler may not have actually made a telescope like he suggested. He was very
nearsighted and probably lacked the practical skill to construct one. Instead he
investigated and wrote down theories on optics, the study of light and its
changes, and dioptics, the study of how different lenses worked. Kepler was the
first to understand the role that light plays in vision. In Kepler's time,
people believed that when you saw something, a beam of light came out of your
eye and in some way touched the object seen. Kepler insisted that the reverse
happens - light bounces off the object and enters the eye, producing an image
inside the eye.
Kepler's work influenced many others in the new scientific age. William
Gascoigne, an amateur astronomer in England, was using a Kepler-style telescope
when part of a spider's web found its way inside the telescope. One small web
line happened to fall right at the focus point, so both the thin line and the
image Gascoigne was viewing were magnified together. Gascoigne realized that he
could more accurately point the telescope using the line as a guide, and he went
on to invent the telescopic sight by purposefully placing wires at the focus
point. This helped astronomers make more accurate observations and measurements
of objects in space, using the thin wires as a reference point. [Telescope with
wires at the focus]
Isaac Newton (1642-1727) also studied Kepler's work and constructed a
telescope along the lines he suggested. Newton also tried to solve the
telescope's color problem. He was the first to realize that white light was a
combination of all the colors, rather than the absence of all colors, as was
generally believed. But Newton eventually decided there was no way to prevent
the breaking up of the different colored images once light passed through a
refracting lens. He was later proved wrong, but unfortunately Newton's
conclusion discouraged others to try. It was 50 years before someone did at last
find a way to remove the color blurs in refracting telescopes.
While he couldn't fix refractors, Newton did have a solution to the color
problem - he created an entirely new kind of telescope. He just used a mirror
instead of a curved lens for the object glass which collects the final image. A
mirror wouldn't refract or bend the light, so there would be no color fringes
around the image. In fact, a parabolic-shaped mirror was later found to solve
the spherical problem, discovered to be a separate problem. Newton's first
reflecting telescope was a great advance in clearer viewing. [Diagram of Newton
reflector with side eyepiece]
But reflectors also had problems at the very beginning. Back then, people
didn't know how to make mirrors that wouldn't tarnish. Newton's mirror was made
of bell-metal, copper, tin, and a little arsenic for whitening. Such a
combination got dull quickly and had to be resurfaced, usually a very expensive
and time-consuming process. It wasn't until two centuries later that Léon
Foucault (1819-1868) discovered how to layer silver on glass, a process used
until layering aluminum on glass was developed.
Newton constructed his reflecting telescope with another smaller mirror
facing the main mirror. The smaller mirror angled the image to the side of the
telescope, where Newton put the eyepiece, the hole through which to view the
image. A Frenchman named N. Cassegrain in 1672 also proposed a reflecting
telescope, but instead of a small mirror angling the image to the side,
Cassegrain's main mirror had a hole in the center, and the smaller mirror
reflected the image back through that hole to an eyepiece behind the main
mirror. Newton ridiculed this arrangement; perhaps he was defensive because some
thought Cassegrain invented his reflecting telescope before Newton did. Newton
was so well respected an authority at the time that Cassegrain didn't challenge
this attack and perhaps even felt his idea wasn't a good one. It's interesting
to note, however, that one of the greatest telescopes of our time, the Hubble
Space Telescope, is designed as Cassegrain suggested. [Diagram of Cassegrain
reflector]
Refracting telescopes were still used even after reflectors were invented,
for a number of reasons. Many more artisans were making lenses than were making
the right kind of mirrors for telescopes. Refractors were easier to get, and
they revealed a larger area of the sky. People continued trying to improve
refractors. In the process, they discovered that magnification increased with
the length of the telescope's tube. To get a brighter image, however, the size
of the lens called the object glass had to be larger in diameter. A wider lens
could also be made with less curvature, which would reduce the annoying color
and spherical problems that still plagued refractors. So the ideal telescope
that everyone sought after was one that had the longest possible tube and widest
available object glass. The problem with getting such a telescope was that
really long telescopes needed scaffolding or long masts and cranes to hold them
up. Some shook when a breeze came along, and others collapsed altogether. They
were hard to maneuver into position. Some astronomers eventually learned that
their problems outweighed their benefits. Advances in firmly securing and
maneuvering telescopes had to occur before very large telescopes would be
practical. [long telescope on a mast]
Despite their awkwardness, longer telescopes with larger lenses helped make
more and more discoveries. Saturn's ring was identified by Christiaan Huygens
(1629-1695) who, with his brother Constantine, constructed telescopes that were
12, 23, and even 123 feet long. Christiaan Huygens also developed an aerial
telescope, an eyepiece joined by a taut thread to the main telescope, that was
itself perched on a tall pole. [Aerial telescope. image 14, p. 143 Hoskin] Many
of Saturn's moons were discovered by Jean Dominique Cassini (1625-1712), who
used telescopes as long as 17, 34, 100, and 135 feet. It seemed every time
Cassini made a longer telescope, he discovered another moon! Cassini also saw
that Saturn really had two rings. He mounted one of his long telescopes on a
water tower that he had the Paris Observatory move to the observatory grounds at
great expense. [Paris Observatory with water tower supporting telescope]
Another astronomer, Adrien Auzout, constructed telescopes that were 300 and
600 feet long. He eventually planned to build a telescope as long as 1,000 feet.
He hoped it would allow him to see animals on the moon! As time passed, the
reports of more moons around planets, moon shadows on Jupiter, and double stars
thrilled people, who then wanted the latest improved telescope so they
themselves could see such wonders. One rich man, Nicholas De Peirese, had over
40 telescopes. There seemed no end to what a bigger and wider telescope would
show.
William Herschel (1738-1822), a musician, became interested in reflecting
telescopes after he used one that was just two feet long. He had some refractors
but liked the clearer image the reflector gave him. Herschel wanted to see how
much better a five- or six-foot reflector would work, but found that no one made
the larger mirrors for them. Herschel decided to try and make his own mirrors.
He was able to obtain some mirror-making equipment from a man who was giving up
the hobby. His first reflector was seven feet long, then he made one 10 feet
long. On March 13, 1781, using just the seven-foot telescope, he discovered a
new planet, later called Uranus. He thought that what he had found was just a
comet, since no one had any clue that there were undiscovered planets. After
that success, he built a 20-foot reflector and finally arranged the construction
of an enormous 40-foot reflector. [Herschel's 20-foot reflector]
Herschel saw farther into space than anyone had before. He found that many
stars were not just simple points of light, but actually quite different from
each other when viewed with a powerful telescope. Some were double stars, and
others seemed cloudy, which he called "nebula." He was the first to suggest that
nebulae might be other galaxies like our own Milky Way. Herschel often examined
over 400 stars in a night. This was the first time that stars were examined for
themselves, and not just as reference points for observing moons and planets.
But his favorite object was Saturn, and he discovered several new moons orbiting
the ringed planet.
Herschel continued to make the mirrors for all his telescopes and even made
and sold smaller telescopes to help pay his expenses. Herschel polished his
mirrors himself, sometimes for 16 hours straight. Often he refused to stop for
meals, so his sister Caroline, who herself became a noted astronomer, would feed
him as he worked. While his 40-foot telescope was a marvel of its age, the
scaffolding he used on his larger telescopes was rickety and dangerous. On more
than one occasion he narrowly escaped structures collapsing on him. In the end,
Herschel decided that using the 40-foot telescope was often more trouble than it
was worth. It took too long to prepare, he had to hire people to help him
uncover and maneuver it, and with the limited clear evenings available for
observing in England, he found he used his smaller telescopes more often.
[Herschel's 40-footer]
Meanwhile, the color problems of refractors was at last solved by putting one
flint glass concave lens up against a crown glass convex lens. The different
types of glass broke up light at somewhat opposite angles, so all the colors
blended together perfectly. [Diagram of flint glass and crown glass different
refracting of colors] Chester Moor Hall (1703-1771) happened on this combination
effect, although it wasn't until John Dollond
[correction made 2001-10-29] manufactured many telescopes
with this correction that the solution became widely known. Another great
improvement was contributed by Pierre Louis Guinand, who developed a way of
stirring the glass when it was forming, making it more defect free. This made
possible the creation of larger and larger glass lenses, which up to that time
always had bubbles or flaws in them when they were made with a very large
diameter.
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