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. This is known as the resolving power of the telescope. The telescopic image is effectively made up of a great number of tiny units, somewhat analogous to the dots in a half-tone photograph. Viewed from a normal distance, the picture appears to be continuous. Examined through a magnifying glass, however, it breaks down into a pattern of individually meaningless large and small dots, and the contours of the photograph are lost.
The effect of very high magnification is not to break up the image in this dramatic way, but to lose contrast. Overmagnifying does not produce new detail on a planet, but merely spreads and dims that which is already visible with a lower power. Having said all this, however, it is still a fact that the atmosphere almost always sets a magnifying limit below this image-degradation limit. In other words, a telescope set up in space could habitually profit more from high magnifications than those regularly used on the Earth's surface. Those rare nights of superb seeing will, frustratingly, tend to support this view!
To fix ideas, magnifications of about twice the aperture expressed in millimeters are necessary to give ready visibility to the resolved detail in a telescopic image. If the atmosphere normally sets a limit of about x400 to what can usefully be employed, it is evident that large instruments are frequently being used below their true potential, while smaller instruments are more likely to give optimum performance. This does not, however, mean that a large instrument will reveal no more detail than a small one, for its extra light-grasp will always be a powerful factor, and even on the worst nights there are moments of relative steadiness when fine detail jumps into view.
Test double stars The night sky contains a great many convenient resolution tests. These are binary, stars: pairs of suns revolving around each other, separated by distances from millions to thousands of millions of kilometers, and taking from years to centuries to complete a revolution. In the sky, some appear to be several seconds of arc apart, while others may be separated by only a fraction of one second. The stars themselves are so distant from the Earth that, despite their large size, they appear only as points of light far smaller than the resolving power of an ordinary telescope.
Point a telescope of small aperture towards a binary star whose components are separated by an angle smaller than the telescope's resolving limit. Then, no matter how high a magnification is used, the object will appear as a single star, though possibly of elliptical outline if it is almost resolved. Now examine the same double star with a telescope of much larger aperture, whose resolving power exceeds the angular separation of the binary. The object should now be seen as a close pair, even though the magnification may be the same!
Aperture and theoretical resolving power go hand in hand, and can be enshrined in a table as shown here:
Aperture of telescope (mm)
Resolving power (" arc)
Having done so, it should be added straight away that such a table can be highly misleading! This does not prevent manufacturers from stating that their particular model will resolve to the limit -- often referred to as the Dawes limit. It may well do so in the laboratory, looking at an artificial double star. But there are plenty of double stars in the sky, wider than the theoretical resolution limit, that it has no hope of resolving. To begin with, the stars must be of approximately equal brightness, or else the fainter one will be obliterated by the brighter. Furthermore, even if they are of similar brightness, they may just be of the optimum magnitude for the particular aperture being used: too faint, and the eye will have difficulty in seeing them sufficiently clearly; too bright, and the individual stars will seem to enlarge and blur together. We shall have much more to say about observing double stars in Chapter 9. For the moment, the best advice is not to be beguiled by tables of "theoretical" performance because they can lead to quite unjustified disappointment.
Refracting telescopes The first telescopes, which appeared back in the 17th century, were of the refracting kind, and there are many people today who say that a good refractor is the best of all telescopes. The refractor uses an object glass or objective, which consists of a pair of lenses mounted in a cell at the upper end of the tube, to collect light and focus the image. The focal point of the objective (in other words, the place where the image is formed) is at the lower end of the tube. Here, the eyepiece fits into the drawtube, that can be moved in and out slowly with a knob to permit sharp focusing. The focal length of the object glass is, as would be expected, about equal to the length of the telescope tube.
Achromatism The performance of any telescope depends critically upon two things: the steadiness of the mounting that carries the tube, and the quality of its image-forming components. We shall have more to say about mountings in due course. In a refractor, then, the object glass must be singled out for particular attention. First of all, it must be achromatic, a word literally meaning "without color." A single glass lens certainly can form an image if it is curved to a suitable cross-section, but this image will be of poor quality because the light entering the lens from the object being studied will emerge as a rainbow-colored blur. The lens not only produces an image but also acts as a prism, splitting the original white light up into its component colors. In fact, the result is a string of images of different color, none of which can be focused without the other, out-of-focus images, interfering. This trouble can be almost entirely overcome by combining the first lens (made of light crown glass) with a second (made of dense flint glass), the pair having been carefully computed so that the color spread produced by the first lens is matched by a color convergence supplied by the second. The result is, or should be, an image free from false color, preserving the tones of the original.
It is a fact that no pair of lenses can produce perfect achromatism, and even a first-class refractor will show a very faint violet halo around a very brilliant object, such as the planet Venus viewed in a dark sky, or perhaps the edge of the Moon. This effect is referred to as secondary spectrum. The obtrusiveness or otherwise of this halo depends partly upon the observer's color vision, but principally upon the aperture and focal length of the objective. The longer its focal length in relation to its aperture (in other words, the greater its focal ratio), the fainter the secondary spectrum becomes. Most commercial refractors, in the 75-mm to 150-mm aperture range, have a focal ratio of about 15, written f/15. This gives a compromise between unwieldy tube length and color-free performance. On the other hand, some modern types of glass have made it possible to produce object glasses which work acceptably at f/10: if you see a fluorite objective advertised, it should have minimal secondary spectrum because it is made from a special and rare type of crown glass -- but it will be very expensive. At least one manufacturer in the United States has recently begun offering object glasses containing three individual lenses, known as triplets, and these too should have reduced secondary spectrum.
However, for most visual work with normal apertures, secondary spectrum is quite unimportant. It becomes serious with increasing aperture, out of the amateur range -- but even the world's largest refractor, the 1-meter (40-inch) at Yerkes Observatory, has been in constant and successful use since 1897! It is certainly a nuisance in photography because photographic emulsions have more of an appetite for violet light than does the human eye -- and in this connection it may be worth remarking that violet sensitivity does vary from one individual to another.
Virtues and drawbacks of the refractor Why, therefore, do so many people have such a deep regard for the refractor, if it has this inherent although certainly not insuperable drawback? Simply because a good refractor can most easily approach the most perfect clarity of definition possible with any telescope. The writer must choose his words very carefully here because the "refractor versus reflector" controversy is a notorious minefield; and he is certainly not saying that the refractor is the best kind of telescope, full stop! Such a judgment would be absurd. For example, not all astronomical work demands exquisite definition of image. But, for the work that does -- which means that we wish to distinguish between those instruments defining adequately from those defining superbly -- there is a good chance that the refractor will out-perform another type of telescope of similar aperture. Three reasons are offered:
1. Optical errors on the surface of a lens have less effect on the final image quality than would the same errors on the surface of a mirror: approximately a quarter as much.
2. The reflecting surfaces of mirrors, particularly as they age, tend to scatter more light around the image than do the glass surfaces of a lens. Therefore, the contrast of the image may be better.
3. A lens is much less sensitive to errors of adjustment than is a mirror.
What these points are saying is this: although it might be hard to distinguish the performance of typical refractors and reflectors under laboratory conditions, what matters is the way they work under the sky. In practice, the refractor may well prove more durable and consistent, able to suffer more mishandling and degradation without its optical performance being compromised.
However, the refractor has a serious drawback: its bulk. At f/15, even a 100-mm refractor can reach an adult's shoulder. Because the eyepiece is at the lower end of the tube, the telescope has to be mounted well above the ground if the eyepiece is to be brought to a comfortable height. Furthermore, for viewing objects at an altitude of more than about 60° above the horizon, an attachment known as a zenith prism, which turns the ocular at right-angles to the drawtube, is practically essential. A zenith prism can be a nuisance because it reverses the image top for bottom. It should be noted, in passing, that most astronomical telescopes completely reverse the image anyway, so that what is up in the sky appears down, and what is right appears left. But this can be overcome simply by turning the map or chart, if one is being used, through 180°. A one-way reversal produced by a zenith prism cannot be accommodated in this way, and the only course of action is to look through the back of the chart, a not always tenable procedure.
Not only is a long tube awkward: it is also difficult to mount steadily. This is a most important consideration, for the excellence of a telescope's mounting is at least as important as the quality of its optics. A refractor is, undeniably, difficult to mount. Whether this tips the final judgment away from it must depend upon the individual observer and the particular telescope; what must be said here is that refractors of about 150 mm aperture and over should be mounted on a solid pillar in an observatory rather than on a wooden or even steel tripod. If both portability and large aperture are considered necessary, the refractor does not come into the reckoning.
Reflecting telescopes A mirror with a slightly concave surface can reflect light to form an image in a way analogous to the refractive formation of a lens. In fact, it can do the job rather better because mirrors do not disperse light into colors -- they are perfectly achromatic. This is a great advantage enjoyed by reflecting telescopes. Another benefit is that the mirror does not need to be made of superfine optical glass because the aluminum coating is applied to the front rather than to the back of the glass, the reason being that passage through a thick glass disk would introduce unwanted color into the image.
Ordinary thick plate glass can be used for making telescope mirrors, but most are made of low-expansion glass, such as Pyrex, or even from pure silica (fused quartz), which has practically zero expansion with temperature change. This can be important because the definition of a slightly expanding or contracting mirror can be ruined. A reflecting surface is also particularly sensitive to flexure, and mirror disks are commonly made very thick, with a diameter: thickness ratio of 6 or 8. If the mirror is large, however, such a cross-section results in an expensive and weighty disk, so the modern tendency is to use thinner glass and to take care to mount it in a cell where carefully designed supports preserve its optical form. Telescope advertisements often refer to 9-, 18-, or even 27-point flotation systems, but a typical thick 150-mm aperture mirror can be mounted perfectly well on just three locating points.
Other agreeable advantages accrue from using a mirror as the light-collector and image-former. It is, for instance, easy to obtain mirror disks up to 400 mm or 500 mm across. A number of amateurs have ground and polished their own mirrors of this order of aperture. Therefore, light-collecting power is there in plenty. Furthermore, because a mirror is perfectly achromatic, there is no need to make reflecting telescopes with the large focal ratios necessary for refractors. Many reflectors work in the range of f/5 to f/6. At f/5 you can have three times the aperture (hence, nine times the light-collecting power) of a typical refractor, for the same tube length. This is of enormous benefit when the telescope mounting is considered.
Newtonian reflectors All reflecting telescopes use a concave mirror, situated at the lower end of an open-mouthed tube, to reflect and focus the light back up the tube. What happens then depends upon the design. In the common Newtonian form, a small, optically plane mirror of rectangular or elliptical outline intercepts the reflected light and reflects it through the side of the tube to where the eyepiece is situated. This mirror, which is referred to as the diagonal or flat (not as the "secondary," which is something else altogether), should preferably be elliptical, so that its outline, when viewed at an angle of 45°, is circular. For most amateur purposes the Newtonian is the most satisfactory, as well as the cheapest, reflecting telescope. Not the least of its advantages is that the eyepiece is near the upper end of the tube, and more or less equally convenient for the observer regardless of the altitude of the object being observed.
The cross-sectional shape of the concave mirror's surface is not part of a sphere, but part of a paraboloid; or at least it should be. A spherical concave mirror will not give a perfect image unless the focal ratio is unusually large -- about f/12 with 150 mm aperture, and greater with larger apertures. Effectively, a parabolic mirror is a spherical mirror with the center deepened very slightly. In amateur sizes, this deepening can be measured only in terms of wavelengths of light: the difference in depth at the center of spherical and parabolic f/8 mirrors of 150 mm aperture is only 0.0003 mm, or 0.6 of the wavelength of yellow light! Yet this minute amount of glass makes a huge difference to the quality of the final image. Controlling such figuring in commercial production is a highly skilled business, and by no means all mirrors are as good as they should be. Typically, makers offer different grades of optical finish, depending upon how closely the surface approaches theoretical perfection.
Mirrors are usually graded in terms of wavelength error. A sample might be quoted as "1/8th-wave quality." This, by itself, does not mean anything very definite. It could mean that some parts of the surface are 1/8th of a wavelength of light deeper than the theoretical paraboloid, while others are up to 1/8 of a wave higher. In other words, the total "ripple"on the surface is up to 1/4 of a wave. This would be a poor quality component. Alternatively, it could refer to a total ripple of 1/8th of a wave, or a maximum deviation of 1/16th of a wave from the paraboloid. Such a mirror should approach theoretical perfection of definition.
But a wavelength of what? Red, yellow, or blue light? Deep red light has almost twice the wavelength of violet light; therefore, a "red light" error will be physically much more objectionable than a "blue light" error. The eye is most sensitive to yellow light, roughly mid-way in wavelength between red and blue, and it is errors measured at this wavelength (about 550 nanometers, or 0.00055 mm) that are significant.
The optical quality of the flat is often ignored, or taken for granted. But its optical tolerance is just as critical as that of the main mirror. Therefore, this needs to be known too.
What we are saying is that there is plenty of scope for sharp practice in the manufacture and sale of Newtonian reflectors. Never judge the optical worth of an instrument -- any instrument -- by a maker's claims. A star test is the only way of establishing its excellence or otherwise, and the section on page 30 is devoted to this important procedure.
Cassegrain-type telescopes The basic Cassegrain design is rarely seen in amateur sizes because very few manufacturers offer this type of instrument for sale. Large professional telescopes, however, almost always belong to the Cassegrain family. The Newtonian diagonal is replaced by a small, convex, secondary mirror that reflects the converging beam from the primary mirror back down the tube. This secondary effectively increases the focal length of the primary without necessitating a corresponding increase in tube length. The returned beam either passes through a small hole cut in the center of the primary mirror, or is reflected by a Newtonian-style plane mirror through a hole in the side of the tube, somewhere near the lower end.
The effective focal length of a typical Cassegrain may be between f/10 and f/30 -- separate secondaries can be supplied to give different amplifications. A long focal length, or large focal ratio, has some definite advantages. If the telescope is being used photographically, the image scale is increased: a focal length of 1000 mm produces a 9-mm lunar image, but this expands to 27 mm if the focal length is 3000 mm. A 3-meter tube would be extremely cumbersome, and the benefits of compressing this focal length into a much shorter tube are obvious. Visually, too, a long focal length means that high magnifications can be obtained without using very short-focus eyepieces, or a Barlow lens.
The larger the focal ratio of an object glass or mirror, the better is an eyepiece likely to perform. It must be remembered that eyepieces, like object glasses, have to be made achromatic by the introduction of separate lenses, and that their residual color is tested more severely when they are being used in conjunction with an object glass or mirror of short focal length. An f/5 Newtonian makes much greater demands upon eyepiece perfection than does an f/10 or f/15 Cassegrain or refractor.
The following table may be found useful. It sets out the eyepiece focal length necessary to produce a given magnification with telescopes of various focal lengths.
The advantage of a Cassegrain-type telescope for high-power work is obvious. On the other hand, an f/20 system would not be convenient for low-power, wide-field work. The diameter of the drawtube into which the eyepiece fits is rarely more than 30 mm; in the United States, one common "standard" eyepiece fitting takes ocular mounts that are exactly 1 1/4 inches across. Used at the focus of a telescope with a focal length of 3000 mm, this width is only a little greater than the diameter of the lunar image (1/2°), so that a view of the sky encompassing a greater diameter than this would be unobtainable.
Catadioptric or "lens and mirror" telescopes The handy compactness of the Cassegrain design has sent manufacturers seeking ways of combining a short, portable tube with an effective focal length that is not too long. The result has been the family of squat-tubed instruments, familiar from advertisements in astronomical and scientific magazines, that at first sight look utterly unlike a telescope at all. The tube of a typical 200-mm aperture catadioptric telescope is less than 450 mm long, as opposed to a probable length of 800 mm for a true, Cassegrain of the same aperture, or perhaps three meters for a refractor! How is this extreme compactness possible -- and what are the benefits to the observer?
It would be possible, both in theory and in practice, to make an ordinary Cassegrain telescope with a tube length of only twice its aperture. It would, however, be very expensive to figure a mirror of such short focal length to the correct parabolic shape. Furthermore, the mirror, once finished, would be extremely sensitive to maladjustment. If it were even slightly shifted from its perfect position, the image quality would be ruined -- parabolic mirrors of very small focal ratio need to be adjusted very critically indeed. It is true that the recent generation of huge observatory telescopes are Cassegrains with primaries of very small focal ratio -- about f/2.5 -- but there is a world of difference between making a single expensive instrument and mass-producing telescopes for the amateur market.
Schmidt and Maksutov systems The seed of the catadioptric idea was planted in the mind of the Estonian astronomical optician Bernhard Schmidt half a century ago. He realized that a spherical rather than a parabolic mirror could give a perfect image if the light first passed through a very thin, almost flat plate of glass. This correcting plate has so slight a curve or "figure" on it that, to a casual glance, it looks just like an ordinary piece of glass. Schmidt was interested in the wide-field photographic possibilities of this device, and the so-called Schmidt camera was born. Much later, it was realized that the same principle could be used for correcting a visual telescope, using a spherical rather than a parabolic primary mirror. There are several advantages:
1. We have already seen that the surface of a lens does not have to be figured as accurately as that of a mirror. Because a spherical mirror can be mass-produced quite easily, it may be cheaper to "figure" it by making a corrector plate.
2. The corrector plate/spherical mirror combination is not so sensitive to misalignment as is a single, short-focus parabolic mirror.
3. The small secondary mirror of the Cassegrain-type system can be mounted directly on the corrector plate, obviating the need for a supporting arm.
4. By sealing the contents of the tube from the outside air, the corrector plate protects the optical surfaces from dust and tarnish.
5. It is possible to design a corrector plate/mirror combination that; gives a wider field of good definition than could an ordinary Cassegrain. However, this is usually significant only with photographic work.
This was the genesis of the Schmidt-Cassegrain telescope, of which the best-known, at the present time, is the Celestron. There is also a rather similar system, in which a very strongly curved lens or shell replaces the almost flat corrector plate. This is the Maksutov-Cassegrain, of which the best-known example is the Questar. In some Maksutov designs, the center of the shell is converted directly into the secondary mirror by coating it with a circle of aluminum. Although there are considerable technical differences between them, both systems embody the same principle of a relatively long focal length compressed into a short tube. For example, the Celestron 8, of 200 mm aperture, has an effective focal ratio of 10, so that a 2000-mm focal length is compressed into a tube only 450 mm long.
Advantages of the catadioptric Even though it contains a refracting element, whether plate or shell, a well-made catadioptric telescope gives an image that is practically as achromatic as a pure reflector. Also, the use of the Cassegrain system adds another dimension to the possibilities of this telescope because, by altering the distance between the primary and secondary mirrors, it can be made to focus on objects near at hand. An ordinary refractor or reflector can also do this, but only by pulling the eyepiece a long way out beyond its normal setting for "infinity." In the case of the Cassegrain, however, the position of the final focus is very sensitive to changes in the inter-mirror distance, and an increase in this distance of only a centimeter or so will allow the observer to focus on a bird in a nearby tree instead of on the Moon! This is accomplished by turning a knob that moves the primary mirror back and forth inside the tube.
As achromatic as a reflector, fantastically compact, and highly versatile...is the catadioptric telescope the ultimate choice for the observer? Before the case can be fairly examined, we need to look at the ways in which the different types of telescopes can be mounted.
Testing a telescope Only someone with a certain amount of experience can make a really worthwhile judgment of a telescope's optical quality. You are, therefore, strongly advised to seek the judgment of such an expert, perhaps through an astronomer friend, or through the local astronomical society. However, should you find yourself without any such aid, the following hints may help.
The telescope's optical components must be well aligned. A catadioptric telescope should have been checked before it left the factory, and should need no subsequent adjustment. If it seems faulty in this respect (in other words, the image is flared on one side, or otherwise unsymmetrical), the instrument should be returned to the agent or manufacturer.
A refractor, too, should already be well aligned because its optical adjustments are the most permanent of any telescope. If the image is showing one-sided flare, or the out-of-focus star disks are elliptical, it will be necessary to adjust the object glass by using the three sets of adjusting screws set around the cell. Very carefully screw one set in or out by a turn or two, and observe the result. Proceed by a process of trial and error until the out-of-focus star disks, as described below, are as circular as they will go. If no amount of adjustment can bring them truly circular, there is something wrong with the object glass.
A Newtonian reflector is quite likely to be out of adjustment when it arrives from the manufacturer, or if it has been purchased secondhand. New instruments usually come with instructions on the method of alignment, but a summary will be given here:
1. Remove the eyepiece from the drawtube and rack it well out. Place a cardboard disk over the open end of the drawtube, perforated with a central hole about 5 mm across.
2. Uncap the mirrors and direct the telescope at some bright surface -- a white wall, or the daytime sky.
3. Look through the perforation. The outline of the flat will be seen. Ensure that its outline appears concentric with the drawtube -- in other words, that the axis of the drawtube passes through the apparent center of the flat. There should be an adjustment device on the flat's mounting for this purpose.
4. Now adjust the tilt of the flat so that the image of the primary mirror appears central within it.
5. Study the image of the primary mirror. The black outline of the flat will be seen, as well as the outline of the flat mounting (sometimes a single strut, more often three or four radiating vanes). Adjust the primary mirror's screws until this outline of the flat appears central in the reflection of the primary.
6. Examine the expanded star disks for symmetry. Any slight errors can be remedied by very delicate adjustment of the screws supporting the primary mirror.
Having brought the optical components to good adjustment, you can begin testing.
There is only one satisfactory test object -- a star. For apertures of up to about 150 mm, a 2nd or 3rd magnitude star is ideal -- a magnitude fainter, perhaps, for a 300 mm telescope. A very bright star often shows so much atmospheric flare that the test sensitivity is lowered. Don't look at the Moon; almost any telescope will make it look so marvelous that you will be tempted to purchase the instrument on the spot! The planet Jupiter is a good test if the atmosphere is steady and you are an experienced observer. But a star test is useful on almost any night, and even someone who has never looked through a telescope before can make some sort of judgment.
When making a star test, you do not look at the focused star. Turn the focusing knob so that a high-power eyepiece (x200-x300, depending upon the aperture) moves just a few millimeters inside and outside the position of best focus. The star image will expand into a small disk. What you must now do is compare the appearance of the two disks. The more alike they are, the better the telescope. In a perfect telescope, they will be indistinguishable. A reflector will reveal a little black spot at the center of both intra- and extra-focal disks: this is the outline of the flat or secondary. A refractor has no central spot, but will produce a faintly-colored fringe -- the remnants of the chromatic aberration. In a well-corrected object glass the intra-focal disk will have a reddish-purple fringe, while the extra-focal disk will be edged with bluish-green. Use a white test star rather than a red one, to show up these color differences.
It is easier for the amateur to judge a bad telescope than a good one. A certain amount of difference between the two images can be tolerated because the test is an extremely sensitive one. But if the two images are clearly different (one may have a hard, bright edge, while the other has a soft blurred outline, for example), then the optical system is poor, and probably not worth bothering with.
It may seem strange that the focused star image is not used as a test. In theory, a perfect telescope will form an image that consists of a tiny disk (equal in diameter to the resolving limit) surrounded by two or three very faint rings. This disk has a sharp edge, and with a very high magnification resembles a minute planet. However, the air is usually so unsteady that this disk is visible only fleetingly, and is smeared and blurred with false rays. Bad seeing has a minimal effect on the out-of-focus test, which is why it is so convenient a resource for the practical amateur.
The expanded-disk test becomes much less sensitive if they are expanded too much. The writer recently came across commercial literature advising the tester to move the eyepiece so far away from the focus that the disks had a width equal to half the field of view! Such a test would conceal rather than reveal errors. Bringing the star up to about half the apparent diameter of the planet Jupiter is likely to give the best results, at least to begin with, although an experienced eye may prefer to expand it less than this.
Mountings for telescopes Concern over the optical quality of a telescope must not lead you into the trap of believing that this is the only important feature of a new instrument. No matter how excellent the image, observation will be nothing but a torment if the instrument wobbles or vibrates every time the observer puts an eye to the eyepiece, or if the image is constantly drifting out of the field of view and cannot easily be recovered. In fact, astronomical work will soon languish altogether!
Whether a perfectly mounted telescope of rather poor optical quality will enjoy any better use is a moot point. However, there is certainly no point in attending only to the optical performance of the instrument when making a selection, because this feature can be appreciated truly only if the telescope is steady and well-mounted.
Any mounting must offer the following features:
1. Rigidity of the tube at any angle of elevation. Ideally the image should not shake at all when the eyepiece focusing knob is turned. If it does move slightly, any vibration should damp down the moment the knob is released.
2. Ease of following the motion of an object, caused by the Earth's spin. A star or planet appears to go round the sky once in twenty-four hours, or at a rate of 1° in four minutes. The field of view of a x250 eyepiece is likely to have a diameter of only about 12' arc, which means that a celestial object will pass right across the view in less than fifty seconds. Extremely sensitive and responsive tracking is, therefore, essential.
To some extent, these two requirements go together, for it is impossible to give a telescope a satisfactory fine motion if it is not steady to start with. The difficulty is not so much in holding the instrument firmly, as in combining steadiness with smooth motion. Mechanically, the most rigid mounting for a tube is to secure it at both ends; the least rigid is to try to hold it at one point only. Yet the demand that the tube be freely moveable, so as to be able to aim at all parts of the sky, means that the telescope must be pivoted around one point.
Altazimuth mountings Most mountings contain two axes, at right angles to each other. The simplest type is known as the altazimuth because one axis permits the tube to be moved in azimuth (horizontally, parallel to the horizon), while the other allows vertical motion in altitude. Mechanically, altazimuths have the advantage of simplicity, stability, and compactness. In recent years, indeed, the altazimuth has made something of a come-back in commercial circles, and the box-like Dobsonian is now popular for short-focus Newtonian reflectors. These are typically low-power, wide-field instruments used for observing clusters, nebulae, and galaxies, and so do not suffer from the altazimuth's main drawback: that it cannot easily be fitted with an automatic drive.
The author finds the rise of the altazimuth particularly pleasing because it confirms what he has maintained for many year -- a mounting need not be finely machined and elaborately finished in order to be effective. The Dobsonian depends for its success on three important factors:
1. The weight of the telescope is "inboard" as much as possible. The less the overhang, the more inherently stable the mounting will be. This is acknowledged by the fact that some of the very latest large telescopes being built for professional use are on altazimuth mountings, with computerized drives to give an "equatorial" motion to the tube.
2. The bearing surfaces are large, being formed from almost friction-free Teflon. Large-diameter bearings encourage stability, and "frictionless" motion leads to minimum backlash. The tube, in a good Dobsonian mount, should respond to finger pressure to move it to a new position, and remain in that position when the pressure is relaxed. Note that this performance cannot be judged by looking at the tube in a showroom, but only by observing a celestial object in its natural surroundings!
3. The "minimum mounting" concept means that larger optics can be purchased for the same total price. With a mirror of 500 mm aperture, one's view of the deep recesses of the sky is transformed when compared with the performance of even a 300-mm reflector.
Equatorial mountings Most commercially-available mountings, however, are of the equatorial form. An equatorial mounting directly counteracts the Earth's axial spin, which is what makes celestial objects follow their daily course around the sky. If one axis of the mounting is tilted until it is exactly parallel to the Earth's and the telescope is made to rotate around this axis (the polar axis) once a day, in a direction opposite to the Earth's spin, then the telescope tube will remain pointing in the same direction in space. The declination axis joins the telescope to the polar axis, and is adjusted only during the initial location of the celestial object. Thereafter, rotation of the polar axis keeps the object within the telescopic field -- or should do, if the mounting is accurately set up.
Types of equatorial There are several ways of arranging these axes, and each arrangement results in a mounting of different character. By far the most common is the German. The two 'axes form a T, the vertical member forming the polar axis and the horizontal forming the declination axis. The declination axis carries the telescope tube at one end, and a counterweight at the other. Due to the necessity for a counterweight, the German is not a particularly good design mechanically; but it is fairly compact, and is suitable for both reflectors and refractors. Practically all Newtonians and refractors are sold on German stands.
The Fork mounting, as its name implies, carries a large fork at the upper end of the polar axis, inside which the tube can swing. Its advantages over the German are that the telescope is attached to the mounting on both sides of the tube, and no counterweight is needed. On the other hand, it is suitable only for short-tubed telescopes because a long tube could not swing through the fork and point to objects near the celestial poles. Not surprisingly, the Fork mounting is favored by the makers of catadioptric instruments.
Many other types of equatorial mounting exist, but they are unlikely to be encountered in the mass market.
Drives and alignment The telescope mounting usually incorporates an electric drive for rotating the polar axis. A synchronous motor, that keeps in exact step with the mains frequency, is used. This frequency is not perfectly stable on an hour-to-hour basis because it can vary by a few percent according to the change of power demand, and the image may slowly drift in the field of view. This difficulty can be overcome by using a hand-controlled oscillator that supplies a steady frequency to the motor. Such oscillators can also be used to make the motor run fast or slow when centering an object in the field of view.
Some sort of slow motion, that can be applied to both axes when making fine adjustments to the direction of the telescope under high magnification, is practically essential. Remember that a movement of just l' arc may make a noticeable difference to the position of an object in the field of view. Slow motions must, therefore, be smooth and responsive, if the observer is to have confident control over the instrument. Cheaper telescopes often fail on this point, either lacking slow motions altogether or having ones that are much too coarse. Another characteristic of cheaply-made instruments is the poor performance of the motor drive. This drive is communicated to the polar axis via either a worm and wheel or a spur gear, and defective machining or assembly of these turning units will result in non-uniform rotation of the polar axis. This is a particularly serious defect if any sort of long-exposure photography is to be undertaken because the resulting star images will be ellipses, or even lines, if the tracking has not been sufficiently accurate.
The alignment of an equatorial mounting, to ensure that its polar axis is parallel to that of the Earth, is very important if the instrument is to be used to its full potential. Misalignment means that a celestial object will steadily drift through the field of view, even if the motor drive is functioning perfectly. The ideal solution is a permanently mounted instrument that can gradually be brought to good adjustment. The need to realign a portable instrument each time it is used means that a part of each observing session must be devoted to getting ready to observe. The next chapter pays more attention to the problem of aligning an equatorial mounting.
Which telescope to choose? Perhaps the best way of helping the reader to come to a decision will be to summarize the features of some typical telescopes in the refractor, reflector, and catadioptric class.
TYPICAL SPECIFICATIONS Object glass aperture 80 mm, focal length 1200 mm (f/15). German equatorial mounting on a tall tripod. Overall weight about 18 kg.
OBSERVATIONAL CAPABILITY An excellent instrument for observing sunspots, and many models include a solar projection screen for safe viewing of the Sun's surface. An enormous amount of detail visible on the Moon -- more, probably, than could be drawn in a lifetime. Adequate regular views of Venus and Jupiter, and of Mars when very near the Earth; excellent views of Saturn's rings, but the disk of the planet appears rather dim under the necessarily high magnification. Hundreds of double stars, nebulae, star clusters, and galaxies can be satisfactorily observed, and plenty of variable stars can be followed in their light changes.
ADVANTAGES Only minimum maintenance of the optical parts is required for the telescope to perform to the peak of its capability. Many refractors of this order of aperture, a hundred years old or more, are still in regular use.
DRAWBACKS Expensive and bulky for its aperture. Awkward to use for high-altitude observations, without a zenith prism.
SUMMARY Although refracting telescopes are necessarily limited in aperture because of their long tubes, they are the most efficient design from the point of view of light-transmission -- that is, the ratio of the amount of light entering the object glass to that finally concentrated into the image is very high, typically about 76 percent. Light losses occur mainly at the air/glass surfaces, about 4 percent being reflected back out of the telescope at each interface; if these surfaces are coated or bloomed (the effect of blooming, which is carried out on most camera lenses to reduce internal reflections, gives the lens a bluish appearance by reflected light) the transmission rises to about 88 percent.
A good refractor gives the "cleanest" star image of any telescope because its optical train is the simplest, and the light scattered around a star, giving it flare or rays and reducing its contrast with the dark sky, is at a minimum. This ignores the effect of secondary spectrum, already mentioned, which normally is so inconspicuous that it will be noticed only around very bright objects.
Nevertheless, it must be recognized that the refractor cannot compete with other designs of telescope where sheer light-grasp is vital. Even a 150-mm aperture refractor would be a massive and expensive instrument.
NEWTONIAN REFLECTING TELESCOPES
TYPICAL SPECIFICATIONS Primary mirror aperture 150 mm, focal length 1200 mm (f/8). German equatorial mounting on a low metal stand. Overall weight about 25 kg. The 200-mm f/6 instrument is also a popular buy.
OBSERVATIONAL CAPABILITY Disk details can be seen on all the planets out to Saturn; even remote Neptune will exhibit a minute disk to distinguish it from a star. Useful for sunspot projection, although the eyepiece may become very hot from the concentrated heat. Many double stars are visible, and more variable stars than one observer could hope to follow come into view. An excellent instrument for observing star clusters, nebulae, and the brighter galaxies in some detail, while numerous fainter objects can be detected well enough for their positions to be recorded. Every year, at least one comet visible with this aperture passes through the sky.
ADVANTAGES The "best buy" in terms of dollars per unit aperture; a representative price list published by a well-known firm quotes a German-mounted, motor-driven, 150-mm Newtonian at $440, and an 80-mm refractor with similar specification at about $500. (It is worth noting that the latter price will also buy a 330-mm, short-focus Dobsonian reflector.) Newtonians are more convenient to use than are refractors, but the eyepiece can assume an awkward position at certain orientations when equatorially mounted, unless the tube can be rotated inside the declination-axis cradle -- this feature is worth paying for.
DRAWBACKS The two mirrors must be carefully protected when the telescope is not in use; and, even with the most scrupulous care, the aluminum coating eventually will tarnish and have to be replaced. The optical alignment of the mirrors may also be disturbed in use, and must immediately be corrected, for the image quality is seriously affected by even a small displacement of the primary mirror.
SUMMARY A useful "all-around" telescope, capable of doing justice to most fields of observational work. Its light-transmission, even with brand-new aluminum coatings, is only about 70 percent, falling rapidly to nearer 60 percent in many locations where the air is damp and dirty. On the other hand, this is compensated by the fact that relatively large apertures are available at a reasonable price. However, mirrors scatter a good deal of light, and although they are truly achromatic it may be found that the image contrast is not as good as with a refractor. The arm, or vanes, supporting the diagonal mirror also produce diffraction rays around bright objects. For planetary work, refractors up to about 200 mm in aperture will almost certainly outperform reflectors of the same aperture, assuming, of course, that both instruments are optically excellent. There used to be a rule of thumb that compared the effectiveness of an 80-mm refractor with that of a 150-mm reflector, but many observers would feel that this is a little hard on the reflector!
Although requiring more care and maintenance than other types of telescopes, a good 150-mm Newtonian can give a lifetime's enjoyable and useful service. There is certainly no question of it being simply a "beginner's telescope."
TYPICAL SPECIFICATIONS Aperture 200 mm, effective focal length 2000 mm (f/10). Fork equatorial mount designed to stand either on a table top or on its own tripod. Overall weight of telescope and mounting, but excluding tripod, about 10 kg. This type of instrument is, of course, available in both smaller and larger sizes, but the 200-mm aperture seems to be particularly popular with amateurs.
OBSERVATIONAL CAPABILITY Generally, as for a Newtonian reflector of similar aperture, but with some reservations.
ADVANTAGES Extremely portable and convenient to use. Aperture for aperture it is more expensive than a Newtonian reflector, but still much cheaper than a refractor. Can easily be adapted for photographic use (makers tend to exploit this potential), and because it gives an upright image, it can be used terrestrially for viewing objects only a few meters away. Very steady.
DRAWBACKS Because the optical specification and the mechanical tolerances are very critical, the image quality of different samples of the same model can vary, and readjustment is a matter for the expert. The "sealed tube" design, while protecting the mirrors, tends to trap warm air inside the tube, and this can lead to particularly unsteady images when the telescope is taken out into a cold night. Dewing of the shell or corrector plate can also be a nuisance in damp climates, and a satisfactory dewcap needs to be practically as long as the telescope tube.
SUMMARY As far as sheer definition is concerned, a catadioptric telescope is unlikely to perform as well as the equivalent Newtonian (for example, comparing a 200-mm, f/10 catadioptric with a 200-mm, f/8 Newtonian). One reason is the difficulty of making and mounting optical components to the required specifications -- indeed, the marvel of these telescopes is that they perform as well as they do! A second reason is the light-scattering effect of the relatively large secondary mirror, that can have a diameter approaching one-third of the total aperture. The serious planetary observer, who does not demand extreme compactness, would almost certainly do better with the larger Newtonian that the same money would buy.
It is also probable that a Newtonian (certainly with fresh mirror coatings) will detect fainter stars, nebulae, and galaxies. It is difficult to prevent some skylight or starlight from leaking down through the catadioptric system and brightening the background field of view. In practice, however, the reflective coatings of the sealed-tube system should stay in good condition for a very long time.
A catadioptric is a delight to use because its short tube is so stable and can be maneuvered so easily; and, remembering the suggestion that the mounting may be even more important than optical quality, there is a strong argument for buying a catadioptric! After using a swaying Newtonian or refractor, the joy of a rock-solid instrument cannot easily be exaggerated. One's final feeling, however, is that the catadioptric telescope has, to some extent, sacrificed ultimate optical performance for convenience and versatility.
THE DECIDING FACTORS The individual will have to weigh up the merits of the different instruments somewhat along these lines:
Refractor Excellent definition, minimum maintenance, restricted aperture
Newtonian Large apertures readily available, good to excellent definition, needs periodic cleaning and adjustment
Catadioptric Moderate to good definition, reasonable aperture, very convenient and versatile
Copyright © 1985, 1988 by James Muirden
How To Use An Astronomical Telescope
Renowned British astronomer and author James Muirden takes the fledgling astronomer by the hand in his new book, offering tips on:
* the purchase, assembly, and orientation of your new telescope
* how to observe and chart the Sun, Moon, planets, stars and comets
* how to investigate the deep-sky objects -- clusters, nebulae, and other galaxies beyond the Milky Way
The final chapter, "Windows into Space," explores ten carefully selected regions featuring noteworthy examples of double stars, galaxies, and nebulae, as well as more obscure objects seldom examined by astronomers.
How to Use an Astronomical Telescope offers completely revised and updated location charts with detailed coordinates, tables, appendixes, and numerous illustrations and photographs, making it the essential volume for one's first exploration of the cosmos.