Reflecting Telescope
Introduction
1.1 Scope
The aim of this book is to describe how a simple reflecting telescope can be made with the materials and facilities available in India. As the primary goal is to popularize telescope making, only the most primitive (but effective) constructions will be considered; the emphasis will be on the practical aspects of telescope making.
Some excellent books on this subject are in print (see, for example, the Bibliography). Yet few people have easy access to these books as they are shelved in specialist libraries or are expensive to buy. Moreover, the methosds discussed are beyond the means of even most middle class Indians. Perfection, indeed, demands the use of thick glass perspx blanks, well graded abrasives, good pitch, sturdy mounts, good eyepiece etc. But a keen beginning amateur can make a useful telescope with which he can see many clusters, nebulae and other interesting objects, using rather ordinary materials. It is for such an amateur that this book is meant.
We shall only consider Newtonian reflecting telescopes of large focal ratios using simple altazimuth or german equatorial mounts. We shall not discuss parabolizing of mirrors, the construction of Cassegrainians, the design of telescope drives etc. These pages would have served their purpose if they encourage beginners to construct simple telescopes for sight seeing the skies. If they make this important beginning it is likely that in course of time they will become serious amateurs for whom the more advanced books will be indispensible; especially recommended is the book by Texereau.
1.2 The Newtonian Reflector
The schematic of a Newtonian Reflector is shown in Fig. 1. It consists of three optical elements : the concave primary mirror, the diagonal or small flat mirror and the eyepiece or ocular, mounted in a tube. The primary mirror gathers light from astronomical objects and focuses it in its focal plane. The small flat mirror turns the image around through 900 so that
1.1 Scope
The aim of this book is to describe how a simple reflecting telescope can be made with the materials and facilities available in India. As the primary goal is to popularize telescope making, only the most primitive (but effective) constructions will be considered; the emphasis will be on the practical aspects of telescope making.
Some excellent books on this subject are in print (see, for example, the Bibliography). Yet few people have easy access to these books as they are shelved in specialist libraries or are expensive to buy. Moreover, the methosds discussed are beyond the means of even most middle class Indians. Perfection, indeed, demands the use of thick glass perspx blanks, well graded abrasives, good pitch, sturdy mounts, good eyepiece etc. But a keen beginning amateur can make a useful telescope with which he can see many clusters, nebulae and other interesting objects, using rather ordinary materials. It is for such an amateur that this book is meant.
We shall only consider Newtonian reflecting telescopes of large focal ratios using simple altazimuth or german equatorial mounts. We shall not discuss parabolizing of mirrors, the construction of Cassegrainians, the design of telescope drives etc. These pages would have served their purpose if they encourage beginners to construct simple telescopes for sight seeing the skies. If they make this important beginning it is likely that in course of time they will become serious amateurs for whom the more advanced books will be indispensible; especially recommended is the book by Texereau.
1.2 The Newtonian Reflector
The schematic of a Newtonian Reflector is shown in Fig. 1. It consists of three optical elements : the concave primary mirror, the diagonal or small flat mirror and the eyepiece or ocular, mounted in a tube. The primary mirror gathers light from astronomical objects and focuses it in its focal plane. The small flat mirror turns the image around through 900 so that
Bibliography: . Yet few people have easy access to these books as they are shelved in specialist libraries or are expensive to buy. Moreover, the methosds discussed are beyond the means of even most middle class Indians. Perfection, indeed, demands the use of thick glass perspx blanks, well graded abrasives, good pitch, sturdy mounts, good eyepiece etc. But a keen beginning amateur can make a useful telescope with which he can see many clusters, nebulae and other interesting objects, using rather ordinary materials. It is for such an amateur that this book is meant. We shall only consider Newtonian reflecting telescopes of large focal ratios using simple altazimuth or german equatorial mounts. We shall not discuss parabolizing of mirrors, the construction of Cassegrainians, the design of telescope drives etc. These pages would have served their purpose if they encourage beginners to construct simple telescopes for sight seeing the skies. If they make this important beginning it is likely that in course of time they will become serious amateurs for whom the more advanced books will be indispensible; especially recommended is the book by Texereau. 1.2 The Newtonian Reflector The schematic of a Newtonian Reflector is shown in Fig. 1. It consists of three optical elements : the concave primary mirror, the diagonal or small flat mirror and the eyepiece or ocular, mounted in a tube. The primary mirror gathers light from astronomical objects and focuses it in its focal plane. The small flat mirror turns the image around through 900 so that it can be viewed thorough the eyepiece. About 50% is of the labour involved in making a telescope of this type goes into grinding and polishing the primary mirror (Chapters 2 & 3 ). The other 50% is accounted for in the fabrication of the telescope tube, the mirror cell and the mount (Chapter 5 & 6). The eyepiece has to be bought readymade and the small diagonal is best obtained from a suitable source (e.g. the ABAA, the Association of Bangalore Amateur Astronomers). We now define certain terms and symbols that will be used later. Let D and F be the diameter (aperture) and focal length of the primary mirror and let fe be the focal length of the eyepiece. It can be shown that the angular magnifying power of the telescope , M is given by M= F/fe Thus the angular magnification can be increased by increasing the focal length of the primary or by decreasing the focal length of the eyepiece; the latter is what is normally done when increased magnification is required. Apart from its magnifying power a telescope is characterised by its light gathering power, which increases as the square of the diameter, D, and the resolving power which also increases with D. In brief, a telescope�s power to see faint objects increases greatly with the aperture D of the primary mirror. A term commonly used in the trade is the focal ratio, F/D, of the primary. When one talks about an f/8 (pronounsed �eff eight�) mirror, one means that F/D=8. For a given aperture, the larger the focal ratio the larger the focal length, the shallower the mirror surface. It is easier to make large f-ratio mirrors than to make small f-raito mirrors. We will in these pages restrict ourselves to mirrors with D