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Unit 16 B- Using telescopesP2-
Describe how different types of telescopes are used for astronomical
observation
What are telescopes?
A telescope is an optical telescopes, but there are telescopes for most of the spectrum of
electromagnetic radiation and for other signal types. An optical telescope gathers and focuses
visible light and other electromagnetic radiation. Telescopes increase the apparent angular
size of distant objects, as well as their apparent brightness.
Telescopes work by employing one or more curved optical elements - lenses or mirrors - to
gather light or other electromagnetic radiation and bring that light or radiation to a focus,
where the image can be observed, photographed or studied. Optical telescopes are often used
for astronomy and in many non-astronomical instruments including theodolites, transits,
spotting scopes, monocular, binoculars, camera lenses and spyglasses.
Lenses in a telescope-
The most simple working telescope needs nothing more than a pair of lenses mounted in a
tube. The lens in front, known as the objective lens, focuses an image; the lens in back,
known as the eyepiece lens, magnifies that image. Although it may seem like a crude device,
a simple telescope nicely illustrates the basic working principles of more powerful
astronomical instruments.
LENSES
All light normally moves in straight lines, but there are cases in which it does not. I am
already familiar with some: for example, the distortions i see looking through the surface of
the ocean occur because light bends as it passes from the water into the air. Long before i
understood why light bends as it passes from one transparent material to another, people had
used this effect to create lenses: optical devices which can gather light together or spread it
apart.
For me to understand how a lens works, i need to know a little about how light behaves in
passing from one material to another. If I imagine a tank of water on the table in front of me;
the surface of the water should be perfectly flat and horizontal. If i shine a ray of
light straight down from above, it will pass through the surface of the water without bending.
But if i shine the light in at an angle, it will bend as it passes through the surface. The
diagram below illustrates two important facts about this effect. First, in passing from air to
water, the light always bends into the water. Second, the smaller the angle between the light
ray and the surface, the more it bends in passing through. The same rules would also apply if
the tank of water was replaced with a block of glass.
,Light rays passing from air into water. If the ray strikes the surface at a 90° angle it does not
bend (left). But if the angle is less than 90° the light does bend (middle); reducing the angle
between the light and the surface increases the bend (right).
Important properties of Telescope:
The two most important properties of a telescope are:
1. Light gathering ability - The better a telescope can gather light, the better you will be
able to see far away stars and faint objects in the night sky. This feature is usually
determined by the size of the aperture of the telescope. The larger the aperture, the
more light the telescope can gather.
2. Magnification - The magnification of a telescope describes how much larger the
telescope can make objects appear.
The main types of telescopes are :
The Refracting telescope –
Most commonly known as refractors, telescopes of this kind are used to examine
the visible-light region of the electromagnetic spectrum. Typical uses include
viewing the Moon, other objects of the solar system such as Jupiter and Mars, and
double stars. The name refractor is derived from the term refraction, which is the
bending of light when it passes from one medium to another of different density--
e.g., from air to glass. The glass is referred to as a lens and may have one or more
components. The physical shape of the components may be convex, concave, or
plane parallel. Figure 2 illustrates the principle of refraction and the term focal
length. The focus is the point, or plane, at which light rays from infinity converge
after passing through a lens and traveling a distance of one focal length. In a
refractor, the first lens through which light from a celestial object passes is called
the objective lens. It should be noted that the light will be inverted at the focal
plane. A second lens, referred to as the eyepiece lens, is placed behind the focal
plane and enables the observer to view the enlarged, or magnified, image. Thus,
the simplest form of refractor consists of an objective and an eyepiece. The basic
refracting telescope has two lenses. The first lens is called the objective lens. This lens is a
convex lens that bends the incoming light rays to a focal point within the telescope. The
, second lens is called the eyepiece. This lens takes the light from the focal point and spreads it
out across the retina of your eye. This makes the object seem much closer than it really is.
This is Figure 2
Advantages of refracting telescope-
1. Reflector telescopes do not suffer from chromatic aberration because all wavelengths
will reflect off the mirror in the same way.
2. Support for the objective mirror is all along the back side so they can be made very
BIG!
3. Reflector telescopes are cheaper to make than refractors of the same size.
4. Because light is reflecting off the objective, rather than passing through it, only one
side of the reflector telescope's objective needs to be perfect.
Disadvantages of refracting telescope-
1. It is easy to get the optics out of alignment.
2. A reflector telescope's tube is open to the outside and the optics need frequent
cleaning.
3. Often a secondary mirror is used to redirect the light into a more convenient viewing
spot. The secondary mirror and its supports can produce diffraction effects: bright
objects have spikes (the ``Christmas star effect'').
The reflecting telescope –
Reflectors are not only used to examine the visible region of the electromagnetic
spectrum but also to explore both the shorter- and longer-wavelength regions
adjacent to it (i.e., the ultraviolet and the infrared). The name given to this type of
instrument is derived from the fact that the primary mirror reflects the light back to
a focus instead of refracting it. The primary mirror usually has a concave spherical
or parabolic shape, and, as it reflects the light, it inverts the image at the focal
plane. The figure below illustrates the principle of a concave reflecting mirror. The
primary mirror is located at the lower end of the telescope tube in a reflector and
has its front surface coated with an extremely thin film of metal, such as
aluminium. The back of the mirror is usually made of glass, although other
materials have been used from time to time. Pyrex (trademark) was the principal
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