Optics

Wow! Look at that! You can see Haley’s Comet from here and it’s not even blurry. This reflecting telescope
works so much better than that old refracting telescope.
For hundreds of years, telescopes were the only things people could use to study the
sky. Today we use telescopes to study distant stars and galaxies. We do use space
probes, but they can only go so far out in space and they take a very long time to get
there.
A telescope is a device that allows you to look at an object that is far away or very
faint, and it makes the object appear much closer to the person looking through it.
Most telescopes work by collecting and magnifying light that was given off by stars or
reflected by the surface of planets. This type of telescope is called an optical
telescope. Optical telescopes use mirrors or lenses that collect the light and make the
image appear larger.
Early History of the Refracting Telescope
In 1608 a Dutch optician (someone who makes lenses) named Hans Lippershy discovered that a distant
object could be seen much clearer if it was looked at through concave and convex lenses. So he stuck two
lenses in a tube and created the first telescope. About a year later Galileo, an Italian astronomer, made his
own refracting telescope and discovered four moons orbiting Jupiter. Galileo also used his refracting
telescope to map the surface of the moon.
Early History of the Reflecting Telescope
In 1663 a Scottish astronomer named James Gregory designed a reflecting telescope, which bounced light
rays off of the mirror instead of bending them like on a refracting telescope. From Gregory’s design, Isaac
Newton built the first reflecting telescope in 1688. After Newton built the reflecting telescope, scientists
discovered that better images were seen through the reflecting telescope instead of the refracting
telescope because the mirror could make larger and clearer images of the object. The telescope reacted
like this because the mirror simply reflects the light and the refracting telescope has to bend the light.
How a Refracting Telescope Works

Refracting telescopes use glass lenses to bend light, magnify it, and bring it into focus. The convex lens is
thickest at the center and thinner towards the edge. This shape allows the lens to bend the light at the edge
of the lens at a greater angle than the light coming through the center, so all of the light rays come

together at a point of focus.
How a Reflecting Telescope Works

Reflecting telescopes use curved mirrors instead of convex lenses to collect light. Reflecting telescopes are
especially helpful for viewing dim objects. Larger reflecting telescopes can detect objects that are a
millionth or a billionth the brightness of stars that can be seen by the human eye without a telescope.
[ Up ]
Space Exploration of the Past, Present, and Future
Bartlett Elementary School 2000
Who Invented the Telescope?
Lauren Cox, SPACE.com Contributor | July 13, 2013 11:40am ET
The telescope is one of humankind's most important inventions. The simple device that made far away things
look near gave observers a new perspective. When curious men pointed the spyglass toward the sky, our
view of Earth and our place in the universe changed forever.
But who invented the telescope? The answer remains a mystery today. It is highly probable that as
glassmaking and lens-grinding techniques improved in the late 1500s, someone held up two lenses and
discovered what they could do.

Hans Lippershey, credited with invention of the telescope.
Credit: Public domain
View full size image
The first person to apply for a patent for a telescope was a Dutch eyeglass maker named Hans Lippershey (or
Lipperhey). In 1608, Lippershey tried to lay claim on a device with three-times magnification. His telescope had
a concave eyepiece aligned with a convex objective lens. One story goes that he got the idea for his design

after observing two children in his shop holding up two lenses that made a distant weather vane appear close.
Others charged at the time that he stole the design from another eyeglass maker, Zacharias Jansen.
Jansen and Lippershey lived in the same town and both worked on making optical instruments. Scholars
generally argue, however, that there is no real evidence that Lippershey did not develop his telescope
independently. Lippershey, therefore, gets the credit for the telescope, because of the patent application, while
Jansen is credited with inventing the compound microscope. Both appear to have contributed to the
development of both instruments.
Compounding the confusion, yet another Dutchman, Jacob Metius, applied for a patent for a telescope a few
weeks after Lippershey. The government of the Netherlands eventually turned down both applications because
of the counterclaims. Also, officials said, the device was easy to reproduce, making it difficult to patent. In the
end, Metius got a small reward, but the government paid Lippershey a handsome fee to make copies of his
telescope.

A 1754 painting by H.J. Detouche shows Galileo Galilei displaying his telescope to Leonardo Donato
and the Venetian Senate.
Credit: Public domain
Enter Galileo
In 1609, Galileo Galilei heard about the "Dutch perspective glasses" and within days had designed one of his
own — without ever seeing one. He made some improvements on his initial design and presented his device to
the Venetian Senate. The Senate, in turn, set him up for life as a lecturer at the University of Padua and
doubled his salary, according to Stillman Drake in his book "Galileo at Work: His Scientific Biography" (Courier
Dover Publications, 2003).

Galileo's ink renderings of the moon: the first telescopic observations of a celestial object.
Credit: NASA
Galileo was the first to point a telescope skyward. He was able to make out mountains and craters on the
moon, as well as a ribbon of diffuse light arching across the sky — the Milky Way. He also discovered the sun
had sunspots, and Jupiter had its own set of moons.

The British ethnographer and mathematician Thomas Harriot also used a spyglass to observe the moon.
Harriot became famous for his travels to the early settlements in Virginia to detail resources there. His August
1609 drawings of the moon predate Galileo's, but were never published.
The more he looked, the more Galileo was convinced of the sun-centered Copernican model of the planets.
Galileo wrote a book ―Dialogue Concerning the Two Chief World Systems, Ptolemaic and Copernican‖ and
dedicated it to the Pope Urban VIII. But his ideas were considered heretical, and Galileo was called to appear
before the inquisition in Rome in 1633. He struck a plea bargain and was sentenced to house arrest where he
continued to work and write until his death in 1642.
Elsewhere in Europe, scientists began improving the telescope. Johannes Kepler studied the optics and
designed a telescope with two convex lenses, which made the images appear upside down. Working from
Kepler'swritings,Isaac Newton reasoned it was better to make a telescope out of mirrors rather than lenses and
built his famous reflecting telescope in 1668. Centuries later the reflecting telescope would dominate
astronomy.
Exploring the cosmos
The largest refracting telescope ever built opened at Yerkes Observatory in Williams Bay, Wis., in 1897. But
the 40-inch wide glass lens at Yerkes was soon made obsolete by larger mirrors. The Hooker 100-inch
reflecting telescope opened in 1917 at Mount Wilson Observatory in Pasadena, Calif. It was there that the
astronomer Edwin Hubble determined the distance of the Andromeda Nebula — far beyond the Milky Way.
With the development of the radio, scientists could start to study not just light, but other electromagnetic
radiation in space. An American engineer named Karl Jansky was the first to detect radio radiation from space
in 1931. He found a source of radio interference from the center of the Milky Way. Radio telescopes have since
mapped the shape of galaxies and the existence of background microwave radiation that confirmed a
prediction in the Big Bang Theory.
In April 1990, the Hubble Space Telescope sailed into orbit. The reflecting telescope took advantage of digital
cameras and satellite communications to give jaw dropping views of space free from the interference of the
earth's atmosphere and light pollution. More than 200 years after Galileo pointed his telescope skyward,
people could see space from the heavens.
— Lauren Cox, SPACE.com Contributor

Inventions: Telescopes
A telescope is a device that lets us view distant objects. Early telescopes (and most today) used glass
lenses and/or mirrors to detect visible light. Some modern telescopes gather images from different parts of the
electromagnetic spectrum, from radio waves to gamma rays. Most telescopes are located on Earth, but others are in space.
Refracting Telescopes
The first refracting telescope was invented by Hans Lippershey in 1608.
Lippershey (1570?-1619) was a German-born Dutch lens maker who
demonstrated the first refracting telescope in 1608, made from
two lenses; he applied for a patent for this optical refracting
telescope (using 2 lenses) in 1608, intending it for use as a
military device.In 1609, Galileo was the first person to use a telescope to observe the skies (after hearing
about Hans Lippershey's newly-invented telescope). Galileo discovered the rings of Saturn (1610), was the
first person to see the four major moons of Jupiter (1610), observed the phases of Venus, studied sunspots, and discovered
many other important phenomena.Isaac Newton improved the design of the refracting telescope (using an objective
mirror, instead of a lens), now called the Newtonian telescope.

New and Improved Lenses
Christian Huygens (1629-1695) was a Dutch physicist and astronomer who developed new
methods for grinding and polishing glass telescope lenses (about 1654). With his new, powerful
telescopes, he identified Saturn's rings and discovered Titan, the largest moon of Saturn in 1655.
Huygens also invented the pendulum clock in 1656 (eliminating springs), wrote the first work on
the calculus of probability (De Ratiociniis in LudoAleae, 1655), and proposed the wave theory of
light (Traité de la lumiere, 1678).

Reflecting Telescopes
James Gregory (1638-1675), a Scottish mathematician, invented the first reflecting telescope in 1663. He published a
description of the reflecting telescope in "OpticaPromota," which was published in 1663. He never actually made the
telescope, which was to have used a parabolic and an ellipsoidal mirror.
Cassegrain Telescope
ACassegrain telescope is a wide-angle reflecting telescope with a concave mirror that receives light and focuses an image.
A second mirror reflects the light through a gap in the primary mirror, allowing the eyepiece or camera to be mounted at
the back end of the tube. The Cassegrain reflecting telescope was developed in 1672 by the French sculptor Sieur
Guillaume Cassegrain. A correcting plate (a lens) was added in 1930 by the Estonian astronomer and lens-maker Bernard
Schmidt (1879-1935), creating the Schmidt-Cassegrain telescope which minimized the spherical aberration of the
Cassegrain telescope.
Schmidt-Cassegrain Telescope
A Schmidt-Cassegrain telescope (SCT) is a wide-angle reflecting telescope with a correcting lens that minimizes spherical
aberration and a concave mirror that receives light and focuses an image. A second mirror reflects the light through a gap
in the primary mirror, allowing the eyepiece or camera to be mounted at the back end of the tube. The Cassegrain
telescope (named for the French sculptor Sieur Guillaume Cassegrain) was developed in 1672; the correcting plate (a lens)
was added in 1930 by the Estonian astronomer and lens-maker Bernard Schmidt (1879-1935).
Radio Telescopes
Karl GotheJansky (1905-1949) was an American radio engineer who pioneered and developed radio astronomy. In 1932,

he detected the first radio waves from a cosmic source - in the central region of the Milky Way Galaxy. GoteReber (a ham
radio operator) made the first true radio telescope (using a 32-foot diameter parabolic dish to focus the radio waves) after
reading of Jansky's discoveries.
Hubble Space Telescopes
The Hubble Space Telescope (or HST) is a powerful telescope in orbit around the Earth. HST transmits
pictures and spectra of objects in space without the interference of the atmosphere (which makes telescopic
images from the ground have less detail). It was launched into space in April 1990 and was repaired in
December, 1993. It was named for the American astronomer Edwin Hubble.
Reflecting vs. Refracting Stargazing Telescopes
By Steve Owens from Stargazing For Dummies
When you‘re ready to invest in a stargazing telescope, start by looking at the different models of telescope
tube – the bits with the optics in. You can find quite a few different designs. Reflectingtelescopes use mirrors to
gather the light. Refracting telescopes use lenses. There are different kinds of reflectors, but in general the
refractors all follow the same basic design.
Stargazing: Refracting telescopes
Refracting telescopes use lenses to collect and focus the light, just like binoculars do. In fact, you can think of
a refracting telescope as one half of a giant pair of binoculars. The light enters a refracting telescope through
the front lens, called the objective lens. It then travels down the length of the telescope to the eyepiece lenses,
which is where the magnifying happens.

This setup can make observing through a refracting telescope quite uncomfortable, especially if you‘re pointing
your telescope high in the sky, because you may have to stoop and crane your neck to see through the
eyepiece. Some telescopes come with a prism adaptor that bends the light through 90 degrees, so looking
through the eyepiece is much more comfortable.
Refracting telescopes have several benefits over other telescopes:
Because the inside of the tube is sealed at both ends (with lenses), refracting telescopes don‘t suffer from
dirt inside.
Because the tube is sealed at both ends, refracting telescopes don‘t have the problem of air moving about
inside the tube, and so have sharper, steadier images.
However, refracting telescopes are longer and more unwieldy than reflecting telescopes, and can suffer from
something called chromatic aberration, where a rainbow of colours appears around the image. Lens coatings
and a longer telescope tube (which increases something called the focal length) can help reduce this problem.
Stargazing: Reflecting telescopes
One of the most common types of reflecting telescope is called a Newtonian reflector, named after the man
who invented it, Isaac Newton (of falling apple fame). In Newtonian telescopes, the tube is open at one end.

The light enters the tube and reflects off a curved mirror (called the primary mirror) at the other end, before
bouncing back up the tube to near the top where it reflects off a smaller mirror (called thesecondary mirror),
which bounces the light out of a hole in the side of the telescope where you attach your eyepiece.

This setup can make looking through the eyepiece a little tricky, especially in the bigger Newtonian telescopes,
where you may need a step or small ladder to get high up enough to see through the eyepiece.
The benefits of Newtonian reflecting telescopes over other telescopes include the following:
Because the mirror can be fixed onto a metal plate, reflecting telescopes can be much bigger than
refractors.
Reflecting telescopes are cheaper to make.
Reflecting telescopes don‘t suffer from chromatic aberration.
A variation on the standard Newtonian telescope is the Cassegrain reflector, which uses a curved secondary
mirror to bounce the light back down the length of the tube to an eyepiece at the bottom end.

Because of their design, Cassegrain reflectors can be more compact than their Newtonian cousins, because
the light is ‗folded‘ twice inside the tube, making two journeys rather than one. Cassegrain telescopes can be
half the length of Newtonians. Cassegrain reflecting telescopes have all the same advantages as a Newtonian
reflector, plus they‘re much more compact
Another variation on the Newtonian reflector theme is the SchmidtCassegrain, in which a thin lens is placed
over the front of the telescope tube. This lens gives the telescope a slightly wider field of view. To all intents
and purposes, the design is the same as the standard Cassegrain.
Invention[edit]
Main article: History of the telescope

This section does not cite any references or sources. Please help improve this
section by adding citations to reliable sources. Unsourced material may be
challenged and removed. (May 2010)
Refractors were the earliest type of optical telescope. The first practical refracting telescopes appeared in
the Netherlands about 1608, and were credited to three individuals, Hans Lippershey andZacharias Janssen,
spectacle-makers in Middelburg, and Jacob Metius of Alkmaar. Galileo Galilei, happening to be in Venice in
about the month of May 1609, heard of the invention and constructed a version of his own. Galileo then
communicated the details of his invention to the public, and presented the instrument itself to
the Doge Leonardo Donato, sitting in full council.
Refracting telescope designs[edit]

All refracting telescopes use the same principles. The combination of an objective lens 1 and some type
of eyepiece 2 is used to gather more light than the human eye is able to collect on its own, focus it 5, and
present the viewer with abrighter, clearer, and magnified virtual image 6.
The objective in a refracting telescope refracts or bends light. This refraction causes parallel light rays to
converge at afocal point; while those not parallel converge upon a focal plane. The telescope converts a
bundle of parallel rays to make an angle α, with the optical axis to a second parallel bundle with angle β. The
ratio β/α is called the angular magnification. It equals the ratio between the retinal image sizes obtained with
and without the telescope.[1]
Refracting telescopes can come in many different configurations to correct for image orientation and types of
aberration. Because the image was formed by the bending of light, or refraction, these telescopes are
called refracting telescopes orrefractors.
Galileo's telescope[edit]

Optical diagram of Galilean telescope y – Distant object ; y’ – Real image from objective ; y’’ – Magnified
virtual image from eyepiece ; D – Entrance pupil diameter ; d – Virtual exit pupil diameter ; L1 – Objective
lens ; L2 – Eyepiece lense – Virtual exit pupil – Telescope equals [2]
The original design Galileo Galilei came up with in 1609 is commonly called a Galilean telescope. It used a
convergent (plano-convex) objective lens and a divergent (plano-concave)(Galileo, 1610) eyepiece lens.[3] A
Galilean telescope, because the design has no intermediary focus, results in an non inverted and upright
image.
Galileo‘s best telescope magnified objects about 30 times. Because of flaws in its design, such as the shape of
the lens and the narrow field of view, the images were blurry and distorted. Despite these flaws, the telescope
was still good enough for Galileo to explore the sky. The Galilean telescope could view the phases of Venus,
and was able to seecraters on the Moon and four moons orbiting Jupiter.
Parallel rays of light from a distant object (y) would be brought to a focus in the focal plane of the objective lens
(F' L1 / y’). The (diverging) eyepiece (L2) lens intercepts these rays and renders them parallel once more.
Non-parallel rays of light from the object traveling at an angle α1 to the optical axis travel at a larger angle (α2
> α1) after they passed through the eyepiece. This leads to an increase in the apparent angular size and is
responsible for the perceived magnification.
The final image (y’’) is a virtual image, located at infinity and is the same way up as the object.
KeplerianTelescope[edit]

Woodcut illustration of a 45 m (150 ft) focal length Keplerian astronomical refracting telescope built by
Johannes Hevelius. From his book Machinacoelestis(first part), published in 1673.
The Keplerian Telescope, invented by Johannes Kepler in 1611, is an improvement on Galileo's design.[4] It
uses a convex lens as the eyepiece instead of Galileo's concave one. The advantage of this arrangement is
that the rays of light emerging from the eyepiece are converging. This allows for a much wider field of view and
greater eye relief, but the image for the viewer is inverted. Considerably higher magnifications can be reached
with this design, but to overcome aberrations the simple objective lens needs to have a very high fratio (Johannes Hevelius built one with a 45 m (150 ft) focal length, and even longer tubeless "aerial
telescopes" were constructed). The design also allows for use of a micrometer at the focal plane (used to
determining the angular size and/or distance between objects observed).
Achromatic refractors[edit]
Main article: Achromatic telescope

The achromatic refracting lens was invented in 1733 by an English barrister named Chester Moore Hall,
although it was independently invented and patented by John Dollond around 1758. The design overcame the
need for very long focal lengths in refracting telescopes by using an objective made of two pieces of glass with
different dispersion, "crown" and "flint glass", to limit the effects of chromatic and spherical aberration. Each
side of each piece is ground and polished, and then the two pieces are assembled together. Achromatic lenses
are corrected to bring two wavelengths (typically red and blue) into focus in the same plane. The era of
the Great refractors in the 19th century saw large achromatic lenses culminating with largest achromatic
refractor ever built, the Great Paris Exhibition Telescope of 1900.
Apochromatic refractors[edit]
Main article: Apochromat
Apochromatic refractors have objectives built with special, extra-low dispersion materials. They are designed
to bring three wavelengths (typically red, green, and blue) into focus in the same plane. The residual color error
(tertiary spectrum) can be up to an order of magnitude less than that of an achromatic lens.[citation needed] Such
telescopes contain elements of fluorite or special, extra-low dispersion (ED) glass in the objective and produce
a very crisp image that is virtually free of chromatic aberration.[5] Due to the special materials needed in the
fabrication, apochromatic refractors are usually more expensive than telescopes of other types with a
comparable aperture.
Compound Microscope Parts
A high power or compound microscope achieves higher levels of magnification than a stereo or low power
microscope. It is used to view smaller specimens such as cell structures which cannot be seen at lower levels
of magnification. Essentially, a compound microscope consists of structural and optical components. However,
within these two basic systems, there are some essential components that every microscopist should know
and understand. These key microscope parts are illustrated and explained below.
STRUCTURAL COMPONENTS
The three basic, structural components of a compound microscope are the head, base and arm.
Head/Body houses the optical parts in the upper part of the microscope
Base of the microscope supports the microscope and houses the illuminator
Arm connects to the base and supports the microscope head. It is also used to carry the microscope.
When carrying a compound microscope always take care to lift it by both the arm and base, simultaneously.
OPTICAL COMPONENTS
There are two optical systems in a compound microscope: Eyepiece Lenses and Objective Lenses:
Eyepiece or Ocular is what you look through at the top of the microscope. Typically, standard eyepieces have
a magnifying power of 10x. Optional eyepieces of varying powers are available, typically from 5x-30x.
Eyepiece Tube holds the eyepieces in place above the objective lens. Binocular microscope heads typically
incorporate a diopter adjustment ring that allows for the possible inconsistencies of our eyesight in one or both
eyes. The monocular (single eye usage) microscope does not need a diopter. Binocular microscopes also
swivel (Interpupillary Adjustment) to allow for different distances between the eyes of different individuals.
Objective Lenses are the primary optical lenses on a microscope. They range from 4x-100x and typically,
include, three, four or five on lens on most microscopes. Objectives can be forward or rear-facing.
Nosepiece houses the objectives. The objectives are exposed and are mounted on a rotating turret so that
different objectives can be conveniently selected. Standard objectives include 4x, 10x, 40x and 100x although
different power objectives are available.
Coarse and Fine Focus knobs are used to focus the microscope. Increasingly, they are coaxial knobs - that
is to say they are built on the same axis with the fine focus knob on the outside. Coaxial focus knobs are more
convenient since the viewer does not have to grope for a different knob.
Stage is where the specimen to be viewed is placed. A mechanical stage is used when working at higher
magnifications where delicate movements of the specimen slide are required.
Stage Clips are used when there is no mechanical stage. The viewer is required to move the slide manually to
view different sections of the specimen.
Aperture is the hole in the stage through which the base (transmitted) light reaches the stage.
Illuminator is the light source for a microscope, typically located in the base of the microscope. Most light
microscopes use low voltage, halogen bulbs with continuous variable lighting control located within the base.

Condenser is used to collect and focus the light from the illuminator on to the specimen. It is located under the
stage often in conjunction with an iris diaphragm.
Iris Diaphragm controls the amount of light reaching the specimen. It is located above the condenser and
below the stage. Most high quality microscopes include an Abbe condenser with an iris diaphragm. Combined,
they control both the focus and quantity of light applied to the specimen.
Condenser Focus Knob moves the condenser up or down to control the lighting focus on the specimen.
Compound Microscopes
The compound microscope consists essentially of two or more double convex lenses fixed in the two
extremities of a hollow cylinder. The lower lens (nearest to the object) is called the objective; the upper lens
(nearest to the eye of the observer), the eyepiece. The cylinder is mounted upright on a screw device, which
permits it to be raised or lowered until the object is in focus, i.e., until a clear image is formed. When an object
is in focus, a real, inverted image is formed by the lower lens at a point inside the principal focus of the upper
lens. This image serves as an "object" for the upper lens which produces another image larger still (but virtual)
and visible to the eye of the observer.
Computation of Magnifying Power
The magnifying power of a lens is commonly expressed in diameters. For example, if a lens magnifies an
object 5 times, the magnification is said to be 5 diameters, commonly written simply "5x." The total
magnification of a compound microscope is computed by multiplying the magnifying power of the objective by
the magnifying power of the eyepiece.
Development and Uses
The invention of the microscope is variously accredited to Zacharias Janssen, a Dutch spectaclemaker,
c.1590, and to Galileo, who announced his invention in 1610. Others are known for their discoveries made by
the use of the instrument and for their new designs and improvements, among them G. B. Amici, Nehemiah
Grew, Robert Hooke, Antony van Leeuwenhoek, Marcello Malpighi, and Jan Swammerdam. The compound
microscope is widely used in bacteriology, biology, and medicine in the examination of such extremely minute
objects as bacteria, other unicellular organisms, and plant and animal cells and tissue—fine optical
microscopes are capable of resolving objects as small as 5000 Angstroms. It has been extremely important in
the development of the biological sciences and of medicine.
Modified Compound Microscopes
The ultramicroscope is an apparatus consisting essentially of a compound microscope with an arrangement by
which the material to be viewed is illuminated by a point of light placed at right angles to the plane of the
objective and brought to a focus directly beneath it. This instrument is used especially in the study of Brownian
movement in colloidal solutions (see colloid). The phase-contrast microscope, a modification of the compound
microscope, makes transparent objects visible; it is used to study living cells. The television microscope uses
ultraviolet light. Since this light is not visible, the apparatus is used with a special camera and may be
connected with a television receiver on which the objects (e.g., living microorganisms) may be observed in
color.

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