17 Introduction to Telescopes

Naseer Iqbal

 

Learning Outcomes

 

After studying this module, you shall be able to

  • Write brief introductory remarks on the Telescopic science and its specific relevance to the observational purposes.
  • How the up gradation of telescopes have taken place.
  • What are the different optical parts of a telescope?
  • Point out the major merits and demerits of reflecting and refracting telescopes.
  • Which type of a telescope is more friendly for observing various objects and what are the specific reasons for such a type.
  • What is the importance of telescopes in Science and Technology?

 

 

1.   INTRODUCTION 

 

Since the inception of Astronomical studies some 400 yrs ago, the need to study fainter and fainter objects has naturally led the interest in studying, constructing and using a telescope of ever large diameter. A telescope is a device whose basic technical purpose is to provide a high quality image of distant objects, which may be point sources or extended objects. It is simply an optical instrument that aids in the observation of remote objects by collecting electromagnetic radiation. The construction of telescope is important in the sense that it should be pointed at the desired object field. Furthermore, because of the moment of the objects due to earth’s rotation, the telescope must provide the means to compensate this movement. First practical telescopes were invented in the Netherlands at the beginning of the 17th century, by using glass lenses. Within a few decades, the reflecting telescope was invented, which used mirrors to collect and focus the light. Hans Lippershey Middleburg, from Holland invented the refracting telescope in 1608. In 20th century many new types were invented. Unlike all other branches of science, astronomy is limited to observations. Aside from the analysis of meteorites, and some use of space probes, no experimentation has been much successful so far. It is believed that the astronomer on earth is a passive observer. Galileo was the first to use a telescope for observational purpose. Telescope designs have been upgraded from the very earlier times. Kepler improved the design of the telescope which was found must friendly while using it for observational purposes.

 

An Astronomical telescope has three important basic functions.

  • High quality imaging
  • Pointing
  • Tracking

 

2.  Types 

 

Telescopes are broadly classified in to two types Reflecting Telescope and Refracting Telescope.

 

2.1  Reflecting Telescope: 

 

Sir Isaac Newton is always remembered as the inventor of the reflecting telescope. The Newtonian telescope is a type of reflecting telescope using a concave primary mirror and a flat diagonal secondary mirror. Newton’s first reflecting telescope was completed in 1668 and is the earliest known functional reflecting telescope.

Figure 1.1  Reflecting Telescope / Newtonian type.

 

In reflecting telescopes the primary mirror reflects the light back to a focus instead of refracting it. The primary mirror has a concave spherical or parabolic shape, and, as it reflects the light, it inverts the image at the focal plane. The basic principle of a concave reflecting mirror is shown in fig 1.2.

Fig 1.2   How the incident light gets reflected.

 

Here the primary mirror is located at the lower end of the telescope tube in a reflector and has its front surface coated with 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 with the advent of new telescopes. Materials with expansion coefficients take care of the shape of the mirror which somehow gets affected as the temperature of the telescope changes during night hours. Reflecting telescopes have few advantages over refractors.

  • Reflector telescopes do not suffer from chromatic aberration because all wavelengths will reflect off the mirror in the same way.
  • Reflector telescopes are much cheaper to construct.
  • Since the light is reflecting off from the objective therefore only one side of the reflector telescope’s objective needs to be perfect.

In addition to these advantages there are however, few disadvantages as well.

  • The optics alignment can get disturbed.
  • The tube is open and therefore it needs continue monitoring.
  • The secondary mirror and its support can produce diffraction effects.

 

One more Inventor namely Cassegrain of France, invented another type of reflecting telescope called the Cassegrainian telescope. Cassegrain has remained a contemporary of Newton and therefore there has been a sharp competition in constructing a reflecting telescope. This instrument employs a small convex mirror to reflect the light back through a small hole in the primary mirror to a focus located behind the primary. Figure 1.3 illustrates a typical Cassegrain reflector. Many large telescopes of this kind do not have a hole in the primary mirror but use a small plane mirror in front of the primary to reflect the light outside the main tube and provide another place for observation. This design permits short tubes relative to their mirror diameter.

 

Figure 1.3 Cassegrain Telescope.

 

Today’s reflecting telescopes have a cage at their prime focus and permit the observer to sit inside the telescope tube while operating the instrument. They have also small guide telescopes mounted parallel to their main optical axis to facilitate in locating the desired object. These guide telescopes have low magnification and a wide field of view.

 

2.2  Refracting Telescope 

 

Telescopes of this kind are used to examine the visible-light region of the electromagnetic spectrum e. g viewing of Moon and few objects of the solar system. The refracting telescope uses a lens to gather and focus the light. The physical shape of the components can be a convex, concave, or plane-parallel. Figure 1.4 shows a typical refracting telescope. The first lens through which light from an object passes is the objective lens. The light gets inverted at the focal plane. A second lens, called as eyepiece lens, is placed behind the focal plane and enables the observer to view the enlarged or magnified image.

Fig. 1.4 A typical refracting telescope

 

Like reflecting telescopes, the refracting telescopes have also some advantages. Few of them are:

  • These telescopes seem are more resistant as a result of which the alignment of the optical system is not disturbed as compared to the reflecting one.
  • It does not need any continue monitoring as the tube inside is sealed and therefore has a less atmospheric effect.
  • As the tube is closed from outside, therefore the atmospheric air currents are negligible as a result of which the images are sharper as compared to the reflecting one.

 

Although the refracting telescopes have above mentioned advantages but still because of some more affected drawbacks the demand of the construction of refracting telescopes in Astronomical research is less. Here we mention few disadvantages of this type of a telescope.

  • Refracting telescopes suffer from chromatic aberration which produces a rainbow of colors around the image. Nowadays people have devised few methods for reduction of the chromatic aberration. Because of this defect earlier telescopes constructed were long in size.
  • UV doesn’t pass through the lens and therefore it also marks an important defect in the telescope.
  • By increasing the thickness of the lens more, it effects the moving of light through it.
  • Since the objective lens can be supported at the end point only therefore the lens will always have an effect due to its own weight.

 

3.  How Telescopes work 

 

We have discussed the two basic types of telescopes, reflecting and the refracting one. Let us first discuss the working of a reflecting telescope. In reflecting telescopes mirrors are used to focus the light together. In the given figure (1.5) a concave mirror is used to focus the reflectedlight.

Figure. 1.5  Shows how the reflected light gets simply focused.

 

As and when the incident light strikes the surface, it bounces back. For a smooth surface, the light gets reflected and for a flat surface, the angle of incidence = angle of reflection (Fig 1.6).

Figure 1.6 Angle of incidence = Angle of reflection Reflector Telescopes

 

We know that curved mirrors bend light and make parallel light rays to converge them to a focus. Focus is in the path of the incoming light, hence there are different ways of making images from the mirror visible. For e..g if we see a Newtonian reflector a flat mirror is used to point the light rays out to an eyepiece Figure (1.7).

Figure 1.7 Shows how the Newtonian telescope works.

 

Working of a Refracting Telescope 

 

Here we use two lenses to focus the light. Convex lenses are used as they bend the light inwards (Figure 1.8).

Figure 1.8   Simple illustration of a refracting telescope

 

When light enters from one medium to another one, it is the refractive index of the medium that plays a role in the refraction of the incident light. Light travels at much slower speeds through different materials, such as glass or air. While traveling from one medium to another, some part of it gets reflected at the surface of the new medium. The light that continues through the new medium will either speed up or slow down depending on how fast it can travel through each medium.

 

Light bends towards the normal when traveling into a medium with a higher index of refraction, and away from the normal when traveling into a medium where it can go faster figure (1.9). In this diagram light is leaving air and entering glass, so it bends towards the normal on the way in, and away on the way out of the glass.

Figure 1.9 Bending of light in the process of refraction

 

4.  Importance of Telescopes in Research and Development. 

 

In today’s scientific world telescope has become an icon of science and technology. It is an instrument of our thought. Some people call it an instrument which is helpful to extend our senses into reality, anything that we observe, and anything that we see, anything that we discover. Telescope has a history of about 400 years; it has really changed our perception in so many ways from observing clusters of galaxies, observing a galaxy itself, observing cluster of stars, observing a star itself or even observing the sun as a whole. The pioneering work of the Galileo’s telescope made clear that the universe was far larger than had been imagined and our place in it is far smaller than had been imagined. One can therefore not hesitate to write that had there been no telescope; there would have been a very limited concept of the universe.

 

Research and Development (R & D) is an important aspect of any nation. With this description we can access the economic growth of the country particularly in the field of science and technology. What makes the difference between the developed and underdeveloped countries is the growth of Research and Development (R and D). From smaller size telescope to a larger one all the types including the ground based and space based defines the technological growth of the country.

 

Our country has definitely progressed in constructing, installing or even launching of various space based probes. If we take the example of the ASTROSAT which is a multi-wavelength satellite, all the science loving societies at International level are presently rolling around the scientific results of ASTROSAT that have become now available on different sites. This has been only possible when the country has responsibly moved in the Wright direction. We still need to take the telescopic science far ahead. There is yet a lot of scope available in the instrumentation science of observations.

 

5.  SUMMARY

 

In this module, you study

  • Telescope, its types, The role of Reflecting and Refracting telescopes in Astronomical science
  • Merits and demerits of both the types of telescopes.
  • Working module of Reflecting and Refracting telescope. How different parameters are important in the functioning of a telescope.
  • The role of telescopes in Research and Development.

For More Details

(on this topic and other topics discussed in Text Module) See

1. Telescopes and Techniques C.R.Kitchin, Springer 1995.

2. Observational Astrophysics R.C. Smith, CUP 1995

3. Observational Astronomy D.S. Birney, G Gonzalez and D Oesper, CUP 2006 2nd Ed ISBN 0-521-85370-2