Physics
Telescopes
Telescopes are optical instruments used to observe distant objects by collecting and focusing light. They work by using lenses or mirrors to magnify and enhance the visibility of celestial bodies such as stars, planets, and galaxies. Telescopes have been crucial in advancing our understanding of the universe and have played a significant role in astronomical research and discovery.
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11 Key excerpts on "Telescopes"
eBook - PDF- Carol Christian, Jean-René Roy(Authors)
- 2017(Publication Date)
- Cambridge University Press(Publisher)
Telescopes Astronomy is essentially a passive science. Aside from exploring the Moon and our nearest planetary neighbors, we cannot make experiments directly, we can only observe and try to understand what we see. And the master tool for making observations is the telescope. 209 How do refracting and reflecting Telescopes differ? Generally speaking, a telescope † is an instrument that enhances the observation of celestial objects by increasing their apparent size and luminosity. This applies to the entire electromagnetic spectrum, from radio waves to gamma rays, including of course the “optical domain,” which covers visible light and radiation in the infrared and ultraviolet. Optical Telescopes can be very diverse, but basically they work just like a photographic camera: they focus the light of a celestial object to form a real image { on film or on an electronic detector. In the visual version, an “eyepiece,” which is basically a magnifying glass, is used to observe the image directly. The terms “refracting” and “reflecting” simply refer to the composition of a telescope’s optics. If lenses are used, the instrument is a refracting telescope; if mirrors are used, it is a reflecting telescope. Which system is better? Well, in general, the larger the telescope – that is, the larger its main mirror or lens – the more light it can collect and the fainter the objects it can observe (Q. 212). Reflecting Telescopes can be built very large indeed, whereas refracting Telescopes have serious size limitations. This is because in a refractor the light must pass unobstructed through lenses, and hence they can be supported only at their rims. The problem is that a lens with a diameter of much more than 1 meter sags under its own weight and this lens deformation quickly becomes optically unacceptable.
eBook - PDF- Andrew Fraknoi, David Morrison, Sidney C. Wolff(Authors)
- 2016(Publication Date)
- Openstax(Publisher)
If a thousand more people were watching, each of them would also catch a bit of each star’s light. Yet, as far as you are concerned, the light not shining into your eye is wasted. It would be great if some of this “wasted” light could also be captured and brought to your eye. This is precisely what a telescope does. The most important functions of a telescope are (1) to collect the faint light from an astronomical source and (2) to focus all the light into a point or an image. Most objects of interest to astronomers are extremely faint: the more light we can collect, the better we can study such objects. (And remember, even though we are focusing on visible light first, there are many Telescopes that collect other kinds of electromagnetic radiation.) Telescopes that collect visible radiation use a lens or mirror to gather the light. Other types of Telescopes may use collecting devices that look very different from the lenses and mirrors with which we are familiar, but they serve the same function. In all types of Telescopes, the light-gathering ability is determined by the area of the device acting as the light-gathering “bucket.” Since most Telescopes have mirrors or lenses, we can compare Chapter 6 Astronomical Instruments 191 their light-gathering power by comparing the apertures, or diameters, of the opening through which light travels or reflects. The amount of light a telescope can collect increases with the size of the aperture. A telescope with a mirror that is 4 meters in diameter can collect 16 times as much light as a telescope that is 1 meter in diameter. (The diameter is squared because the area of a circle equals πd 2 /4, where d is the diameter of the circle.) After the telescope forms an image, we need some way to detect and record it so that we can measure, reproduce, and analyze the image in various ways. Before the nineteenth century, astronomers simply viewed images with their eyes and wrote descriptions of what they saw.
eBook - PDF- Bradley W. Carroll, Dale A. Ostlie(Authors)
- 2017(Publication Date)
- Cambridge University Press(Publisher)
The proper design of a telescope must take into account the principal applications that are intended for the instrument. 6.2 OPTICAL Telescopes In the last section we studied some of the fundamental aspects of optics in the context of astronomical observing. We now build on those concepts to consider design features of optical Telescopes. Refracting Telescopes The major optical component of a refracting telescope is the primary or objective lens of focal length f obj . The purpose of the objective lens is to collect as much light as possible and with the greatest possible resolution, bringing the light to a focus at the focal plane. A photographic plate or other detector may be placed at the focal plane to record the image, or the image may be viewed with an eyepiece, which serves as a magnifying glass. The eyepiece would be placed at a distance from the focal plane equal to its focal length, f eye , causing the light rays to be refocused at infinity. Figure 6.15 shows the path of rays coming from a point source lying off the optical axis at an angle θ . The rays ultimately emerge from the eyepiece at an angle φ from the optical axis. The angular magnification produced by this arrangement of lenses can be shown to be (Problem 6.5) m = f obj f eye . (6.9) Clearly, eyepieces of different focal lengths can produce different angular magnifications. Viewing a large image requires a long objective focal length, in combination with a short focal length for the eyepiece. Recall, however, that the illumination decreases with the square of the objective’s focal length (Eq. 6.8). To compensate for the diminished illumination, a larger-diameter objective is needed. Unfortunately, significant practical limitations exist for the size of the objective 6.2 Optical Telescopes 155 f obj f eye FIGURE 6.15 A refracting telescope is composed of an objective lens and an eyepiece.
eBook - PDF- Michael Seeds, Dana Backman(Authors)
- 2016(Publication Date)
- Cengage Learning EMEA(Publisher)
NASA/ESA/STScI Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 113 Chapter 6 LIGHT AND Telescopes astronomers have solved these problems in a number of ways. Look at Modern Optical Telescopes on pages 114–115 and notice three important points about telescope design and ten new terms that describe optical Telescopes and their operation: 1 Conventional-design reflecting tele-scopes use large, solid, heavy mirrors to focus starlight to a prime focus, or by using a secondary mirror, to a Cassegrain focus (pronounced KASS-uh-grain ). Other Telescopes have a Newtonian focus or a Schmidt-Cassegrain focus. 2 Telescopes must have a sidereal drive to follow the stars. An equatorial mount with motion around a polar axis is the conventional way to provide that 1 2 Radio astronomers face a problem of radio interference comparable to visible light pollution. Weak radio waves from the cosmos are easily drowned out by human-made radio noise— everything from automobiles with faulty spark plugs to poorly designed communication systems. A few narrow radio bands are reserved for astronomy research, but even those are often con-taminated by stray signals. To avoid that noise and have the radio equivalent of a dark sky, astronomers locate radio tele-scopes as far from civilization as possible. Hidden in mountain valleys or in remote deserts, they are able to study the Universe protected from humanity’s radio output. As you have already learned, astronomers prefer to put optical Telescopes on high mountains for several reasons.
eBook - PDFThe Cosmos
Astronomy in the New Millennium
- Jay M. Pasachoff, Alex Filippenko(Authors)
- 2019(Publication Date)
- Cambridge University Press(Publisher)
41 Everybody knows that astronomers use Telescopes, but not every- body realizes that the Telescopes astronomers use are of very differ- ent types. Moreover, very few modern Telescopes are used directly with the eye. In this chapter, we will first discuss the Telescopes that astronomers use to collect visible light, as they have for hundreds of years. Then we will see how astronomers now also use Telescopes to study gamma rays, x-rays, ultraviolet, infrared, and radio waves. We will see how the James Webb Space Telescope, at this writing scheduled for launch in 2021, is expected to transform many areas of astronomy. 3.1 THE FIRST Telescopes FOR ASTRONOMY Over 400 years ago, a Dutch optician put two eyeglass lenses together, and noticed that distant objects appeared closer (that is, they looked magnified). The next year, in 1609, the English scientist Thomas Harriot built one of these devices and looked at the Moon. But all he saw was a blotchy surface, and he didn’t make anything of it. Credit for first using a telescope to make serious astronomical studies goes to Galileo Galilei. In 1609, Galileo heard that a telescope had been made in Holland, so in Venice he made one of his own and used it to look at the Moon. Perhaps as a result of his training in interpreting light and shadow in drawings (he was surrounded by the Renaissance and its developments in visual perspective), Galileo realized that the light and dark patterns on the Moon meant that there were craters on its surface (■ Fig. 3–1). With his tiny Telescopes – using lenses only a few centimeters across and provid- ing, with an eyepiece, a magnification of only 20 or 30, not much more powerful than a modern pair of binoculars and showing a smaller part of the sky– he went on to revolutionize our view of the cosmos, as we will further discuss in Chapter 5. Whenever Galileo looked at Jupiter through his telescope, he saw that it was not just a point of light, but appeared as a small disk.
eBook - PDF- Michael Seeds(Author)
- 2018(Publication Date)
- Cengage Learning EMEA(Publisher)
Because the amount of detail that a telescope can discern is limited generally either by its resolving power or the seeing conditions, very high magnification does not necessarily show more detail. The magnifying power of a telescope equals the focal length of the primary mirror or lens divided by the focal length of the eyepiece. M 5 1 F p F e 2 Example: What is the magnification of a telescope with a primary mirror focal length of 80 cm if it is used with an eyepiece with a focal length of 0.50 cm? Solution: The magnification is 80 divided by 0.50, or 160 power. Radio Telescopes, of course, don’t have eyepieces, but they do have instruments that examine the radio waves focused by the telescope, and each such instrument would, in effect, have its own magnifying power. As was mentioned previously, the two most important powers of the telescope—light-gathering power and resolving power—depend on the diameter of the telescope that is essentially impossible to change. In contrast, you can change the magnifica-tion of a telescope simply by changing the eyepiece. This explains why astronomers describe Telescopes by diameter and not by magnification. Astronomers will refer to a telescope as a 4-meter telescope or a 10-meter telescope, but they would never identify a research telescope as being, say, a 1000-power telescope. 6-3 Observatories on Earth: Optical and Radio The quest for light-gathering power and good resolution explains why nearly all the world’s major observatories are located far from big cities and, especially in the case of optical Telescopes, usually on top of mountains. Astronomers avoid cities because light pollution , which is the brightening of the night sky by light scattered from artificial outdoor lighting, can make it impossible to see faint objects ( Figure 6-11 ). In fact, many resi-dents of cities are unfamiliar with the beauty of the night sky because they can see only the brightest stars.
eBook - PDF- Barbara Ryden, Bradley M. Peterson(Authors)
- 2020(Publication Date)
- Cambridge University Press(Publisher)
6 Astronomical Detection of Light As the previous chapter revealed, there is a wealth of information to be gained from observing the stars. In particular, spectroscopy of stars yields temperatures, elemental abundances, stellar rotation rates, and magnetic field strengths, among other information. While stars are intrinsically luminous, they are (except for the Sun) at extremely large distances, and hence appear to be very faint. The challenge facing astronomers over the centuries has been to collect the faint light from distant stars and other astronomical objects, and preserve and analyze the information it contains. Large Telescopes and sophisticated instrumentation are required. 6.1 THE TELESCOPE AS A CAMERA In many ways, a telescope and its associated instrumentation can be thought of as a camera, albeit one with a very large and unwieldy telephoto lens. A review of how cameras work will thus be useful for understanding the basics of imaging science. The word “camera” is the Latin word for “room,” and is a shortening of the term “camera obscura,” or “darkened room.” A camera obscura, the earliest and simplest of all cameras, was an unlit room with a tiny hole cut in one wall; Figure 6.1 is an illustration of a camera obscura from the sixteenth century, before the invention of the telescope. Because the hole in the wall is small, only light rays headed in a specific direction can reach the far wall of the room. Thus, there is a one-to-one mapping between points on the object (the source of light) and the image. A compact version of the camera obscura is the pinhole camera (Figure 6.2), an opaque box with a tiny pinhole in the middle of one wall. If a permanent record of the projected image is required, an electronic detector or a piece of photographic film can be placed on the wall where the image is located. From Figure 6.2, we see that the image is inverted, and its size is proportional to the length F of the box, called the focal length.
eBook - PDFFundamentals Of Imaging, The: From Particles To Galaxies
From Particles to Galaxies
- Michael Mark Woolfson(Author)
- 2011(Publication Date)
- ICP(Publisher)
Chapter 8 Imaging the Universe with Visible and Near-Visible Radiation 8.1. Optical Telescopes The first practical Telescopes, which could magnify the appearance of distant objects by a factor of three, were certainly the work of Dutch lens-makers and opticians, although to whom the credit as inventor is due is not clear. Strong contenders for this honour are the spec-tacle makers Hans Lippershey, also credited with inventing the com-pound microscope ( § 4.3), and Zacharias Janssen (1580–1638), but the optician Jacob Metius (1571–1630) seems to have been develop-ing the same idea at the same time. The date of the invention is usually quoted as 1608 and barely one year later, in July 1609, the English astronomer, Thomas Harriot (1560–1621) was viewing the Moon through a telescope and making drawings of what he saw. The most famous of the early users of Telescopes was Galileo Galilei (1564–1642; Fig. 8.1) who also observed the Moon in late 1609 and who developed much-improved Telescopes, with magnifications of up to 30. From that time, Telescopes were used for both astronomical and terrestrial observations; Galileo had a profitable sideline selling Telescopes to merchants who could reap financial benefit by spotting returning merchant vessels long before they reached port. Driven by mankind’s strong desire to understand the nature of the Universe, over the centuries that have followed the design and construction of optical Telescopes, especially for astronomical obser-vations, have made great advances, the nature of which will be the topic of this chapter. 147 148 The Fundamentals of Imaging: From Particles to Galaxies Figure 8.1 Galileo Galilei. 8.2. Refracting Telescopes There is a story that Hans Lippershey was led to his discovery of the telescope because children playing with lenses in his workshop accidentally discovered that a large image of an object could be seen if it was viewed through two lenses.
eBook - PDF- D. Scott Birney, Guillermo Gonzalez, David Oesper(Authors)
- 2006(Publication Date)
- Cambridge University Press(Publisher)
When the human eye is the detector at the telescope, an eyepiece is needed to take the converging light rays produced by its objective and make them parallel again. In the simplest terms we may think of an eyepiece as a single lens used as a magnifier. The objective forms an image in the focal plane, and the eyepiece magnifies that image as one might use a hand lens to magnify some small object. The magnification tells us how many times bigger or how many times closer the object appears as compared with the view using the unaided eye. For example, a magnification of 48× means that the object appears 48 times larger or that the object appears as if it were 48 times Optical Telescopes 117 primary mirror secondary mirror tertiary mirror hollow altitude axis Nasmyth focus declination axis Coudé focus polar axis (a) (b) Figure 6.9. (a) Nasmyth focus on an alt–az mounted telescope. (b) Coud´ e focus on an equatorially mounted telescope. Each of these foci is stationary with respect to the telescope mount and thus allows heavy instruments to be attached. closer. But, a telescope does not actually make an object larger or bring it closer. A telescope only magnifies an object’s angular size. This is illustrated in Figure 6.10 for a simple refractor with a positive lens as an eyepiece. The objective is at O, and the eyepiece is at E. The image formed by O is indicated at I. The distances OI and EI are respectively the focal lengths of the objective and the eyepiece. The figure includes several rays that indicate how the objective forms a real image and the eyepiece forms a virtual image. Parallel rays from the object at infinity are refracted by the lens and converge to form part of the image. 118 Observational Astronomy eye E I f o f e O a a b Figure 6.10. Principles of telescope optics. Shown is a simple Keplerian design with two positive lenses.
eBook - PDF- Daniel Malacara(Author)
- 2001(Publication Date)
- CRC Press(Publisher)
In the astronomical telescope the objective is known as the primary mirror, because it has the primary or critical function of collecting the light. The eyepiece becomes the secondary mirror, because it is really not needed as an eyepiece. Its secondary function, in terms of its importance, is to present the image in a suitable form for recording and data processing. In astronomical applications, the objective lenses in the Galilean and Keplerian telescope configuration are replaced with the mirrors of the same power in order to have a large aperture without the associated weight and fabrication challenges. In a mirror system completely equivalent to the refractive version, both the primary mirror and the secondary mirror are located on the optical axis: the sec- 234 Paez and Strojnik ondary mirror obstructs the incident beam and projects the image inside the central section of the primary mirror. This procedure requires the construction of a physical hole in the primary mirror which adds to the risk in the fabrication and the amount of polishing. Astronomical Telescopes generally have an insignificant field of view com- pared with that of a terrestrial telescope. They tend to be designed for a specific spectral region, from ultraviolet (UV) to far-infrared. A most informative review of the requirements for the imaging in UV and some of the solutions to those is offered in the review article by Courtes [28]. The term 'camera' is sometimes used when referring to the telescope with wide-angle capabilities, such as Schmidt camera, with good half-field performance of up to more than 5 degrees. This term has arisen due to the similar requirements of the photographic camera [29]. A majority of the telescope configurations built in recent years take advantage of the excellent performance of the Ritchey-Chretien design. Its resolution is much higher than that actually achieved due to the atmospheric aberration.
- Ian Morison(Author)
- 2014(Publication Date)
- Cambridge University Press(Publisher)
1 1 Telescope and Observing Fundamentals This chapter will discuss the fundamentals of Telescopes and observing which are independent of the telescope type or, as in the case of the contrast of a telescope image, dependent on aspects of the telescope design. Later sections in the chapter will discuss the effects of the atmosphere on image quality due to the ‘atmospheric seeing’ and the faintness of stars that can be seen due to its ‘transparency’. The final sections will give details as to how the stars are charted and named on the celestial sphere and how time, relating to both the Earth (Universal Time) and the stars (sidereal time) are determined. There is one problem that can cause some confusion: the mixing of two units of length: millimetres and inches. Quite a number of US and Russian Telescopes have their apertures defined in inches, and the two common focusers have diameters of 1¼ and 2 inches. However, the focal lengths of Telescopes and eyepieces are always speci- fied in millimetres, as are the apertures of more recent US, Japanese and European Telescopes. In this book I have used the unit which is appropriate and have not tried to convert inches into millimetres when, for example, referring to a 9.25-inch Schmidt- Cassegrain telescope. In calculations and where no specific telescope is referred to, millimetres are always used. 1.1 Telescope Basics Focal Ratio A telescope tube assembly will have an objective of a given diameter, D, and have a focal length F. The ratio of the two, F / D, is called the focal ratio, f (Figure 1.1). Typical focal ratios range from 4 to 15. It is easier to design an optical system with a larger focal ratio, so Telescopes whose focal ratios are towards the lower end may need more complex – and hence expensive – optical designs or, as in the case of a Newtonian telescope with a short focal ratio, additional corrector lenses.
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