ASTRONOMICAL TELESCOPE
Name:__________ Roll Number:____________ School:_________________________________
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Index 1. Certificate
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2. Acknowledgements
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3. What is an Astronomical Telescope?
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4. Parts Required
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5. Working Principle of an Astronomical Telescope
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6. Uses
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7. Future Aspects
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8. Bibliography
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Certificate This is to certify that ______, student of Class XII X, __________ has completed the project during the academic year 20__-20__ towards partial fulfillment of credit for the Physics practical evaluation of CBSE board 20XX, and submitted satisfactory report, as compiled in the following pages, under my supervision.
Mr. ______________ (Physics teacher)
Examiner
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Acknowledgements The completion of any project is not complete without thanking the people behind the venture & this project is no exception. I take this opportunity to express my profound gratitude and deep regards to my Physics teacher Mr. __________ for his exemplary guidance, monitoring and constant encouragement throughout the course of this thesis. The blessing, help and guidance given by his time to time shall carry me a long way in the journey of life on which I am about to embark.
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What is an Astronomical Telescope? An Astronomical Telescope is a simple device based on optical refraction through spherical lenses, that is used to view heavenly bodies such as the moon, stars and distant objects.
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Parts Required Two Convex Lenses are required: Objective Lens: This lens is fitted at the front side of the telescope. It should have a large aperture (diameter) and a large focal length. Eyepiece: This lens is fitted at the back side of the telescope. It should have a smaller aperture (diameter) and focal length as compared to the objective lens.
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Working Principle of an Astronomical Telescope
As this telescope uses a lens as an objective, it is called a refracting telescope or refractor. The objective lens forms a real, reduced image I of the object. This image is the object for the eyepiece lens, which forms an enlarged, virtual image of I. Objects that are viewed with a telescope are usually so far away from 7
the instrument that the first image I is formed very nearly at the second focal point of the objective lens. If the final image I’ formed by the eyepiece is at infinity (for most comfortable viewing by a normal eye), the first image must also be at the first focal point of the eyepiece. The distance between objective and eyepiece, which is the length of the telescope, is therefore the sum of the focal lengths of objective and eyepiece, ƒ 1 +ƒ2. The angular magnification M of a telescope is defined as the ratio of the angle subtended at the eye by the final image I’ to the angle subtended at the eye by the object. We can express this ratio in terms of the focal lengths of objective and eyepiece. The object (not shown) subtends an angle Ө at the objective and would subtend essentially the same angle at the eye. Also, since the observer’s eye is placed just to the right of the focal point F2’, the angle subtended at the eye by the final image is very nearly equal to the angle Ө’. As bd is parallel to the optic axis, the distances ab and cd are equal to each other and also to the height y’ of the real image I. Because the angles Ө and Ө’ are small, they may be approximated by their tangents. From the right triangles F1ab and F2cd, 𝜃=
−𝑦′
𝜃′ =
ƒ1 𝑦′ ƒ2
And the angular magnification M is 8
𝑀=
𝜃′ 𝜃
=
𝑦 ′ /𝑓1 −𝑦 ′ /𝑓2
= −
𝑓1 𝑓2
The angular magnification M of a telescope is equal to the ratio of the focal length of the objective to that of the eyepiece. The negative sign shows that the final image is inverted. This equation shows that to achieve good angular magnification, a telescope should have a long objective focal length ƒ1.
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Uses Astronomical telescopes are being used worldwide for scientific as well as educational purposes. The refracting telescope design was originally used in spy glasses. It is also used in long focus camera lenses. With the addition of another convex lens (inverting lens) at an appropriate position, this telescope can be modified into a terrestrial telescope.
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Modified Models & Designs Galileo’s Telescope Keplerian Telescope Achromatic Refractor Apochromatic Refractor
Image of a refracting telescope from the Cincinnati Observatory in 1848 11
Bibliography Physics (Part I) – Textbook for Class XII; National Council of Educational Research and Training Sears and Zemansky’s University Physics by Hugh D. Young and Roger A. Freedman
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