What is a telescope? It is an instrument that forms an image of a distant object, and it is thanks to a marvelous property of light rays - that they can be bent or "refracted" by a piece of glass, or reflected by a shiny surface - that telescopes are possible. With mirrors, or glass lenses, we can manipulate light rays in any way we wish, casting images of remote objects onto the eye's highly sensitive screen, the retina. Countless nerve endings then transmit the color and intensity responses from different parts of this image to the brain, which in turn decodes the information and presents the viewer with a mental image of the physical image produced by the telescope.
The observer's task The view produced by the telescope will be both larger (the magnification aspect) and brighter (the light-collecting aspect) than what is seen with the unaided eye. However, it must never be forgotten that the telescope's task is only to throw the view onto the retina; it does not, itself, "see" anything. Unscrambling the image is the observer's job. The most perfect image will be wasted if the observer does not put it to good use, and the act of observing is a highly personal one. Set two people down side by side to look at the same object and to draw what they see, and you will notice enormous differences between the results. Some of these differences will be due to fluency with the pencil and general artistic competence, but outside of these effects lie real differences in what is perceived. Some people are very sensitive to color differences, others to symmetry, and so on. Some may try hard to detect fine details while others could rind themselves more concerned with overall proportion.
It is true that some people have such defective eyesight that no reasonable comparison is possible. But the differences outlined above do not refer to the clarity, of vision. It is the way in which the image - as far as we know, the identical image - is used by different people that is so intriguing. The reason for mentioning this individuality of vision so early is to emphasize what a telescope cannot do. It can produce an image, but it cannot see; unless it is being used as a camera - and astronomical photography is a very important branch of amateur astronomy - it is only as good as the observer who is looking through the eyepiece. Nobody would expect someone who has just passed the driving test to get the most out of a high-performance automobile; on the other hand, someone with particular aptitude for driving will rapidly overtake (in both senses of the word!) another individual who is merely competent. It isn't easy, however, to convince someone buying a fine new telescope that he or she is going to have to work hard and persistently in order to get the best out of it. Advertisements have a habit of making astronomy look easier than it really is!
However, this chapter is about how telescopes work, so let us defer discussion of the observer until the appropriate place, and take a look at the very important considerations of magnification and, to begin with, aperture.
Light-collecting power The eye's light-collecting power is controlled by the pupil, a variable aperture that opens to its widest amount (about 8 mm in diameter) in dim light, and closes down to about 2 mm in sunlight. At night, then, we are observing with an 8-mm aperture "telescope." Even this modest instrument is sufficient to reveal some 2000 stars at any one time if the air is very clear, there are no nearby artificial lights, the Moon is absent, and the eye focuses sharply. However, there are innumerable stars which are too faint to be seen with the keenest unaided eye, and to detect them we have to use an aperture larger than 8 mm, so that more light can be collected and focused, and fainter stars therefore have a chance of energizing the nerve endings.
The area of a circle is proportional to the square of its diameter. Logic suggests, therefore, that a telescope with a light-collecting aperture of 16 mm will collect four times as much light as will the naked eye, making the same stars appear four times as bright, and revealing stars that are only a quarter as bright as the dimmest naked-eye stars. The same reasoning suggests that a telescope with an aperture of 100 mm - which is modest by amateur standards - will show the same star looking (100/8)2 or about 156 times as bright as when seen with the naked eye, or reveal stars 156 times fainter than those visible without any optical aid. This is an enormous increase in light-gathering power, and it is not surprising that even a relatively small telescope utterly transforms our view of the universe.
Does 156 times the light-gathering power mean that a 100-mm aperture telescope will reveal 156 times as many stars in the sky? To investigate this question, it is necessary to understand how star brightnesses are graded, and this is important enough to be worth examining straight away. The brightness of a star is called its magnitude, and the magnitude scale is based on an ancient system of measurement in which the brightest naked-eye stars were called "1st magnitude" and the faintest were called "6th magnitude" - so the higher the magnitude number, the fainter the star. This was, originally, a very approximate grading, but modern brightness-measuring devices known as photometers permit the brightness of a star to be measured to within a hundredth of a magnitude unit. One magnitude step now corresponds to a brightness ratio of 2.512 times. The reason for choosing this number is that five magnitudes correspond to a brightness difference of exactly 100 (or 2.512 to the power of 5).
Theoretically, a 100-mm aperture telescope will gain about 5.2 magnitudes over the unaided eye. Therefore, whereas the naked eye will normally see stars no fainter than the 6th magnitude (although some observers, under extraordinarily good conditions, have reported stars of magnitude 6.5 or even fainter), the eye and telescope combined should reach the 11th magnitude. On any one night, there are several million stars of the 11th magnitude and brighter above the horizon. The telescope will, therefore, reveal perhaps a thousand times as many stars as the naked eye, and not just 156 times as many: far more than you could hope to observe, individually, in a lifetime. Imagine a thousand separate skies full of stars, and that is the girl to your eye of a 100-mm telescope.
We have already stated that this aperture is a modest one by commercial standards. Most amateur-owned telescopes fall in the 75- to 250-mm range, but some are much larger. In any of these instruments, the view is at first bewildering: the crowds of stars cannot be related to anything seen with the naked eye. A small low-power telescope attached to the main instrument, known as a finder, is of great value in locating objects, and an instrument of any reasonable size needs one. The task of the finder is to negotiate between what the eye sees and what the telescope reveals. If it is too small, it won't show enough telescopic stars; if too large, it will defeat its own purpose and confuse the eye with too many. For most amateur instruments, an aperture of about 50 mm and a magnification of about eight times is ideal.
Magnification "How much does it magnify?" is the frequently heard inquiry when a telescope is mentioned. The answer to this is "It depends." With the exception of small hand telescopes and binoculars, which have a fixed magnification, the magnifying power of a telescope depends upon the eyepiece or ocular that is used with it. Most astronomical telescopes are equipped with several eyepieces, giving a range of magnification. A telescope requires several different magnifications or "powers" because the answer to the question "How much should it magnify?" is not always the same. At first sight, it might seem obvious that the highest possible power will give the most detailed view of an object. While this may be true if a finely marked planetary disk is being examined, it certainly is not true if we want to survey a scattered cluster of stars or a large, hazy nebula. A lower magnification shows a larger area of sky at one view than does a higher one, and for some purposes it is important to have a wide rather than a narrow view.
Field of view In astronomy, the diameter of the field of view (which is the width of sky that can be seen at one time, without moving the telescope) is reckoned in angular measure. An angle of 1° corresponds to twice the diameter of the Moon or Sun in the sky. Binocular fields of view are usually rated in terms of the number of meters that can be seen at a distance of 1000 meters. This is all right for terrestrial viewing, but astronomically the same field could show a few craters on the Moon or encompass a whole group of galaxies! This is why the astronomer normally uses angular units of distance - in other words, how far apart objects appear to be in terms of degrees, minutes of arc, and seconds of arc. These latter units are more correctly styled arcmin and arcsec, expressions which the writer finds particularly ugly, and the old-fashioned symbols ' arc and " arc will be used in this book to signify angular minutes and seconds respectively. One degree (1°) is equivalent to 60' arc, each one of which is equivalent to 60" arc. Although 1" arc may seem a tiny angle, it is one that can be divided or resolved by most good amateur telescopes. It corresponds, approximately, to the thickness of a human hair viewed from a distance of 2 1/2 meters.
As a rough guide, a magnification of about 50 times - which means that the apparent width or height of an object is 50 times larger than when viewed without the telescope - will reveal a circle of sky about 1° across. If it is doubled to 100 times (more conveniently written as x100), the diameter of the field of view is halved, to 1/2° or 30' arc, and the whole of the Moon's disk could just be included in one view. At x200, the diameter of the field of view is halved again, to about 15' arc, and so on. The diameter of the field of view depends only upon the design of eyepiece and the magnification it provides - it has nothing to do with the size or type of telescope. Some designs of eyepiece give fields of view noticeably wider than those quoted here, while others have a narrower inherent field of vision.
Eyepieces What, then, is an eyepiece? It is a powerful magnifying glass, consisting usually of at least two small lenses arranged close together inside a metal or plastic mount. (The lens closest to the eye is called the eye lens, while the one facing the telescope's upper end is known as the field lens, its function being to increase the useful field of view.) An eyepiece works by permitting the eye to be brought very close to the telescopic image formed by its large lens or mirror. The closer we are to something, the larger that thing appears to be. The normal human eye cannot focus - at least not without some discomfort - upon any object that is less than about 25 cm away. It is the function of a magnifying glass (the ordinary kind) to allow the eye to be brought much closer to whatever is being examined, so that it appears larger and more detailed. For instance, a "x5" magnifier makes an object appear as if it were being viewed from a distance of 5 cm rather than 25.
This value of 5 cm corresponds pretty closely to the focal length of the magnifying glass. If the focal length were only 10 mm, then this would be the effective distance from which the object were being viewed, equivalent to a magnification of x25 compared with the closest naked-eye view. Therefore, the shorter the focal length of the magnifying glass or eyepiece, the more powerful a magnifier it becomes.
Astronomical eyepieces are available in a considerable range of focal lengths. The longest are up to 50 mm or so, the shortest may be as little as 4 mm. There are many different optical designs, some optimizing critical viewing of fine detail, others offering a very wide field of view - some very expensive ones, such as the Plossl, claiming to achieve both simultaneously! Normally, a telescope comes ready equipped with standard eyepieces, which should be of reasonable quality. Designs such as the Achromatic Ramsden, Orthoscopic, Erfle, and Monocentric are all frequently encountered, but the design on its own means little if the eyepiece is poorly made, and a sky test by an expert is the only reliable guide if you are not happy with the performance of the telescope, because image faults can arise in the telecope's optical system as well as in the eyepiece. However, if you are using a reflecting telescope, and the image of a planet or bright star has a noticeable colored fringe around it, then the fault must lie in the eyepiece, which should be rejected.
Many smaller telescopes, that come with a standard set of eyepieces, have the magnifications given by these eyepieces already listed. If they are not, then it is necessary to know the focal length of the telescope because this determines the size of the image that the eyepiece is called upon to magnify. More will be said about focal length later in this chapter, but it can be pointed out here that the longer the focal length, the larger the image of a celestial object formed by the telescope; with a focal length of one meter, the image of the Sun or Moon is about 9 mm across, and telescopic focal lengths of from one to two meters are the most common. The magnification given by a particular eyepiece is determined by dividing its own focal length into that of the telescope.
High powers and the Barlow lens Suppose that you have a telescope of 1500 mm focal length, and wish to use a magnification of x300. The focal length of the eyepiece will therefore need to be 1500/300 or 5 mm. The eye lens of such an eyepiece is so small that some difficulty may be experienced in seeing anything through it at all because the eye must be placed in exactly the right position behind it - a location not achieved without some practice. Even then, tiny eyepieces are not comfortable to use.
Many experts therefore recommend the use of a Barlow lens for high-power work. This is a device which is placed a little way inside the focal point of the telescope, having the effect of increasing (usually doubling) the effective focal length. With a Barlow in place, the magnifying powers of all existing eyepieces are doubled, and so the more comfortable, longer-focus oculars can be used on all occasions.
Resolving power Magnification cannot be increased without limit. The Earth's flickering atmosphere rarely allows powers higher than about x350 or x400 to be used with any telescope. There is, however, another and more basic limit to the useful range of magnification, set by the amount of detail which the telescope itself imprints into the image.