| Astronomical Telescopes:
The Earliest Telescopes.
The first important telescope user was Galileo
who was able to show that Venus displayed phases which proved that it orbited the Sun. This was the clinching demonstration needed to prove the correctness of Copernicus's heliocentric model of the solar system. As I told you earlier, Galileo was relying on the fact that telescopes provide magnified images so that he could see the phases of Venus. (To the unaided eye, Venus looks merely like a point of light.) But nowadays we make telescopes large for different reasons, as I will explain.
Galileo used a telescope with lenses - a refracting telescope (or simply a refractor ). The figure on page 177 of your text remind you why this works. The light from a distant object changes direction as it passes from air into (and then right through) the glass in the lens. With a carefully shaped lens, you can cause the light of a star to come to a focus, as shown.
The big lens at the front end of the telescope is called the objective, and its width, or aperture, determines how much light is collected. An instructive way to consider what happens next is to imagine using such a lens all by itself to create a little disc-like image of (say) the moon, hovering in the air. In fact, you could imagine holding up a sheet of paper onto which you could project the image of the moon (in the way that a slide projector casts an image onto the screen). Alternatively, you could put a piece of film there and take a photograph, just as your camera focusses an image onto the film inside it. But suppose you wanted to look at the image with your eye? What then?
The Role of the Eyepiece - and Its Irrelevance.The big objective lens has focussed the light from the moon (or whatever your target is) into a small image floating in the air, just as though there were a little tiny celestial body floating there in space, with light coming out from it. Suppose there were such a micro-moon in front of you, hovering in the air. How would you examine it in detail? The answer, of course, is that you would hold up a magnifying glass and look through it. In other words, for your eye to examine the image which has been created, you must have an eyepiece, the other important part of a simple refracting telescope. This already gives you an idea of why astronomers do not consider magnification as the predominant factor in building big telescopes. If the image looks too small to your eyes, you can simply use a more powerful `magnifying glass' - that is, change the eyepiece! For a given telescope, you can adjust the magnification by swapping eyepieces. It is true that larger telescopes in general provide greater magnification, but this is only a small part of the reason for their construction. More to the point, today's research astronomers do not even look at the objects being studied, at least not with their eyes. Instead, the light is collected with an electronic instrument of some sort, and analyzed later. The important aspect is the size of the image compared to that of the detector -- there is no point in making the image of the moon a lot larger than the piece of film onto which you are going to focus it! -- not how big one could make it look to the eye. For amateur astronomers interested in star-gazing or planet-watching, of course, magnification does matter. A couple of paragraphs farther down the page, however, we will learn that there are practical considerations which limit the maximum magnification anyway.
Why Make Big Telescopes?For historical reasons, optical telescopes (those with which we study visible light) came first, followed by radio astronomy (post World War II) and the later study of other wavelengths. In the present discussion, I am speaking only of optical telescopes, the development of which has been featured by a desire and need to build ever-larger telescopes. Why is this so? There are several reasons, of which one is predominant. Magnification: Is it to provide greater magnification? Yes, this is partly the reason. As we have seen, a bigger telescope can provide a bigger image; but the magnification also depends on the eyepiece used, as I have described. Thus in principle one could put an appropriately-chosen eyepiece on a small telescope to get a greatly magnified image, and never bother to build a large telescope at all. By the way, magnification is not as important as you might think. Suppose, for instance, you point a telescope at Mars and get a hugely magnified image in front of your eyes - one which looks as big as a beach ball would if it were held a foot in front of your face, say. Would this help your study of Mars? The answer is no, for two reasons. First of all, there is a limited amount of light coming from Mars, and you are spreading it out into a large image which will look very dim overall. Secondly, the turbulence of the Earth's atmosphere blurs out the details of Mars's surface, and if you magnify the image you will simply see magnified blurriness, not more detail. Resolution: Is it to resolve finer details? Yes, partly it is, although less so with optical telescopes than at some other wavelengths. The details you can detect in any image will be limited because of the wave nature of light. This imposes an inherent `fuzziness' which depends on the particular wavelength at which you are working (the longer the wavelength, the poorer the resolution). To get around this limitation, you can use the fact that the larger the aperture of your telescope, the sharper the image which it forms, at least in principle. (See the 'Mathematical Insight' on page 181 of your text.) This sounds like a pretty good reason for making larger telescopes! - and indeed, this is exactly the explanation for the very large size of many radio telescopes. The wavelength of radio radiation is so long that the resolution is very poor, and to compensate we build enormous telescopes, sometimes hudreds of metres across.(I will come back to this point later). But for optical telescopes this has not generally been a big consideration, up until at least the last decade or so. The turbulence in the Earth's atmosphere means that the resolution we can attain is not limited by the aperture (size) of the telescope at all, but rather by the atmospheric blurring. Of course, something like the Hubble Space Telescope (HST), which is orbiting above the Earth's atmosphere, is free of these effects, and can work to the theoretical limits of resolution; thus the HST sees much finer detail than any ground-based optical telescope, even bigger one, unless we can compensate for the atmospheric blurring in some other way. (As we will see, this kind of compensation is now becoming standard practice, using some clever new techniques. As a result, ground-based telescopes can attain all the resolution of which they are theoretically capable, and this provides extra incentive to make them yet bigger.) Light Grasp? Is it to collect more light? Yes! This is the principal reason for making telescopes big. I used an analogy in class: if you wanted to collect rain water to do your laundry in pure untreated water, you would not put thimbles out in the back yard. Instead, you would use the biggest washtubs you could find. So too in astronomy: we want to study faint stars and galaxies, so faint that they cannot be seen except through the accumulation of as much light as possible. Hence big telescopes!
How Telescopes Are Used.The stereotypical view of astronomers is that they are male (almost always), elderly, bearded (well...), dressed in white lab coats, and rather eccentric (I'll let you be the judge of that). They are usually portrayed as peering through a telescope and steadily scanning the night sky, as though in search of brand new objects. Indeed, I am often asked by people whether I have ``found any new stars lately?'' This is not what most professional astronomy research is like, in a variety of ways. Let me enumerate a few of them: The survey mode: You would not expect to learn of a botanist that she spends her time crawling around on the lawn looking for a blade of grass that has not been recorded before. So too in astronomy: we do not typically sit at the telescope, sweeping across the sky in search of new objects to add to some list. This is not to say that we do not do some surveys, or that we aren't on the alert for new types of objects. But when we go to the telescope, it is generally with a fixed program in mind. For instance, we might be planning to study how the stars are moving around near the centre of a particular nearby galaxy, perhaps to determine whether their motions are affected by the presence of a massive object in the core of the galaxy -- a big black hole, maybe. The telescope time is very keenly competed for, and part of the job is to write applications which have to be reviewed and ranked by a panel of expert astronomers to see if the project is feasible and of scientific interest. I hasten to add that there are indeed some worthwhile observing programs which are entirely devoted to the finding of new objects! A good example is provided by the many mid-sized telescopes which are now being used to search for previously-unknown asteroids. The objective here, of course, is to find potential Earth-crossers so that we can have forewarning of any that are likely to hit us and cause global devastation. But not much modern astronomical research is in this 'pure discovery' mode. The observing technique: As I noted earlier, we very seldom "look through" the telescope with our unaided eyes. Indeed, it is possible to carry out a whole observing program without touching or even seeing the telescope: one simply sits in a computer room and runs the equipment and instruments by remote control. The eye is a notoriously unreliable detector, not least because it does not accumulate light, which means that faint objects stil look faint even if you stare at them for a long time. Typically, then, the light is focussed onto a sensitive electronic detector which accumulates the data for many minutes or hours (since we are studying faint sources). We come away with electronic data recorded on magnetic tapes, and a single night's observing can give you enough to keep you busy literally for months in reducing and analysing the data. The sociology: Astronomy is an international discipline, supported by all developed countries. I don't know the breakdowns, but have the impression that there are more women in astronomy than in many of the physical sciences (although still not enough). But one interesting aspect is that there are literally only a few thousand really active research astronomers in the world, and in one's own area of the subject one can get to know almost all the people who are active - you meet them at scientific conferences, at the observatories, and so on. This is in contrast to something like protein research, where there are many tens of thousands. This makes astronomy a very `friendly' science (although there are keen rivalries as well). When I go off on an observing trip to some remote telescope, I almost invariably run into old friends and colleagues from other parts of the world.
The Problem with Refractors.As noted above, Galileo used a refracting telescope (i.e. one with lenses). For a long time thereafter, this was the standard way to make telescopes; but there were problems with refractors, and especially with trying to make them big so that you could study faint objects. Here are the main problems: Chromatic Aberration: `Chromatic' means `having to do with colour', and if you think back to our discussion of the way a prism splits light up into its component colours, you will see the problem. As the light from a star enters the objective lens of a telescope, the various wavelengths (colours) are bent by different amounts and therefore are not brought to a common focus. Thus if you bring the yellow light of a star into crisp focus, the blue light will be out of focus and you will see a fuzzy blue patch surrounding the yellow star. You may wonder how your ordinary camera can give you a sharp picture, with red shirts and blue jeans and so on all in good focus. This is accomplished by using combinations of several lenses of different shapes and made of different kinds of glass. In astronomy, this is not a workable proposition, except for smaller telescopes -- the big lenses are simply too expensive. In the largest refractors, about the most one can afford to do is combine two big lenses made of different materials so that, by careful design, various wavelengths can be focussed at once (red and blue light following different paths through the combination). This approach leads to the use of what are called cemented doublets. The two lenses are made of different materials, called crown glass and flint glass, which might differ in the amount of lead, for example, which is put into the molten glass mix before it is allowed to cool and then formed into a lens. The lenses are also made to different shapes, and in combination, they eliminate a lot of the chromatic aberration (although it cannot be completely removed). The lenses, by the way, are literally stuck together with a clear cement - hence the name. As it happens, the longer the focal length of a lens, the less important the chromatic aberration. For this reason, astronomers of a few centuries ago used refractors of very long focal length, often very cumbersome in design and use, to try and get images of good quality. The Length of the Telescopes: The best images come from lenses that are quite thin. (A thick lens bends the light a lot, but in doing so it may also introduce a lot of unwanted distortions.) Unfortunately, the good image quality comes at quite a cost. Since a thin lens introduces only a very slight change in the direction that the the light is travelling, it is not brought to a quick focus. Instead, the telescope has a very long focal length - the final image is a long way from the objective lens! - and the telescope structure is physically very cumbersome, heavy, and expensive. Since the telescope must be sheltered from the elements, this means in turn that you have to construct a a big (expensive!) dome. By way of example, consider the largest refractor in the world, the Yerkes telescope. It has an objective lens - which is in fact a cemented doublet - which is only 40 inches (1 metre) across; yet the tube of the telescope is fully 60 feet long. (See page 177.) It is housed in an elaborate dome with an elevating floor which allows you to get at the eyepiece. Believe it or not, the size and cost of the dome can be a major factor in the decision to build a telescope or not - rather as though you decided against buying a car because you could not afford to build a garage to keep it in. On the positive side, the long focal length helps to minimize chromatic aberration, as I noted above. But the other inconveniences and costs are not outweighted by this consideration. The Expense of the Lenses: Big lenses are not cheap: you need pieces of glass of very high purity, free of cracks and bubbles (since the light goes right through them); and you have to grind and polish the faces of each lens to make a very precise shape. This makes four demanding polishing jobs, if you are building a doublet. It is not cheap. Only One Focal Position The light is bent as it passes through the lens(es) and is brought to a focus at the bottom of the telescope tube. At that spot, you might first look through the eyepiece to see if you have pointed the telescope correctly at the star you want. But if you want to take a photograph, you must now remove the eyepiece and put on a camera. If you then want to get a spectrum, you must remove the camera and mount a spectrograph in its place. In other words, since the light is brought to only one focus, it is hard to accommodate a varied set of instruments in a convenient way: the instruments must be mounted and dismounted individually. This can be a real operational inconvenience and leads to wasted observing time. The Limited Size: This is the critical factor, because modern astronomy is really driven by the need to capture lots of light. Lenses simply cannot be made very large, because they sag and slump under their own weight. Here is an analogy: imagine a butter tart and an apple pie side-by-side on the table. You can easily pick up the butter tart with one hand, spreading out your fingers to grip it from above by holding the rim of crust which surrounds it. If you try to do that with the apple pie, it will break under its own weight -- it has to be supported from beneath. The problem with lenses is analogous. You can only afford to hold them around the edges, because putting some sort of support behind them prevents some of the light getting through. But a big lens, if it is held only around the rim, will sag so much under its own weight that the images it forms are badly distorted.
The Advent of Reflecting Telescopes.The problems associated with refracting telescopes can be eliminated by the use of reflectors (i.e. telescopes using mirrors instead of lenses). It was Isaac Newton who first recognized the merit in using reflectors, which is what all the big telescopes in the world are today. Newton's original motivation, however, was not to build a big telescope, but simply to get rid of chromatic aberration. He realized that visible light of all wavelengths reflects at the same angle off a shiny surface. (This is quite different from the way a lens refracts light.) So Newton decided to build a telescope with a dish-shaped mirror designed so as to bounce the light back and to bring it to a focus above the mirror. The problem now is an obvious one: how do you look at the image which is formed? If you stick an eyepiece there and put your head in to look through the eyepiece, you will block off the incoming light and will see nothing. Newton came up with a simple solution: put in a flat mirror just in front of where the light would focus, and reflect the light out to a new focus to the side - the so-called Newtonian focus, shown in a figure on page 178 of the text. Such an arrangement is still commonly used for amateur telescopes of modest size.
Solving the Problems Posed by Refractors.Before I discuss other arrangements and designs of optical telescopes, let us consider the general merits of reflectors, especially insofar as they solve the problems posed by refractors. Chromatic Aberration: As Newton realised, visible light of all wavelengths reflects at the same angles, so there is no chromatic aberration at all. (In actual use, big reflectors usually have some lenses somewhere - in the eyepiece, or perhaps in a camera. Thus there is always a bit of a problem, but it is not as extreme as with the big pure refracting telescopes, and can be solved with the use of multiple small lenses if necessary.) The Limited Size of the Telescope: In our quest for larger apertures to collect lots of light, we gave up on refractors because the big lenses sag under their own weight. This is not a problem for big mirrors, because the light only bounces off the surface: it does not pass through the glass. Consequently, you can put all sorts of supports behind the mirror to give it strength. Today the largest single telescope mirrors ever made are about 8 metres in diameter (the Keck 10-metre mirrors are made up of smaller sub-panels), so they have 64 times the collecting area of the objective lens of the Yerkes refractor, the largest refractor of all. The Long Focal Length: One problem with refractors was that the light comes only gradually to a focus, so that you inevitably get a long skinny telescope which is cumbersome and which must be housed in a large expensive dome. But reflectors allow you to bounce the light in various directions, as the figures on page 178 show. Thus, while the light can travel a long distance in coming to a focus, it can be made to do so while it bounces "back and forth" in what is known as "folded optics." The net result is that one can have a big mirror, lots of light-gathering power, but a short stubby telescope in a small and less costly dome. Compare the long, skinny silhouette of the Yerkes refractor (page 177) to that of a modern reflecting telescope (shown in schematic cross-section on page 178). Today's most modern reflecting telescopes are built to very 'stubby' designs, and are remarkably squat in appearance. A Variety of Focal Positions: With refractors, there is only one focus, so once you have looked at the star with an eyepiece, say, you have to remove that to strap on a camera, and so on. Reflecting telescopes can be made more versatile: you can have electronic instruments or cameras in various locations, and feed the light to them simply by inserting an extra mirror oriented in the right way. After you have finished with one instrument, you could in principle merely rearrange or remove the flat mirror to let the light fall onto something else. This allows you to have a whole set of instruments standing ready for varied astronomical investigations. The Expense of Producing Lenses: As noted above, big lenses are costly and difficult to make. Big mirrors have to ground and polished to just the right shape, which is not a trivial job, but on the other hand there is only one surface to be done, and the glass does not have to be absolutely perfect throughout, since the light does not pass through it. It is very much cheaper to build a one-metre mirror than a one-metre lens, and larger telescopes must have mirrors, so it is helpful that they are relatively cheap. Per unit of collecting area, mirrors are cheaper -- but I should emphasise that 8-metre mirrors are still not cheap, by anybody's standards!
Varied Designs of Reflectors.We have discussed the various benefits of reflectors, one of which is that we can focus the light at a variety of locations. What determines where of these focal positions should be used? One possibility would be simply to make a giant scaled-up version of a Newtonian telescope, like the one shown on page 178, but this is not generally practical because we often need to use rather heavy instruments (like spectrographs, to obtain spectra of the stars). If you were to hang such an instrument on the side of the telescope way up near the top, it would make the telescope sag under the extra weight and compromise its performance. (Imagine holding a heavy bag of flour in your hand with your arm extended straight out: there is a considerable strain on your muscles and joints.) Moreover, it would be inconvenient to get at the instrument. The Cassegrain Focus: One goal, then, is to find an arrangement in which heavy instruments can be mounted fairly low down on the telescope, near the axis which supports it. One way of accomplishing this is to use what is called the Cassegrain focus (see page 178). In this arrangement, the light bounces twice and winds up passing through the big mirror, a feat that is accomplished by having a hole in the centre of the primary (big) mirror, so that it is in fact doughnut-shaped. Heavy instruments located here are near the floor and not suspended way out on the end of the telescope structure. By the way, the new 15-inch telescope on the roof of Ellis Hall here at Queen's is a Cassegrain. The Coudé Focus: Instruments mounted at the Cassegrain focus are still attached to the telescope and move with it; thus their weight can cause some strains on the telescope. Suppose you have a colossally heavy instrument: where else might you put it? An obvious answer would be to place it in a fixed location, perhaps in a separate room, and use moveable mirrors to get the light to it. As the telescope moves around, these mirrors would need constant adjustment, but that is feasible. This focus is called the coudé focus, with the name coming from the French word for "bent" (think about the fact that your elbow is called "le coude" in French). A diagram is shown on page 178 of the text, although the figure does not make clear the fact that the focus position can be kept in a constant location as the telescope moves. The Prime Focus: As we saw, Newton's small telescope had to have a flat mirror to cast the light out to the side if the stars and planets were to be observed at all! - if you were to stick your head in to where the image is, you would block the light from reaching the mirror. But suppose you had an absolutely enormous mirror? Then it might be possible to sit up at the top end of the telescope, right in the middle of it, suspended in what is called the "observer's cage", and observe there. This is called the prime focus. A schematic example is shown at the top of page 178. The lucky astronomer sits in a 'cage' high at the top end of the telescope. Of all the ways of observing, I think that this is perhaps the most fun, and I have been fortunate enough over the years to do a lot of such observing on telescopes in Chile, Hawaii, Australia, and Arizona. The telescope is canted over so that you can climb into the cage, along with a flashlight, a box of photographic plates (in the old days!), and a warm parka. Then the operator in the control room moves the telescope so that you are pointing at the target galaxy. The lights are all extinguished, of course, and the dome is open to the starry sky. You make sure the target is well-centered in the field of view, and open the shutter so that you photographic exposure begins. It may last 90 minutes or more, as the telescope slowly tracks across the sky; so you ask the night assistant to put on some music. It can be very peaceful... except when it is -20 degrees in the dome, and there is a cold wind blowing, and things go wrong! Anyway, the instruments we use now are almost all electronic (not much photography any more) and remotely controlled. Sad to say, it is very rare to ride the prime focus these days.
Innovations in Optical Astronomy.One of the limiting factors in optical astronomy used to be the inefficiency of the detectors (rather than the telescopes). For instance, a photographic plate or film may record considerably less than one percent of the photons landing on it. Building larger telescopes provides more photons, but it is just as worthwhile to develop better detectors, so that the present telescopes can be used more efficiently. As it happens, modern electronic detectors such as Charge-Coupled-Devices, or CCDS (see page 176 of the text) are now very efficient - indeed, almost perfectly efficient - so if we want to study yet fainter objects, such as very remote stars and galaxies, we need inevitably to get more photons. The obvious way of doing this is to make telescopes as big as possible. There are a few innovative approaches worth mentioning. Segmented Mirrors: Rather than turning big monolithic slabs of glass into mirrors, it has become possible to use smaller pieces which can be mounted individually and then adjusted to bring the light to a single focus. The Keck Telescopes have taken this to the limit: small panels of glass are used in a mosaic, as shown on page 180 of your text, to make the largest single optical telescopes (a pair of them) in the world. The technology is challenging, because of the need to keep the little mirrors all carefully aligned. (Interestingly, this approach was pioneered by the MMT -- the Multi-Mirror Telescope --- but they have now abandoned the technique and replaced the MMT's six mirrors with one big one.) Liquid Mercury Mirrors: This technique has been championed by Ermanno Borra, of Laval University, and is now being explored further at UBC. If you take a bowl of liquid and spin it at a regular rate, it will form a paraboloidal shape, perfect for focussing the light of a star which is directly overhead. Mercury is very reflective, so a large spinning bowl of it will act like a telescope pointing straight up. Moreover, such a telescope can be made very cheaply. The obvious problem is that you can only see things directly overhead, but as the Earth rotates a lot of different things pass through the field of view. Moreover, such a telescope could be mounted on a truck and driven to a new location farther north or south so that one could study other strips of sky. This is proving a very interesting and novel approach. Thin Mirrors: One of the problems of earlier large reflectors was that the pieces of glass used were very thick, shaped like a big round slab. This meant that: It was time-consuming and expensive to grind the front face of the mirror to the right shape. Indeed, it took eleven years to complete the job for the famous Palomar 200-inch telescope, although that was partly because some of the technology was new and untried in those days. The resultant mirror was very heavy, and needed a lot of strong, heavy support structures. Any such mirror is so thick that heat only slowly diffuses into or out of it. This means that the mirror takes ages to cool down once it warms up, and as a result the images will be distorted, both because the warm air rising from the mirror makes the image shimmer, and also because the mirror itself may change size and shape when its temperature changes. To combat the second of these problems, modern telescope mirrors are made of special zero-expansion glasses which retain their shapes and sizes even when the temperature changes rapidly. You may have encountered something similar with CorningWare, a material which is used to make casserole dishes which can go straight from the freezer into a hot oven. Ordinary glass would shatter as it expanded unevenly - the outer parts of it would feel the full heat of the oven, while the inner edges would be in direct contact with the still-cold casserole contents. This does not happen to the CorningWare. A new technique, applied to the Gemini telescopes (in which Canada is a partner), is to make the mirrors thin (about 20 cm) but big (about 8 metres across). In profile, such mirrors look like gigantic contact lenses! They can be cast into this shape by heating and 'slumping' a thin disk of glass onto a mould which is itself curved. In another approach, the mirror can be created in a huge rotating furnace into which chunks of glass have been placed. They melt in the heat, and then slowly re-solidify in a curved shape as the temperature is lowered. The Gemini mirrors, by the way, were made at the Corning glass works, just south of us in New York state, using the first of these approaches.
The Theoretical Resolution Limits.As noted earlier, the wave nature of light means that there is a limit to our ability to resolve fine detail, even in the absence of atmospheric turbulence and even given a perfectly shaped mirror. This can be proven mathematically, and what comes out is the following: (the smallest resolvable angular detail) = (the wavelength of the light) divided by (the aperture of the telescope) (Remember that the aperture of the telescope is the diameter of the primary mirror or the objective lens.) The equation on its own is sterile! Let me give you some sense of what it actually implies, without belabouring the numerical details. Let us suppose, for instance, that you were observing the moon using a radio telescope working at a wavelength of 1 metre, using a radio telescope which is itself 1 metre across -- a telescope which might look something like a backyard satellite dish. Then the smallest detail you would be able to resolve would be about one radian across, with a 'radian' being the (probably unfamiliar) unit of angular measure which is appropriate to the equation given above. The interesting thing is that a radian is an angle which is about 57 degrees is size (whereas the moon itself is only half a degree across). Let us imagine, for instance, that the moon had a couple of dark craters which give off lots of radio radiation, and several other regions that don't give off any. Well, your radio telescope would never be able to resolve all those interesting details. Everything would be blurred into a fuzzy blob of indeterminate shape. About all you would be able to do is wave your hand and say something like "the moon is out there in that general direction, and we are detecting some radio radiation from its vicinity." You might say that this is no problem. If you can't resolve interesting details at radio wavelengths, then just observe in the visible, where the wavelengths are much shorter and the resolution is inherently better. But this doesn't make sense: you choose a wavelength at which to observe because it gives you a particular piece of astrophysical information. (For instance, if we want to study clouds of cool hydrogen gas between the stars, we are forced to work at the long radio wavelengths.) If we can't change wavelengths, how can we improve the resolution and see more details? Well, if you examine the equation above you will see that if the wavelength is fixed, increasing the size of the telescope makes the angle smaller -- which means, in other words, that you will resolve finer detail. This is why radio telescopes in particular are built so very large. I have given the example of radio radiation because that is where the wavelengths are longest and the resolution problem is most acute. But similar considerations hold for visible light as well. I remind you, though, that this has not historically been our main motivation for building ever-larger optical telescopes: until recently, other factors beyond our control have made such considerations irrelevant. The turbulence in the atmosphere gives rise to a "blurring" of the images which cannot be gotten rid of simply by increasing the diameter of the mirror. Little cells or pockets of air, with different temperatures and densities, swirl around above the telescope. As rays of light pass from one cell to another, they change direction a bit. The net image is irretrievably smeared out -- or at least that used to be the case, until the development of the science of `adaptive optics.' But that's a topic for a later day. Previous chapter:Next chapter
0: Physics 015: The Course Notes, Fall 2004 1: Opening Remarks: Setting the Scene. 2: The Science of Astronomy: 3: The Importance of Scale: A First Conservation Law. 4: The Dominance of Gravity. 5: Looking Up: 6: The Seasons: 7: The Spin of the Earth: Another Conservation Law. 8: The Earth: Shape, Size, and State of Rotation. 9: The Moon: Shape, Size, Nature. 10: The Relative Distances and Sizes of the Sun and Moon: 11: Further Considerations: Planets and Stars. 12: The Moving Earth: 13: Stellar Parallax: The Astronomical Chicken 14: Greek Cosmology: 15: Stonehenge: 16: The Pyramids: 17: Copernicus Suggests a Heliocentric Cosmology: 18: Tycho Brahe, the Master Observer: 19: Kepler the Mystic. 20: Galileo Provides the Proof: 21: Light: Introductory Remarks. 22: Light as a Wave: 23: Light as Particles. 24: Full Spectrum of Light: 25: Interpreting the Emitted Light: 26: Kirchhoff's Laws and Stellar Spectra. 27: Understanding Kirchhoff's Laws. 28: The Doppler Effect: 29: Astronomical Telescopes: 30: The Great Observatories: 31: Making the Most of Optical Astronomy: 32: Adaptive Optics: Beating the Sky. 33: Radio Astronomy: 34: Observing at Other Wavelengths: 35: Isaac Newton's Physics: 36: Newtonian Gravity Explains It All: 37: Weight: 38: The Success of Newtonian Gravity: 39: The Ultimate Failure of Newtonian Gravity: 40: Tsunamis and Tides: 41: The Organization of the Solar System: 42: Solar System Formation: 43: The Age of the Solar System: 44: Planetary Structure: The Earth. 45: Solar System Leftovers: 46: The Vulnerability of the Earth: 47: Venus: 48: Mars: 49: The Search for Martian Life: 50: Physics 015 - Parallel Readings.
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(Wednesday, 21 April, 2021.)