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Tesseract, v. 2, issue 4, April 1937

tesseract    13

TELESCOPE TOPICS
By ARTHUR R. MINK

[DIAGRAM OF TELESCOPE]

MAGNIFICATION OF A TELESCOPE
The degree of magnification of any telescope depends on the focal lengths—the distance from the lens at which rays
passed thru a lens (or reflected by a mirror) meet--of the mirror M and the
eyepiece E. There are certain exceptions to this rule but they may be safely ignored in practice. The formula # 1
beside the drawing is used to compute the value of magnification. As seen in the
illustration MF equals the focal length, of the mirror and EF equals the focal length of the eyepiece. Taking as an example a six inch mirror with a focal length of 48 inches and a one inch eyepiece (eyepieces are referred to by their focal lengths while mirrors are designated by their diameters). The magnification
of this combination (X) is 43 diameters, that is, an object soon in this telescope will appear 43 times as great in every dimension as it would if viewed with the eye alone.
     The maximum magnification of which a
telescope is capable is limited by the fineness of the surface of the mirror. Thus, a poorly made mirror can hardly
stand the magnification of 20 diameters per inch. With a six inch mirror of this poor quality the maximum power will be
120 diameters. If an eyepiece of greater power is used the image will break down into a blur of light which is of no use
to the observer. At the other extreme, if the mirror is perfect, or nearly so, it may safely be pushed to 100 diameters per inch. In a six inch mirror with a perfect surface the maximum magnification is 600 diameters. In use, however, it is
unusual for an observer of any experience to crowd on power merely for power's sake.    Seldom are powers exceeding 50 per inch used in practical work because the greater the magnification the dimmer the
image. It is even theoretically possible to use all the light for magnification and thus leave none to illuminate the field of view. This cannot, however, be
accomplished practically. 
     In the case of usefulness of a telescope we count not so much the magnifying power as the light gathering power, be-
cause, as has just been said one can have too much magnification but for astronomical purposes he can never get too much light on the image. Considered in this
way the size of the mirror is of paramount importance. The formula # 2 is that used to compute the light gathering
power of a telescope. The number obtained always signifies that the telescope
gathers that many times as much light as the unaided eye.   Using this formula we find the following light gathering powers for mirrors of various sizes. For a one inch mirror the value is nine; for a six inch mirror it is 324; for a ten inch mirror, 900; for a 100 inch mirror, 90,000; and for the soon-to-be 200 inch mirror, 360,000 times the light gathering power of the human eye. And in these calculations the human eye is considered
as being at its best, which it seldom is.

Continued from page 12
excellent phantasy by R. H. Barlow. The story,  "The Night Ocean", is a superb word picture that equals many of the contemporary published works of this nature.
     "The Forbidden Room" by Duane f. Rimel has been reprinted by the "Scarlet
Cockerel".
     This trend speaks well for the future of fantasy and science fiction. It is to be hoped that more of the amateur publishers can be induced to print s-f.

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tesseract 13 TELESCOPE TOPICS By ARTHUR R. MINK [DIAGRAM OF TELESCOPE] MAGNIFICATION OF A TELESCOPE The degree of magnification of any telescope depends on the focal lengths—the distance from the lens at which rays passed thru a lens (or reflected by a mirror) meet--of the mirror M and the eyepiece E. There are certain exceptions to this rule but they may be safely ignored in practice. The formula # 1 beside the drawing is used to compute the value of magnification. As seen in the illustration MF equals the focal length, of the mirror and EF equals the focal length of the eyepiece. Taking as an example a six inch mirror with a focal length of 48 inches and a one inch eyepiece (eyepieces are referred to by their focal lengths while mirrors are designated by their diameters). The magnification of this combination (X) is 43 diameters, that is, an object soon in this telescope will appear 43 times as great in every dimension as it would if viewed with the eye alone. The maximum magnification of which a telescope is capable is limited by the fineness of the surface of the mirror. Thus, a poorly made mirror can hardly stand the magnification of 20 diameters per inch. With a six inch mirror of this poor quality the maximum power will be 120 diameters. If an eyepiece of greater power is used the image will break down into a blur of light which is of no use to the observer. At the other extreme, if the mirror is perfect, or nearly so, it may safely be pushed to 100 diameters per inch. In a six inch mirror with a perfect surface the maximum magnification is 600 diameters. In use, however, it is unusual for an observer of any experience to crowd on power merely for power's sake. Seldom are powers exceeding 50 per inch used in practical work because the greater the magnification the dimmer the image. It is even theoretically possible to use all the light for magnification and thus leave none to illuminate the field of view. This cannot, however, be accomplished practically. In the case of usefulness of a telescope we count not so much the magnifying power as the light gathering power, be- cause, as has just been said one can have too much magnification but for astronomical purposes he can never get too much light on the image. Considered in this way the size of the mirror is of paramount importance. The formula # 2 is that used to compute the light gathering power of a telescope. The number obtained always signifies that the telescope gathers that many times as much light as the unaided eye. Using this formula we find the following light gathering powers for mirrors of various sizes. For a one inch mirror the value is nine; for a six inch mirror it is 324; for a ten inch mirror, 900; for a 100 inch mirror, 90,000; and for the soon-to-be 200 inch mirror, 360,000 times the light gathering power of the human eye. And in these calculations the human eye is considered as being at its best, which it seldom is. Continued from page 12 excellent phantasy by R. H. Barlow. The story, "The Night Ocean", is a superb word picture that equals many of the contemporary published works of this nature. "The Forbidden Room" by Duane f. Rimel has been reprinted by the "Scarlet Cockerel". This trend speaks well for the future of fantasy and science fiction. It is to be hoped that more of the amateur publishers can be induced to print s-f.