A telescope is an instrument that collects electromagnetic radiation to aid in observing distant objects. There are two main types of telescopes: refracting telescopes, which use lenses, and reflecting telescopes, which use mirrors. Refracting telescopes were invented first in 1608 and helped discoveries like Galileo's observation of Jupiter's moons, while reflecting telescopes were developed later due to producing clearer images. Both telescope types work by collecting and focusing light using the principles of refraction for lenses or reflection for mirrors to magnify distant objects.

1. TELESCOPE
 A telescope is an instrument that aids in the observation of remote objects by
collecting electromagnetic radiation.
 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.
TWO TYPES OF TELESCOPE
1. REFRACTING TELESCOPE –this uses lenses to form an image.
2. REFRACTING TELESCOPE – this uses an arrangement of mirrors to form an
image.
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 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.
The Principle of Reflection
The following figure illustrates the principle of reflection: the angle of incidence
(measured from the perpendicular to the reflecting surface) is equal to the angle of
reflection. The right side of the figure illustrates the use of a mirror to make a reflecting
telescope.

2. Principle of reflection and the reflecting telescope
Focus for Reflecting Telescopes
One problem that must be surmounted with a reflecting telescope is how to place an
observer at the focus. In the example shown above, the focus is inside the telescope. This is
called the prime focus, and in some large telescopes observations can be made at the prime
focus. More commonly, various mirror arrangements are used to transport the light from
the focus to an external observer. Two common ones are a Cassegrain focus and
a Newtonian focus.
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.
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.
Principle of Refraction
The direction of light propagation is changed at the boundary of glass and air
by refraction. By designing lenses having the right curvature, this principle can be used to
gather and focus light. The following figure illustrates the use of a lens to gather and focus
light, and the use of two lenses to make a simple refracting telescope.
Principle of refraction and the refracting telescope
Chromatic Aberration

3. One problem with refracting telescopes is that there is frequency dependence for
refraction, so the amount of refraction at each surface of the lens depends on the
wavelength. Thus, different wavelengths focus at slightly different points. This is
called chromatic aberration, and causes objects like stars to be surrounded by fuzzy,
rainbow colored halos. Chromatic aberration can be corrected by using a second carefully
designed lens mounted behind the main objective lens of the telescope to compensate for
the chromatic aberration and cause all wavelengths to focus at the same point.
Powers of Telescopes
Note that there are 3 ways in which telescopes help us. They can make images
brighter (collecting area), they can make them more detailed, (resolution) and they can
make them larger (magnification).
Light gather power describes how much light a telescope can collect. This is directly related
to the area of their OBJECTIVES (= the primary mirror for a refracting telescope or the
primary lens for a reflecting telescope). The area is proportional to the square of the
diameter, so we have the ratio of LIGHT GATHERING POWER between two telescopes a and
b to be
LGPa/LGPb=(Da/Db)2
Note that a factor of 10 increase in the diameter increases the collected light by a factor of
100.
The second power of telescopes is the RESOLVING POWER. This describes how effectively a
telescope can measure fine detail. Since light acts as a wave, as described earlier, it
produces a DIFFRACTION FRINGE around each point in the image and we cannot see any
detail smaller than the fringe. The larger a telescope (i.e. the larger the OBJECTIVE) the
smaller the fringe and the smaller the fringe and the better the resolving power. The
resolving power is proportional to the wavelength divided by the telescope's diameter. For
optical wavelengths, the resolving power in arc seconds is
a = 2.5 x 10^5 x (wavelength/Diameter of telescope)
For middle optical wavelength at 500nm (=5 x 10^(-5) cm), this is approximately
a = 12/D
where D is the telescope diameter in centimeters. (Again, telescope diameter means
diameter of the collecting lens or mirror).
It is important to note that the lens quality and the atmospheric conditions play an
important role in a telescope's ability to resolve: The lenses and mirrors cannot have
imperfections. In addition, because the air in the atmosphere is "turbulent" that is has a lot
of random motions, light passing through this suffers random deflections and slight
blurring. (This is why the stars twinkle.) On a night when the images are blurred we say
that the SEEING is bad. When the images are not blurred we say the SEEING is good. Mostly
it is the seeing which limits the resolution of big telescopes.
The third power of telescopes is called MAGNIFICATION POWER and is the ability of the
telescope to make the image larger. It is determined by the ratio of the FOCAL LENGTH of
the OBJECTIVE to the FOCAL LENGTH of the eyepiece
M = Fo/Fe