21 April 2011 0 Comments

The Telescope

The telescope was one of the central instruments of what has been called the Scientific Revolution of the seventeenth century. It revealed hitherto unsuspected phenomena in the heavens and had a profound influence on the controversy between followers of the traditional geocentric astronomy and cosmology and those who favored the heliocentric system of Copernicus. It was the first extension of one of man’s senses, and demonstrated that ordinary observers could see things that the great Aristotle had not dreamed of. It therefore helped shift authority in the observation of nature from men to instruments. In short, it was the prototype of modern scientific instruments. But the telescope was not the invention of scientists; rather, it was the product of craftsmen. For that reason, much of its origin is inaccessible to us since craftsmen were by and large illiterate and therefore historically often invisible.
Although the magnifying and diminishing properties of convex and concave transparent objects was known in Antiquity, lenses as we know them were introduced in the West [1] at the end of the thirteenth century. Glass of reasonable quality had become relatively cheap and in the major glass-making centers of Venice and Florence techniques for grinding and polishing glass had reached a high state of development. Now one of the perennial problems faced by aging scholars could be solved. With age, the eye progressively loses its power to accommodate, that is to change its focus from faraway objects to nearby ones. This condition, known as presbyopia, becomes noticeable for most people in their forties, when they can no longer focus on letters held at a comfortable distance from the eye. Magnifying glasses became common in the thirteenth century, but these are cumbersome, especially when one is writing. Craftsmen in Venice began making small disks of glass, convex on both sides, that could be worn in a frame–spectacles. Because these little disks were shaped like lentils, they became known as “lentils of glass,” or (from the Latin) lenses. The earliest illustrations of spectacles date from about 1350, and spectacles soon came to be symbols of learning.

The telescope was unveiled in the Netherlands. In October 1608, the States General (the national government) in The Hague discussed the patent applications first of Hans Lipperhey of Middelburg, and then of Jacob Metius of Alkmaar, on a device for “seeing faraway things as though nearby.” It consisted of a convex and concave lens in a tube, and the combination magnified three or four times.[4] The gentlemen found the device too easy to copy to award the patent, but it voted a small award to Metius and employed Lipperhey to make several binocular versions, for which he was paid handsomely. It appears that another citizen of Middelburg, Sacharias Janssen had a telescope at about the same time but was at the Frankfurt Fair where he tried to sell it.

A second theoretical development came in 1672, when Isaac Newton published his celebrated paper on light and colors. Newton showed that white light is a mixture of colored light of different refrangibility: every color had its own degree of refraction. The result was that any curved lens would decompose white light into the colors of the spectrum, each of which comes to a focus at a different point on the optical axis. This effect, which became known as chromatic aberration, resulted in a central image of, e.g., a planet, being surrounded by circles of different colors. Newton had developed his theory of light several years before publishing his paper, when he had turned his mind to the improvement of the telescope, and he had despaired of ever ridding the objective of this defect. He therefore decided to try a mirror, but unlike his predecessors he was able to put his idea into practice. He cast a two-inch mirror blank of speculum metal (basically copper with some tin) and ground it into spherical curvature. He placed it in the bottom of a tube and caught the reflected rays on a 45� secondary mirror which reflected the image into a convex ocular lens outside the tube (see fig. 12). He sent this little instrument to the Royal Society, where it caused a sensation; it was the first working reflecting telescope. But the effort ended there. Others were unable to grind mirrors of regular curvature, and to add to the problem, the mirror tarnished and had to be repolished every few months, with the attending danger of damage to the curvature.

Magnification = Telescope Focal Length = 720 mm = 72 power
Eyepiece Focal Length 10 mm

If your telescope has a 720 mm focal length then an eye piece of 10 mm would give 72 power and 6 mm gives 120 power. Most telescopes come with a BARLOW lenses that doubles the power of each eyepiece so that the 10 mm eyepiece would double to 144X. BUT BEWARE! A 60 mm telescope can ONLY produce magnifications of 120X!!

ATMOSPHERIC LIMITS We observe the stars through a constantly moving ocean of air. The currents in the air blur and distort the images we see. It is a rare night that you can observe with powers over 200x, and Resolution is seldom better than 1 arcsec even with a very large telescope. When the wind is active or the temperatures unstable, 100X may be more than you can use effectively. Light Grasp still gets better with larger telescopes however.

Below is a table showing the performance of some popular sizes of telescopes. (The EYES column refers to Light Grasp compared to the Human eye.)

So you have a NEW TELESCOPE!! What a wonderful instrument for viewing the wonders of God’s universe. Most of the members of the Astronomy Club of Tulsa have shared your eagerness to venture forth into the cosmos. The sky is filled with many marvelous wonders but unfortunately there are no flashing signs to tell you where to point your telescope. We would like to share with you some of the lessons we have learned to help you enjoy the magnificence of the night sky.

LEARN THE NIGHT SKY The first step is to learn to name and locate the brightest stars and major constellations. Just as you need a road map to set off on a trip to unfamiliar places on earth, so a good star chart can be you guide through the night sky. There are many good books available which include star charts. I suggest a simple one to start with and then move up to more detailed charts later.
Telescope Diameter Eyes Magnitude Limit Resolution (arcsecs) Smallest Detail Visible on Moon in Miles
Eye 7 mm 1 6.0 60 225
60 mm 73 10.6 3.2 10.0
3 inch 118 11.2 1.5 5.5
4.25 inch 238 11.9 1.0 3.8
6 inch 474 12.7 0.8 3.0
8 inch 843 13.3 0.6 2.0

The reflecting telescope therefore remained a curiosity for decades. In second and third decades of the eighteenth century, however, the reflecting telescope became a reality in the hands of first James Hadley and then others. By the middle of the century, reflecting telescopes with primary mirrors up to six inches in diameter had been made. It was found that for large aperture ratios (the ratio of focal length of the primary to its aperture, as the f-ratio in modern cameras for instance), f/10 or more, the difference between spherical and paraboloidal mirrors was negligible in the performance of the telescope. In the second half of the eighteenth century, in the hands of James Short and then William Herschel, the reflecting telescope with parabolically ground mirrors came into its own.


Since the objective is the most important part of your telescope take special care to keep it safe from damage and clean. Store your telescope in a safe place where it will not fall over and break. If your telescope has a dust cap keep it on when not in use or tie a clean plastic bag around the objective to keep it clean. NEVER attempt to take the objective lens apart!!. Small specks of dust usually do not affect your seeing. If you must clean the objective use a soft CLEAN camel hair paint brush to whisk away small bits of debris. Finger prints may be removed by a soft cotton ball barely damp with rubbing alcohol. DO NOT RUB the objective!! Follow up by gently dabbing dry with several dry cotton balls. To avoid liquid running between the lenses, point refractor (lenses) telescopes front face straight

How It Works

Light enters through the front objective lens and then passes through the eyepiece lens before reaching your eye.
Refracting telescopes depend on one amazing fact. As light passes through glass, it slows down. Slowing down a light beam makes it bend. Why? Imagine you’re pulling a wagon along a sidewalk, when the wheels on one side slip off into the grass. The wheels turn slower in the grass than they do on the sidewalk, and the wagon moves toward the grass. In the same way, when a light beam passes through a glass lens inside a telescope, it moves toward the lens. When the light beam comes out the other side, it’s bent!
The shape of the lens means light near the top of the lens is bent down and light near the bottom of the lens is bent up. Somewhere inside the tube the light beams cross, but before they can spread out again the eyepiece lens bends the light beams again and sends them to the eye.
Because the light beams cross, the image ends up upside-down. This doesn’t matter much when you’re looking at Mars or the Moon (remember there’s no real up or down in space), but refracting telescopes used to see objects here on Earth often have another set of lenses to flip the image right-side up again.
Refracting telescopes are simpler than reflecting telescopes, but they have an important limitation. Remember that the light passing through the glass lens gets bent. It turns out that different colours are bent different amounts, and that causes the light to become unfocused. Isaac Newton solved this problem by replacing the lenses with mirrors.
When light hits a mirror, it doesn’t bend. Instead, it bounces off. Just like a ball bouncing off a wall, a light beam comes off a mirror the same way it comes in. In other words, the angle in equals the angle out. And that rule is true for all the light, no matter its colour.
The primary mirror in a reflecting telescope is curved just the right amount to bounce all the light onto the secondary mirror. From there, the light passes through the eyepiece lens, which bends the light into the eye.

Telescope Facts
Probably the world’s most famous telescope today is the Hubble Space Telescope. It is a reflecting telescope that orbits 600 kilometres above the Earth. Because it is above the Earth’s atmosphere, Hubble is able to see faraway objects more clearly than any telescope in history.

The Very Large Array is a radio astronomy observatory in New Mexico.
Telescopes aren’t limited to just the light we can see. Invisible kinds of light, like radio waves and x-rays, are important to astronomers, too. Every time astronomers use a new kind of light to look at the universe, they make new and unexpected discoveries. When scientists first used x-rays to look at the sky, they discovered black holes. When they used radio waves, they found the light left over from the birth of the universe, the event called the Big Bang.
X-ray telescopes are some of the strangest devices ever invented. Because x-rays are so powerful, they’d pass right through any mirror they hit straight on. To capture x-rays, scientists use the same effect you use when you skip a rock across a pond. The x-rays come in at an angle, hitting the mirror with a glancing blow that focuses them onto the detector. To increase the number of x-rays collected, the telescopes are built like nested barrels, with the insides of the barrels all covered in mirrors.

by sara

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