TELESCOPES-TYPES OF TELESCOPES-MODE OF OPERATION

A telescope is an instrument used for viewing distant objects. When in normal use/adjustments, the final image is at infinity and when not in normal use, the final image is formed at the near point, D thus the working of a telescope is the opposite of the working a microscope.
Types of Telescopes
There are basically two types of telescopes and these are:-
(i). Refracting telescopes.
(ii). Reflecting telescopes

1. TELESCOPES

2. Telescopes and Astronomical Instruments The 2 main points of telescopes are To make images with as much angular information as possible To gather as much light as possible to allow study of faint things Where you put the telescope can also be important because It’s better to have nice weather (or no weather) It’s better to have as little absorption by atmospheric blocks It’s better to have very stable (or no) air to minimize blurring

3. A telescope is an instrument used for viewing distant objects. When in normal use/adjustments, the final image is at infinity and when not in normal use, the final image is formed at the near point, D thus the working of a telescope is the opposite of the working a microscope. Types of Telescopes There are basically two types of telescopes and these are:- ( i ). Refracting telescopes. (ii). Reflecting telescopes

4. REFRACTING TELESCOPES These consist of objective and eye piece lenses of either convex lens or concave. They include;- ( i ).The Astronomical Telescope (A.T) (ii). The Terrestrial Telescope (T.T) (iii). The Galileo's /Galilean Telescope (G.T)

5. (An Astronomical Telescope (A,T) in normal use An astronomical telescope consists of an objective and eye piece lenses both of which are convex in nature. The objective lens is of long focal length and eye piece has short focal length for a reason that will be later stated. The telescope is in normal use when the final image is formed at infinity. The image I of the distant object is formed at the principal focus, Fo of the objective lens since rays incident on it are parallel. Since the object to the eyepiece is at Fe, then the final image is at infinity as illustrated below.

6. Sample questions 1.Draw a ray diagram to show how two converging lenses, one of focal length,s , and the other of shorter focal length fa can be arranged to make an astronomical telescope in normal adjustment (2mks 2. ( i ) Derive the expression for the magnifying power of telescope in this setting (3mks) 3.Draw a ray diagram to show the action of an astronomical telescope in normal adjustment (3mks) 2007 2b (1) 2004 1b, 1996 2c 4. (ii) Derive an expression for the magnifying power of the telescope in b ( i ) above in terms of focal length fo . and fe . of the objective and eyepiece respectively (3mks)

7. Astronomical telescope not in normal use When an A.T is not in normal use adjustment, the final image is formed at the near point least distance of distant vision, D. The object forms an intermediate imege or distant object at Fo . This image acts as an object to the eye piece. This object is within the F and thus acting as a magnifying glass with respect to the eye piece there by forming a virtual, magnified and inverted image at the near point, D as shown below.

8. DISADVANTAGES OF AN ASTRONOMICAL TELESCOPE It forms an inverted final image.