Physics
Reflecting Telescopes
Reflecting telescopes use mirrors to gather and focus light, allowing for the observation of distant objects in space. They are designed with a concave mirror that reflects light to a focal point, where it can be viewed or captured by instruments. This design eliminates chromatic aberration and allows for larger apertures, making reflecting telescopes popular for astronomical research.
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10 Key excerpts on "Reflecting 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 - PDFThe Cosmos
Astronomy in the New Millennium
- Jay M. Pasachoff, Alex Filippenko(Authors)
- 2019(Publication Date)
- Cambridge University Press(Publisher)
This Newtonian telescope (■ Fig. 3–8) is a design still in use by many amateur astronomers. But many telescopes instead use the Cassegrain design, in which a secondary mirror bounces the light back through a small hole in the middle of the primary mirror (■ Fig. 3–9). Note that the hole, or an obstruction of part of the incoming light by the secondary mirror, only decreases the apparent brightness of the object, without altering its shape or producing a hole in it. Every part of the mirror forms a complete image of the object. Spherical mirrors reflect light from their centers back onto the same point, but do not bring parallel light to a good focus (■ Fig. 3–10). This effect is called spherical aberration. We now often use mirrors that are in the shape of a paraboloid (a parabola rotated around its axis, forming a curved surface) since paraboloids bring parallel light near the mirror’s axis to a focus (■ Fig. 3–11). However, light that comes in from a direction substantially tilted rel- ative to the mirror’s axis is still out of focus; thus, simple Reflecting Telescopes generally have a narrow field of view. 46 3 Light and Telescopes: Extending Our Senses Kyle Cudworth, Yerkes Observatory ■ Figure 3–12 The Yerkes refractor (in Williams Bay, Wisconsin), the largest refractor in the world, has a 1.02-m-diameter (about 40 inches) lens. Through the nineteenth century, telescopes using lenses (refract- ing telescopes, or refractors) and telescopes using mirrors (reflect- ing telescopes, or reflectors) were made larger and larger. The pinnacle of refracting telescopes was reached in the 1890s with the construction of a telescope with a lens 40 inches (1 m) across for the Yerkes Observatory in Wisconsin, until 2018 part of the University of Chicago (■ Fig. 3–12). It was difficult to make a lens of clear glass thick enough to sup- port its large diameter; moreover, such a thick lens may sag from its weight, absorbs light, and also suffers from chromatic aberration.
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- 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.
eBook - PDF- Vern Ostdiek, Donald Bord(Authors)
- 2017(Publication Date)
- Cengage Learning EMEA(Publisher)
Telescope mirrors have as their basic functions the gathering of light and the concentration of that light to a point. The ability of a mirror to collect light increases with its surface area. To acquire enough radiation to study faint objects adequately, astronomers have sought to build instru- ments with larger and larger apertures (openings) and, hence, larger light- collecting areas. The quality of the images produced by telescopes is greatly affected by the shapes of the mirrors. The easiest curved mirror to make is one that has a sur- r r face in the shape of portion of a sphere. But such a spherical mirror is not perfect for the task of focusing light rays. Figure 9.26a shows that parallel light rays reflecting off a spherical mirror are not all focused at the same point. An im- age formed using such a mirror will be somewhat blurred. This phenomenon is called spherical aberration. We will see in Section 9.4 that the same thing happens with lenses. As the name implies, spherical aberration is a defect associated with spherical surfaces. A concave mirror in the shape of a parabola does not have this aberration. (You may recall that we saw the parabola in Section 2.7.) A parabolic mirror will concentrate all the rays coming from a distant source at the same point (Figure 9.26b). Thus, the ideal surface for a telescope mirror (or for that matter, reflectors in auto headlamps and household flashlights) Convex mirror Plane mirror Field of view Field of view Figure 9.23 The field of view of a convex mirror is much larger than that of a plane mirror. Figure 9.24 This convex mirror on a bike path at the University of Minnesota allows quick surveillance of a large area that would normally be hidden from view. Secondary mirror Primary mirror F Figure 9.25 This is a basic design of a large astronomical telescope. The large, concave primary mirror and the small, convex secondary mirror combine to focus incoming light at the focal point F.
eBook - PDFSolar Planetary Systems
Stardust to Terrestrial and Extraterrestrial Planetary Sciences
- Asit B. Bhattacharya, Jeffrey M. Lichtman(Authors)
- 2016(Publication Date)
- CRC Press(Publisher)
415 19 The Telescope—The Essential Tool 19.1 Introduction In this chapter, we discuss the telescope evolution, their properties, and function of telescopes from Galileo to NASA’s Great Observatories [1–3]. In addition, we talk about some of the supporting instrumentation as well as some new programs and discoveries. Figure 19.1 illustrates a rough timeline of telescope technology. Space science researchers of NASA control some of the most sophisticated space probes and orbiting telescopes to get images of objects in space which we have also summarized. 19.2 Lippershey and Galileo’s Refracting Telescope In 1608—Hans Lippershey applies for a patent for the first recorded functional refractor telescope. A year later in 1609, Galileo refractor used two lenses for the purpose of concen-trating the light from celestial objects. The system was designed in a manner so that it can deliver more light to the human eye than it can gather on its own. The light was refracted through a lens of spherical shape, forming an image. The spherical primary lens made the images blurry. The lens also split light into colors, producing a fringe of color around bright objects. A simple arrangement of the refracting telescope is shown in Figure 19.2. As shown in Figure 19.2, the parallel light rays from the star are collected by the primary lens which is a convex spherical lens. After passing through the lens light is being con-centrated on a small area on a concave eyepiece (parallel light rays) lens and then to the observer’s eye. 19.2.1 Kepler’s Design for an Improved Refractor A spherical-type lens brings light rays to different focal points, causing blurry images. Kepler constructed a less curved spherical “primary lens” with a larger “focal length” than Galileo had used (1611). The less curved lens would make light rays move farther before they crossed thus bringing the focal points closer together.
eBook - PDF- D. Scott Birney, Guillermo Gonzalez, David Oesper(Authors)
- 2006(Publication Date)
- Cambridge University Press(Publisher)
A compari- son of the basic telescope types is shown in Figure 6.4. The first refractors, beginning with Galileo, were singlets, where the objective consisted of a single converging lens. The next major improve- ments came with the introduction of doublets (two lenses) and then triplets (three lenses), which greatly improved image quality (see below). Because a lens can be supported only on its periphery, it will sag notice- ably at its center if it is made too large. Larger lenses also absorb more Optical telescopes 109 Newtonian Cassegrain Schmidt-Cassegrain Film Schmidt Camera Refractor Maksutov Prime focus Gregorian Figure 6.4. The major types of telescope optical designs. For each diagram, parallel light from a distant object enters from the left. An eyepiece is shown only for the refractor. light, since they must be thicker. For these reasons, the largest refractor ever built is only 40 inches (∼1 m) in diameter (Yerkes Observatory). The first reflecting telescope was a Newtonian design. A Newtonian telescope employs a parabolic shaped objective (primary) mirror and a flat secondary mirror that redirects the light to focus at a place outside the telescope tube and at a right angle with respect to the long axis of the telescope tube (see Figure 6.4). A large class of telescope designs employ a secondary mirror to redirect the light back through a hole in the objective mirror. This has the advantage of reducing the physical length of the telescope by “folding” the light’s optical path. Two related designs are the Classical Cassegrain and Gregorian designs, each of which employs a parabolic shape for the primary mirror. They differ in their secondary mirrors. The Gregorian employs a concave mirror placed beyond the focal point of the primary, while the Classical Cassegrain employs a convex mirror placed inside the focal point of the primary. As a result, a Gregorian telescope pro- duces an erect image, which makes it better suited for terrestrial viewing.
eBook - PDFFundamentals Of Imaging, The: From Particles To Galaxies
From Particles to Galaxies
- Michael Mark Woolfson(Author)
- 2011(Publication Date)
- ICP(Publisher)
8.3. Reflecting Telescopes Imaging can be carried out not only with lenses but also with mir-rors of convex or concave shape. The action of a concave mirror to produce a real image is shown in Fig. 8.4. When a light ray strikes the mirror the reflected ray leaves the mirror making the same angle with the normal to the mirror as the incident ray. This condition is independent of the wavelength of the radiation so there is no chro-matic aberration. If the shape of the mirror is part of a spherical surface then there is spherical aberration just as for a lens. This can be removed by having a parabolic mirror (Fig. 7.7). The final aber-ration, coma, can be reduced by using a subsidiary lens system. The advantage of mirrors over lenses in giving aberration-free optical systems was appreciated quite soon after the telescope was invented but it was not until 1688 that Isaac Newton (Fig. 3.1) con-structed the first telescope using mirrors, of a type now called a Newtonian reflector . The telescope he constructed is illustrated in Fig. 8.5; the small plane mirror prevents some of the incoming light Imaging the Universe withVisible and Near-Visible Radiation 151 Object Image Figure 8.4 An image produced by a concave lens. Objective mirror Plane mirror Distant object Eyepiece First image Figure 8.5 A schematic representation of Newton’s 1688 telescope. from reaching the objective mirror but this makes little difference to the quality of the final image. The mirror material used by Newton was speculum metal , an alloy of two parts of copper to one part of tin. While it has fairly good reflectivity and gives an effective mirror surface it has the disadvan-tage of tarnishing so that it occasionally needs cleaning. The mirror had a diameter of about 1 1 3 in (34 mm) and a focal length of 6 in (152 mm). Although Newton understood the advantage of having a parabolic mirror the one he produced was spherical and hence much easier to grind.
No longer available |Learn more- (Author)
- 2014(Publication Date)
- University Publications(Publisher)
Galileo used this design the following year. In 1611, Johannes Kepler described how a telescope could be made with a convex objective and eyepiece lens and by 1655 astronomers such as Christiaan Huygens were building powerful but unwieldy Keplerian telescopes with compound eyepieces. Hans Lippershey is the earliest person documented to have applied for a patent for the device. Isaac Newton is credited with building the first practical reflector in 1668 with a design that incorporated a small flat diagonal mirror to reflect the light to an eyepiece mounted on the side of the telescope. Laurent Cassegrain in 1672 described the design of a reflector with a small convex secondary mirror to reflect light through a central hole in the main mirror. The achromatic lens, which greatly reduced color aberrations in objective lenses and allowed for shorter and more functional telescopes, first appeared in a 1733 telescope made by Chester Moore Hall, who did not publicize it. John Dollond independently developed achromatic lenses and produced telescopes using them in commercial quantities, starting in 1758. Important developments in Reflecting Telescopes were John Hadley's production of larger paraboloidal mirrors in 1721; the process of silvering glass mirrors introduced by Léon Foucault in 1857; and the adoption of long lasting aluminized coatings on reflector mirrors in 1932. Almost all of the large optical research telescopes used today are reflectors. The era of radio telescopes (along with radio astronomy) was born with Karl Guthe Jansky's serendipitous discovery of an astronomical radio source in 1931. Many types of telescopes were developed in the 20th century for a wide range of wavelengths from radio to gamma-rays. Optical telescopes Invention ________________________ WORLD TECHNOLOGIES ________________________ Optical foundations Optical diagram showing light being refracted by a spherical glass container full of water, from Roger Bacon, De multiplicatione specierum .
eBook - PDFPractical Astronomy
A User-Friendly Handbook for Skywatchers
- H R Mills(Author)
- 2014(Publication Date)
- Woodhead Publishing(Publisher)
Sec. 4.5] Refractors and reflectors 151 Declination axis Equatorial axis Fig. 4.4.1. An equatorial reflector telescope, showing its two axes. The optics for refractors and reflectors are similar in that they each provide a principal focus I as shown in both Fig. 4.5.1 and Fig. 4.5.2. This image is observed in a refractor telescope by means of an eyepiece directly, but in the case of the reflector telescope, the image of I has to be deflected by a small plane mirror M and is observed through an eyepiece at E from the side of the telescope tube. The refractor has an objective lens of focal length F, while the reflector instrument has a parabolic mirror of focal length F. We are familiar with the properties of a para-bolic mirror in reverse as car headlights have a light bulb at the focus of a parabolic mirror which projects a parallel beam of light (see Section 5.12). 152 Light and Basic Optics [Ch.4 Fig. 4.5.1. A refractor showing a degree of colouration, or dispersion, of light first studied by Newton. < / 1 ^4 E ; : : ^ : ---------Y ::.V-Î.V.,-J | B I e Fig. 4.5.2. A reflector, showing its parabolic mirror BC. 4.5.3 Measuring altitudes and azimuths with an alt-az telescope Many small alt-az telescopes, although they are mounted so that they move about a horizontal axis for altitude and a vertical axis for azimuth, rarely seem to have the means of measuring these important parameters, possibly because transforming altitudes and azimuths into Right Ascensions, Declinations and Hour Angles involves the local time and Sidereal Time and resort to spherical trigonometry and tables. With the use of calculators, computers or planispheres with graticules these transformations can readily be made. An alt-az instrument can be used to measure azimuths by mounting it at the cen-tre of an azimuth table which is a circular disc (see Fig. 4.5.3) marked in degrees round the periphery, from 0° at North, to 90° at East, 180° at South and 270° at
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