| Making the Most of Optical Astronomy:
The Need to Go Faint.
As we have seen, it is expensive and challenging to build big modern telescopes, and it is important to make every dollar count in acquiring the data which astronomers need for their research. The real goal, of course, is to make telescopes as efficient as possible at collecting light from faint objects, those at the limits of detection and study. Let us reconsider for a moment why this is so. There are four different reasons.
The real faintness of some objects . This is fairly obvious: some objects, like the very smallest stars, are intrinsically faint and would be hard to study even if they were right on your doorstep. They give out so little light! Another example is provided by the tiny asteroids. To study these rocky objects requires a big telescope even though they are right inside our own solar system. Here the problem is that the asteroids are so small that they reflect almost no sunlight back to us on Earth.
The rarity of some objects. If every star was like the sun, we would have no problem. Study the sun and you understand all stars! But in fact the universe is full of objects of different kinds, some of which are extremely rare, and to develop a full understanding we must examine a good number of objects of all kinds. Since the universe is vast, however, a rare object will typically be quite far away. If only one star in a million is a neutron star, for instance, the nearest neutron star is probably quite far away and therefore faint.
The desire to study other locales. We live in the outskirts of a spiral galaxy, the Milky Way. We would like to know what the circumstances are in other places, like in the heart of a big elliptical galaxy, or in the void between the galaxies. To do that, we necessarily have to study stars and gas which in regions very far removed from our own 'solar neighbourhood.'
The need to look to great distances for cosmological reasons. Astronomy allows us to `look back into the past'' by examining remote objects, since the light which we receive now left these objects a long time ago. In this way, if we can examine objects fifteen billion light years away, we can study the nature of the universe itself as it was fifteen billion years ago, shortly after it all came into existence in the ``Big Bang'' (about which we will learn much later). At such enormous distances, even whole galaxies of stars are scarcely detectable, and huge efficient telescopes are required.
Accomplishing This Worthwhile Goal.To collect and use light more efficiently, it may seem that ever larger telescopes are the only answer. Photons from remote stars and galaxies stream towards the Earth, and any photons which fall onto the ground are merely wasted (from an astronomer's point of view). Why not erect huge slabs of glass everywhere and bring all that light to a common focus? The ``why not'', of course, is that it costs a great deal of money. Are there other ways? The answer is ``yes.'' Let us consider several of these other approaches, interspersing the discussion with some comments on my own area of research to provide some representative examples.
Moving from Inefficient to Efficient Detectors.In the class, I showed you examples of photographic plates with images which I had taken at a couple of different telescopes. Until about fifteen years ago, photographic plates were very commonly used in astronomy. Let me remind you of a few of the remarks I made about them: The `plates' are literally that, glass plates. These are used in preference to film because they are more stable: sheets of film can stretch or crinkle. Like conventional photographs, the plates yield negative images. In other words, the photographic emulsion is darkened where the light arrives, so a photograph of a field of stars shows black dots on a light background. This is just like the negative made by a photographer in a conventional camera; the difference is that astronomers do not take the subsequent step of going to the darkroom to make a positive print (except occasionally for publicity purposes, to `make pretty pictures'). There are two reasons for preferring the original negative images: Every processing step introduces some `noise', some imperfection in the reproduction. By working with the negatives, we minimize the importance of such effects. Moreover, it is psychologically and physiologically easier to study and examine a few dark spots on a clear background than a few light spots on an broad intensely dark expanse. The photographic plates can be made big and quite uniform in their properties over large areas. (The plates I showed you in class were about 25 cm on a side, but they can be made even larger.) This generous size means that photographs can be taken of fairly wide areas of the sky (like a wide-angle snapshot), so that we can study many stars or galaxies at once. The uniformity means that stars of the same brightness will all show up equally well no matter where on the picture they fall, so intercomparisons are valid and meaningful. This all sounds good, but there are several disadvantages to the photographic plate, of which I want to focus on the two most serious. The most important of these is that the efficiency is very low. In fact, for every one thousand photons focussed onto the plate only a few may be recorded (by which I mean that only a few lead to a perceptible darkening of the emulsion and the formation of a picture). For this reason, the exposure times are necessarily very long: one of the plates I showed you was a ninety-minute exposure! Compare this to the pictures taken for your family photo album, where a typical shutter speed might be a sixtieth of a second. The difference in that family snapshots are taken in bright light, or with flashbulbs. The remote stars, on the other hand, are very faint. The second serious disadvantage is that the photographic plate merely provides you with an image; what you really want and need is data. By eye inspection, you can tell (for instance) which star looks brighter on the photographic plate, but to quantify that impression is rather difficult. The information stored on the plate has to be extracted and digitized somehow, a job which is not simple. The ideal, then, is to find a detector which is much more efficient so that the photons which are steered onto the detector by the telescope actually count for something. (This would clearly be just as good as increasing the size of the telescope itself.) It would be especially helpful if such detectors also converted the incoming light to digital (i.e. numerical) data, perhaps by `counting each photon' as it arrives. Such detectors exist! The best-known example is the CCD, the Charge-Coupled-Device. These little electronic chips are at the heart of everyday video cameras. (See page 176 of your text for some further discussion.) The efficiency of CCDs can be quite high: indeed, it can approach 100%, which means that every arriving photon gets recorded, although this may be the case only over some limited range of wavelengths. Using a CCD, the images on the photographic plates I showed you in class could have been obtained in literally only a few seconds of exposure. This means two things: We can observe many more objects in a given time, by moving the telescope quickly from one to another and taking only brief exposures in each location. This allows us to study many more stars and galaxies than we could have done photographically. Alternatively, we could spend exactly as much time as before, but now record many more photons from the target. Very faint objects which would have failed to register at all on the photographic plate will now show up clearly, which is the desired result. We can study fainter objects without the expense of building a larger telescope. Another welcome feature of the CCD is that it instantly provides digital data, a count of the number of photons recorded at each spot in the image. Mind you, the efficiency of the CCD sometimes means that we use exposure times which are only a few seconds in length. The consequence is that in a few nights of observing one can accumulate many, many images! This enormous amount of data, in digital form, is recorded on magnetic disks and tapes. We need high-speed computers to make sense of it all later.
The Problem with CCDs.From what I have said, CCDs may sound ideal, but they have one real constraint: they cannot be made very large. The very biggest single CCD chips that I know of are rectangular in shape, and about 5 cm by 10 cm. This is, of course, very much smaller than the photographic plates I described earlier. For some applications, this does not matter: if we want to get an image of a remote galaxy which looks very small because of its enormous distance, we may not need to cover a large part of the sky. But obviously there are general advantages in studying wide areas, so a goal of long standing has been to produce larger CCD cameras. The emphasis has lately shifted from the development of larger single chips to the construction of mosaic cameras which contain a great number of CCDs placed side-by-side (like tiles on a bathroom floor) to create a large light-sensitive array. There is, a `Megacam' which has been built for the CFHT, a mosaic device containing about fifty individual CCDs. With such an instrument, one can take images as big across as those provided by the old photographic plates but with hundreds of times greater efficiency. Of course, this merely worsens the `data glut' -- a single exposure with the Megacam produces gigabytes of data, and of course there are hundreds of images taken every night! Speedy data processing is thus even more important than before such devices came into general use.
Multiplexed Observing -- But First a Digression.Before I describe another way in which we can accomplish our goal of improving observing efficiency, let me digress briefly to describe some of my own research interests. You will see that they benefit directly from the multiplexing I will be describing, so the relevance of this will become clear quite soon.
Globular Star Clusters in the Milky Way.In class, I showed you an image of a globular star cluster, a cluster of about one hundred thousand stars held in a mutual gravitational embrace. You can visualise such a star cluster as something like a swarm of luminous bees moving around independently. The cluster is held together by gravity, of course, with each star moving in some complicated orbit which is determined by the combined gravitational effects of all the other stars. (That is, the stars do not orbit some single massive central object.) Your textbook contains a brief discussion of globular clusters, and a nice picture, on page 537. The globular star clusters are intrinsically very bright, hundreds of thousands of times brighter than the sun, since they contain so many stars. The fact that we don't see such clusters conspicuously shining in the night sky is attributable to the fact that they are so very far away -- a typical globular cluster is tens of thousands of light years from the Earth. (From the Southern hemisphere, it is just possible to see one of the nearest globular clusters as a fuzzy patch in the sky.) The astrophysical importance of the globular clusters is that they are the oldest known objects we can observe. In fact, the stars within these clusters seem to have been formed about fifteen billion years ago, shortly after the universe itself came into existence and ten billion years before the Solar System formed. In a sense, they are 'fossil remnants' of the very earliest stages of star and galaxy formation. Our Milky Way galaxy contains about one hundred and fifty globular star clusters. The Milky Way itself is something like a pinwheel of stars, of unimaginably great size, with the Sun and its planets located about two-thirds of the way out to one side. (We will explore this subject in much greater depth in Physics 016, but for now you should look at page 598 of the text to see a nice representation of what our galaxy would look like if we could somehow position ourselves outside it.) The pinwheel is slowly turning, like a long-playing record, so that the sun goes once around every two hundred and fifty million years. The sun and most of the stars in the Milky Way are found in the flattened disk (the `pinwheel'). The globular clusters are not like this: they tend to be found distributed all over the place, in what is called the halo of the galaxy. Naturally, these star clusters cannot simply be sitting out there at rest. If they were, they would fall in towards the disk, attracted by the gravity of all the myriad stars in the Milky Way. (This is the same reasoning that tells us that a planet in the Solar System needs to be moving. For instance, if you were to bring Mars to a complete halt, it would subsequently fall towards and into the sun.) No, the globular clusters must be moving, orbiting the galaxy itself on paths which are determined by the gravitational influence of all the matter in the galaxy. We can determine masses in astronomy by using what are called `test particles': that is, we see how some small object moves in response to the gravitational force of a larger body to determine the total mass of that larger body. (In this way, we can measure the downward motion of a piece of chalk to determine the mass of the Earth, or study the motion of the moons of Jupiter to determine the mass of Jupiter itself.) In our own Milky Way galaxy, the globular clusters act as such `test particles.' (Although each of them contains tens or hundreds of thousands of stars, they are still tiny compared to the galaxy itself!) A study of their motions will tell us about the total mass of the Milky Way and how that mass is distributed in space.
Globular Star Clusters in Other Galaxies.There are galaxies of other kinds, some of which are not pinwheels like our own Milky Way, but rather elliptical in form. An image of M87, one such galaxy, can be seen on page 630 of your text. That galaxy is at a distance of about forty million light years from us. You should not be surprised to learn that this galaxy also contains globular star clusters -- after all, if our galaxy has them, why not in other galaxies as well? Indeed, M87 is surrounded by more than ten thousand globular clusters. But the enormous distance from us means that even the clusters themselves now appear just as little dots of light, barely detectable through the largest telescopes. You can see some of them as the `little dots' of light surrounding the galaxy itself (page 630). It is sobering to realise that each of those dots is a star cluster containing perhaps one hundred thousand stars. As was true in the case of the Milky Way, these clusters must be on the move, orbiting the parent galaxy; and the orbits can tell us something about the mass and nature of the galaxy itself. One of my research interests, then, is to measure the motions of the star clusters around such galaxies. How can we measure these motions? We certainly cannot see the points of light slowly change position, because a given globular cluster might take one hundred million years to go around the parent galaxy! Instead, we can take the light of a single cluster and spread it out into a spectrum within which we will see the characteristic absorption lines produced by the stars out of which the cluster is made. (The light from all the stars is `mixed together' but the absorption lines still show up.) From the Doppler shifts of the absorption lines - are they shifted to longer or shorter wavelengths than usual? - we can work out the present radial (forward or backward) speed of the cluster. Suppose, for instance, that we discover that all the clusters to the `left' of the galaxy are coming towards us at an average speed of 300 kilometers per second, but that those on the right are moving away at an average speed of comparable size. (This is just like what we discover when we study the spectrum of light from the left and right edges of the sun and deduce that it is rotating, although in the case of the sun the rotation speed is only about 1 km/sec.) Such a discovery in the family of globular clusters would tell us that the whole halo of the galaxy, including its many globular clusters, is rotating; and we would be able to work out the mass of the galaxy itself. There is rather more to this kind of research than my simplified discussion suggests, but the important points are the following: to accomplish these particular research goals, I need to determine the velocities of many of the globular clusters around remote galaxies (studying just one or two of them will not do). and I determine these velocities by studying the spectra of many of the target clusters. This sounds straightforward, and would be for the few very nearest galaxies, but there are difficulties.
The Practical Challenges.As you can see, the globular clusters in these remote galaxies look like faint points of light, just detectable at the limits of the world's large telescopes. But these images are the result of piling up as many photons as we can onto a single spot on the CCD detector, to make a perceptible image of the star cluster. How much harder it will be, then, to get a spectrum for such an object! The light now has to be spread out into its various wavelengths, with the red photons in one location, the blue in another, and so on. Each part of the spectrum will be much fainter than the original image was, and getting enough light to allow a reasonable study of the spectrum will be difficult. In short, we will need to take very long exposures in order to get a good spectrum at all. There was a time, not so many years ago, that I did observing of exactly this sort. We would point the telescope at one of these tiny little points of light and allow the light to pass into a spectrograph. After several slow hours of exposure, we might have collected enough photons at all the different wavelengths to produce a reasonable spectrum, and we would aim the telescope at the next target. In this way, we were able to achieve some modest success on such projects. But what an inefficient use of telescope time! If only it were possible to collect the light of many targets all at once!
The Problem Solved: Multiplexed Observations.As I have described it, the real problem is that the points of light (the target globular clusters) are sprinkled at random around the galaxy of interest and it is not possible to feed the photons from more than one of them into the spectrograph at once. A nice way to handle this is provided by a device called Autofib, at the Anglo-Australian Telescope. (If you are particularly attentive, you may recall that in class I discussed a different technique, but in these notes I thought it would be clearer to describe the use of fibre optics. The principle of 'multiplexing' is the same.) In essence, Autofib is a machine which allows you to place, very precisely, a small 'button' into exactly the location where the light from a single target (a single star cluster) is brought to a focus by the telescope. The button is magnetic so that once it is correctly placed it can be held firmly against a metal plate, just like a refrigerator magnet clings to the door of the fridge. Attached to the button is a long piece of `fibre optic', a thin glassy tube down which light will pass even if the tube is bent or moved around. In this way, the light from the target can be carried off to some other location and allowed to pass into the spectrograph. (Some of you may have seen fibre optics in `new age' lamps.) But here is the punchline: an instrument like Autofib contains hundreds of buttons at the ends of hundreds of fibres, each of which carries the light from a target object into some central location. Although the targets are very widely spread out on the sky, and the buttons need to be widely spaced on the metal plate to sit exactly where the light will land on them, the other ends of the fibres can be closely packed together in such a way that the light from all the fibres can be fed simultaneously into the spectrograph in side-by-side fashion. These side-by-side beams of light pass through a prism and are spread out into a set of side-by-side spectra -- perhaps a hundred or more of them! You can see that this multiplexed mode of operation is literally just as efficient as if we had one hundred telescopes in carrying out my research program in the painfully slow way I used to have to do it, one target at a time. Digression: optical fibres are used in a host of other ways, including medicine. Doctors can insert a narrow fibre optic into the damaged elbow of a baseball pitcher, for instance, to see what is happening in there -- are there floating bone chips, or bone spurs, or tears in the cartilege? The Internet connections on the campus are mostly made using fibre optic links through which modulated optical or infrared signals are passed to transmit data. 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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(Tuesday, 20 October, 2020.)