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Unformatted text preview: 25/11/2021, 21:58 Lesson 4 – Techniques of Astronomy Lesson 4 – Techniques of Astronomy Site: UNSW Moodle Course: PHYS1160-Introduction to Astronomy - T3 2021 Book: Lesson 4 – Techniques of Astronomy Printed by: Meg Hutchison Date: Thursday, 25 November 2021, 9:58 PM 1/30 25/11/2021, 21:58 Lesson 4 – Techniques of Astronomy Table of contents Techniques of Astronomy 4.1 Telescopes 4.2 The Development of the Telescope in Modern Times 4.3 Observing Techniques - Some History 4.4 Imaging 4.5 Spectra and Spectroscopy 4.6 Spectral Lines 4.7 The Doppler Effect 4.8 Timing Observations 4.9 Observing Modes 4.10 Observatory Sites and Radio Telescopes 4.11 Telescopes in Space 4.12 Particle Astronomy and Gravitational Waves 2/30 25/11/2021, 21:58 Lesson 4 – Techniques of Astronomy Techniques of Astronomy To show the whole lesson as a single page, use the "Print book" command in Moodle. Lesson 4 Overview 3/30 25/11/2021, 21:58 Lesson 4 – Techniques of Astronomy 4.1 Telescopes Telescopes have been the key tool for astronomical research since they were first used by Galileo in 1609. The telescopes used by Galileo and many other early telescopes were what are known as refracting telescopes. Although, today for professional astronomy, the most common form of telescope is what’s called a reflecting telescope. Refracting Telescopes Galileo's Telescope. A refracting telescope is a telescope that uses a glass lens as its main light collecting element. The picture here shows the 1m refracting telescope at the Yerkes observatory in Chicago. 4/30 25/11/2021, 21:58 Lesson 4 – Techniques of Astronomy The 1 m refractor telescope at Yerkes Observatory in Chicago is the largest such telescope. This is the largest refracting telescope to be built, and it illustrates one of the problems with refracting telescopes: this lens has to be mounted at the top of the telescope tube. A glass lens 1 m across is a very heavy piece of glass; It has to be supported only at its edges in order for the starlight to be able to get through, and this means that it’s really not feasible to make refracting telescopes any bigger than about 1 m in aperture, because the lens simply deforms under its own weight. The diagram below shows how the image of a faraway object forms in the most simple refracting telescope. Note the image is upside down. The objective lens defines an aperture of the telescope. The eyepiece magnifies an image. The Fo and Fe are the foci, while fo and fe are focal lengths of the of the objective and eyepiece lenses respectively. Copyright: College Physics · Openstax College Physics Reflecting Telescopes Today the commonest design for telescopes, and particularly large professional astronomy telescopes, is the reflecting telescope. Reflecting telescopes use a curved mirror instead of a lens to focus light. The big advantage of using a mirror is that you can support the mirror at its back as well as at its sides, and this means you can make a much larger mirror and hold it steadily compared with the lens – where you have to support it only at its edges. 5/30 25/11/2021, 21:58 Lesson 4 – Techniques of Astronomy The 8 m Gemini North Telescope at Mauna Kea in Hawaii is a reflector. The telescope shown here is the Gemini North 8m telescope at Mauna Kea in Hawaii. This is a large telescope that Australia has a share in, and so is used by Australian astronomers. Gemini Observatory Intro Here, in the video, we can see the telescope’s mirror. Telescope mirrors are made of large blocks of glass or ceramic material that’s ground down and then polished to a very precise curved shape. Then it is coated with a reflecting layer of silver or aluminium to provide the mirror surface. The main mirror is mounted at the bottom end of the telescope tube, and then there is a smaller secondary mirror at the top that reflects light back down to the instruments which are mounted behind the back of the mirror. The light reaches them through a hole in the primary mirror. Key Features of a Telescope Let’s look at some of the key properties of a telescope. The first is the light collecting power of the telescope, which is determined by the area of its collecting mirror or lens, which in turn depends on the aperture of the telescope – the diameter of the mirror or lens. So a lens that’s twice as big will collect four times as much light because its area is four times larger. And if a bigger telescope collects more light that means we can see fainter stars or galaxies, which is generally what we want to do. 6/30 25/11/2021, 21:58 Lesson 4 – Techniques of Astronomy The other property of the telescope is its resolving power, and resolving power or angular resolution, is the telescope’s ability to see fine detail. You can think of resolving power as the ability to resolve two nearby stars to see them as two separate stars rather than one. This also depends on the telescope’s aperture, so for these reasons bigger is always better when it comes to telescopes. Astronomers are always wanting to build bigger telescopes. 7/30 25/11/2021, 21:58 Lesson 4 – Techniques of Astronomy 4.2 The Development of the Telescope in Modern Times Mount Wilson 100" If we look at some of the history of the development of the telescope, one of the first big telescopes to be built was the 100-inch telescope (the Hooker telescope) at Mount Wilson observatory, which is near Los Angeles. This is a telescope 2.5 m in diameter. it was completed in 1917, and it made many important discoveries. In particular this was the telescope Edwin Hubble used for his work on the distances of galaxies. The 100" (2.5 m) telescope at Mount Wilson Observatory near Los Angeles was completed in 1917. 5 m Hale Telescope This was followed by an even bigger telescope, the 5m telescope at Mount Palomar (also in California) that came in to operation in 1948. This was also used for many major discoveries, and for quite a while it was the largest telescope available. 8/30 25/11/2021, 21:58 Lesson 4 – Techniques of Astronomy The 5 m (200") Hale Telescope at Mount Palomar in California came into operation in 1948. 10 m Keck Telescopes, Mauna Kea Hawaii More recently much bigger telescopes have been built. Now there are a number of telescopes with apertures of 8 to 10 m. Among these are the Keck telescopes which are in Mauna Kea in Hawaii. These are two 10 m telescopes mounted next to each other. These telescopes use an interesting primary mirror design, instead of a single mirror, as we saw for the Gemini telescope, the Keck telescopes use what’s called a segmented mirror. The mirror is built up from many hexagonal segments, which are joined together to effectively make a large mirror 10 m across. The 10 m Keck Telescopes, Mauna Kea Hawaii. 9/30 25/11/2021, 21:58 Lesson 4 – Techniques of Astronomy The segmented mirror of the Keck Telescope. Extremely Large Telescopes Even larger Extremely Large Telescopes (or ELTs) are now being developed. These are telescopes with apertures of 20, 30 or 40 m. The movie below shows some features of the the European Extremely Large Telescope (EELT) with a 39 metre segmented mirror. The pictures below that show the designs for a the Giant Magellan Telescope in which Australia is a partner (a telescope with seven 8.4 metre mirrors giving an effective aperture of 24.5) and the Thirty Metre Telescope (or TMT with a 30m diameter segmented main mirror). ELT trailer An artist's impression of the Giant Magellan Telescope (GMT) - Video of it can be found . 10/30 25/11/2021, 21:58 AAT Lesson 4 – Techniques of Astronomy An artist's impression of the Thirty Metre Telescope (TMT). The largest optical telescope we have here in Australia is the 3.9 m Anglo Australian Telescope (AAT) which is at Siding Spring Observatory in NSW. This is a telescope with a mirror of 3.9 m across. The Anglo Australian Telescope has proven to be a very scientifically productive telescope, and has several excellent astronomical instruments, including the HERMES spectrograph being used to obtain spectra for a million stars in our galaxy, and the Veloce planet hunting instrument led by a team at UNSW. The 3.9 m AAT at Siding Spring in NSW. 11/30 25/11/2021, 21:58 Lesson 4 – Techniques of Astronomy 4.3 Observing Techniques - Some History Visual Observations So, how are telescopes used? In the early days, astronomers would use an eye-piece, they’d look through the telescope and record their observations as drawings and notes. But astronomy hasn’t been done this way for a long time. There were problems with visual observing. The observations were subjective, observers could be influenced by their own pre-conceptions, by optical illusions, and just by poor eyesight. Percival Lowell used visual observations to describe the Canals of Mars. The picture here shows Percivel Lowell making visual observations. This illustrates one of the problems. Percival Lowell observed Mars and he drew a network of fine lines on the surface of Mars, which he interpreted as canals that had been built by intelligent martians and were being used to transport water from the polar caps to irrigate the deserts of Mars. But subsequent observations show these canals just didn’t exist. Photography Photographic techniques for astronomy were developed in the 19th century and became standard for much of the 20th century. Glass photographic plates coated with light sensitive chemical emulsions were used to record images. Photography had lots of advantages. Photographic plates gave an objective record. You could use long exposures – expose the photographic plates for many hours, sometimes even exposures that continued over a number of nights, and these enabled recording much fainter objects than could be seen by the eye. With photographic plates you could photograph a large area of sky in a single exposure, and you could use the plates subsequently to make accurate measurements of the brightness or positions of objects. Electronic Devices In the 1980s electronic detectors largely replaced chemical photographic techniques. These were detectors called Charge Coupled Devices (CCDs) and they’re essentially the same type of sensor as is used in today’s digital cameras. They allowed digital images to be recorded and stored in a computer. These detectors are much more sensitive than photographic plates and allow much fainter objects to be observed. the sensitivity of these detectors can be measured by what’s called their quantum efficiency, which is the fraction of the photons of light that are detected by the sensors. CCDs can be made to have quantum efficiencies of more than 90%, whereas the quantum efficiency of a photographic plate is at best only about 1 or 2%. So, CCDs are much more sensitive devices. 12/30 25/11/2021, 21:58 Lesson 4 – Techniques of Astronomy 4.4 Imaging Imaging When a telescope is used for imaging observations it’s essentially being used as a giant camera, with the mirror of the telescope being used to form an image on the detector. So we could compare a telescope with an ordinary single lens reflex (SLR) camera. An SLR camera might have a standard lens with a focal length of 50 mm, or might use a telephoto lens with a focal length of 300 mm. If we compare with the Keck telescope, when used for imaging, its focal length is 150 m (150,000 mm!). That’s effectively what is being used when we take photographs with the telescope. It is a camera lens, but with a focal length far longer than any lens you can buy for a normal camera. An SLR camera might have a standard lens with a focal length of 50 mm, or a telephoto The Keck telescope, used for imaging, has a focal length of 150 m, or 150,000 mm! lens with a focal length of 300 mm. CCD Cameras The CCD detectors used in astronomy are similar to the detectors used in ordinary cameras. Some of the detectors in astronomy actually use arrays of detectors to obtain images with very large numbers of pixels. So there are cameras being used for astronomy that have a hundreds of mega-pixels. The other main difference between astronomical cameras and those in your phone or SLR, is that they are usually cooled to very low temperatures. This is because even with the largest telescopes, astronomers still work with detections using small numbers of photons. The read-noise and dark current levels in commercial cameras would swamp these very small signals. So astronomical detectors are usually cooled with either liquid nitrogen, or with what are called closed cycle coolers. These reduces the noise and the dark count (the signal present in the absence of light) of the CCD cameras to below the very fain levels astronomers need to detect. An astronomical CCD detector being installed in its dewar. This CCD has 4096 by 4096 pixels (16 megapixels) – RSAA/ANU 13/30 25/11/2021, 21:58 Lesson 4 – Techniques of Astronomy So an astronomical CCD is usually mounted in a dewar, something that allows it to be kept very cold. This picture shows an example of an astronomical CCD detector, in this case one with a CCD array that has 4096 x 4096 pixels – 16 mega pixels – being mounted in the dewar being used for cooling it down to lower temperatures for use. Imaging Observations The sorts of imaging observations that you can make with these systems are useful for many aspects of astronomy. One of the things that can be done is a survey of large areas of sky and try to identify interesting objects for more detailed study. Imaging observations can be used to study the structure of objects such as galaxies and nebulae. The images can be used to measure the brightness of stars and by using images made at different wavelengths through different filters we can measure the colours of stars – this is a technique that is called photometry. You can use the images to measure positions of objects and this is useful, for example, for looking for the motion of the objects that can be due to stellar parallaxes or proper motion. 14/30 25/11/2021, 21:58 Lesson 4 – Techniques of Astronomy 4.5 Spectra and Spectroscopy A spectrum is obtained when light (or other electromagnetic radiation) is split up into its constituent wavelengths. For light this can be done by passing it through a prism or by using a diffraction grating. An example of light passing by prism: An example of light passing by diffraction grating: The tracks of CD act as a diffraction grating, where the white light is separated into colours. There are usually hundreds groves per millimetre on the diffraction gratings. Such methods produce light that is split up according to its wavelength or colours, so we see light ranging from the violet and blue at one end of the spectrum to the red at the other end of the spectrum. Spectrographs Spectroscopy on a telescope is done with an instrument called a spectrograph. 15/30 25/11/2021, 21:58 Lesson 4 – Techniques of Astronomy A diagram showing the operation of a spectrograph. This picture shows the typical layout of a spectrograph. The spectrograph is sitting at the base of the telescope receiving the light from the telescope. And it consists of a slit, which is a narrow opening that allows the light from a single star to pass through into the spectrograph. the light from the slit goes to a collimator which makes the rays of light parallel, and then that parallel beam of light is fed to the diffraction grating, and the dispersed spectrum comes off the diffraction grating and goes to another curved mirror – which is a camera mirror – and focusses the spectrum onto a CCD detector. The CCD then records an image of the spectrum of the light of the star. Spectroscopy Spectroscopy is a very powerful technique in astronomy, it can tell us all sorts of things about objects. One of the things it tells us about is composition, since different elements and molecules have different patterns of spectral lines. This can tell us about the composition of stars, the composition of the atmospheres of planets, and so on. It can also tell us about the motion of objects, because we can use Doppler effect – we can look at whether the lines are red shifted or blue shifted – and that can tell us whether stars are moving toward or away from us. And it can tell us about the physical conditions in an object such as temperature and pressure, because temperature and pressure have effects on the relative strengths and widths of spectral lines. 16/30 25/11/2021, 21:58 Lesson 4 – Techniques of Astronomy 4.6 Spectral Lines The importance of a spectrum is that we can see the signatures of individual atoms and molecules through what are called spectral lines. Spectral lines are discrete wavelengths at which a particular atom or molecule can emit or absorb radiation and they are related to energy levels in the atom or molecule. Electrons in an atom can gain or loose only a specified amount of energy (not any random amounts). This specific amounts of energy are govern by the rules of quantum mechanics. Each atom or molecule has a set of energy levels that an electron can jump onto. As a result of these quantum rules electron can only absorb or emit exactly as much energy as it is needed to account for a difference between two different energy levels. When the electron in an atom jumps between one energy level and another it can emit light of a particular wavelength and it can absorb light of a particular wavelength. When light is emitted this means that the electron lost energy and jumped to a lower energy level. When the light is absorbed the electron gained energy and it is now on a higher energy level. These spectral lines show as the dark or bright lines across the spectrum. The pattern of bright or dark lines in the spectrum is a distinctive fingerprint of a specific element (in the case above we can see hydrogen lines). Let us consider a different types of spectra in more detail. A continuous spectrum is what we see when we look at the spectrum of an ordinary light globe. We can see light of all wavelengths and this is just the thermal radiation that we already looked at, and it obeys Wien's displacements law. As we change the temperature of the globe, we also change the peak of wavelength on the spectrum. We move it further to the blue if we increase temperature. The hot filament of the globe contains densely packed atoms, that are emitting thermal radiation. They are emitting light of all wavelengths because in such a packed environment atoms collide very often and electrons can exists in many different energy states that depend only on the amount of thermal energy available for them, which in turn depends on the temperature of the filament. Another type of spectrum is an emission line spectrum. This is what we observe when we look at the spectrum of a rarified gas that is hot, so it is glowing. The spectrum emits light at a few specific wavelengths. And we look at different gases we see a different spectrum. So for example neon has a different pattern of lines than hydrogen. In a gas of low density collisions are relatively rare. The atoms that gain energy above their normal levels are called excited. This energy could have been gained by higher energy radiation from a nearby source, or just due to these rare collisions. When electrons loose this excess energy, emission lines are observed. Certain energy levels are more likely to be occupied than others in a low density gas (with limited collisions). This is why we are more likely to see emission from these level that translate into specific wavelengths at which lines are observed. The third type of spectrum is an absorption line spectrum that arises where we have a source of the continuous spectrum (like a light globe or the Sun) and we look at it through a cloud of gas. In this case gas absorbs light of specific wavelengths. These wavelengths are the same as the wavelengths of emission lines in a hot gas of the same composition. The result is that we have a spectrum that is a combination of the continuous spectrum from ...
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