Wednesday, April 3, 2019
Reflecting And Refracting Telescopes
Reflecting And Refracting TelescopesThe com sneak outs reconcileion is practically pegged in 1608 with the award of a patent to Lippershey by the States-General, the name for fantan in the Netherlands. However, an Englishman, Thomas Harriott constructed an early, low-power version of the orbit and utilize it in terrible 1609 to observe the moon about, at the kindred time when Galileo presented a similar minuscule instrument to the Venetian Senate. Galileo undertook his own serious observations in October or November of that same year with a big ambit.Hans Lippershey , a Dutch eyeglass manufacturer,is most(prenominal) often associated with the invention of the telescope. Lippershey was awarded a patent for his device in October 1608 by the parliament in the Netherlands.Credit for the invention of the telescope is also extended to Jacob Metius, a Dutch optician, though he was reluctant to allow the Dutch parliament to review his patent claim and even prohibited anyone fro m visual perception his device. disrespect his reluctance, Metiuswas eventually awarded a micro sum from parliament, also in 1608, when he applied for a patent on his device a hardly a(prenominal) weeks after Lippershey.However, the Dutch parliament onlyallowed Lippershey to construct a binocular version of his telescope. So, Lippershey is also the inventor of the binocular ( note Galileo Galilei did not invent the telescope)TELESCOPE DIMENSIONSAperture The diameter of the primary reverberate or genus Lens. This determines the confining magnitude and the angular resolution.Focal Length The length it takes the a crystallise to meet to a single point. A smaller central length increases enlargement and swank, whereas a longer focal length has the opposite effect. This makes a variance only for extended headingives, not winds.Magnifying Power (focal length of ocular)/(focal length of telescope).F/Ratio (focal length of telescope)/aperture. A ratio of 8 is written f/8.Focal Plane The plane perpendicular to the point of convergence.PARAMETERS OF TELESCOPEThe public-service corporation of a telescope depends on its faculty to collect large quantities of glint and to resolve fine details. The brightness of an stick out is comparative to the area of the ethereal-gathering element, which is proportional to the square of that elements aperture. The brightness also depends on the area over which the protrude is spread. This area is inversely proportional to the square of the focal length (f) of the genus Lens. The brightness of the look therefore depends on the square of the f/ratio, just as in an ordinary camera. The resolving power of a telescope depends on the diameter of the aperture and the wavelength observed the larger the diameter, the smaller the detail that basin be resolved.TYPES OF TELESCOPESWe will be primarily concerned with optical telescopes which have devil basic subdivisionsRefracting TelescopesRefraction works on the principle t hat light has various bending properties in different media (glass,water, air, etc.). Refracting Telescopes use a glass lens system to cause the convergence of the light.Reflecting TelescopesReflecting telescopes use reverberates (concave or convex) to direct incoming light to converge to a point.REFRACTING TELESCOPESSmall refracting telescopes are used in binoculars, cameras, gunsights, galvanometers, periscopes, analyse instruments, rangefinders, astronomical telescopes, and a prominent variety of other devices. Parallel or nearly analogue light from the distant mark enters from the left, and the accusative lens forms an inverted image of it . The inverted image is viewed with the aid of a min lens, called the eyepiece. The eyepiece is adjusted (focused) to form a parallel bundle of rays so that the image of the object whitethorn be viewed by the eye without strain. The objective lens is typically compound that is, it is made up of two or more pieces of glass, of differe nt types, introductioned to correct for aberrations such as chromatic aberration. To construct a visual refractor, a lens is placed beyond the images formed by the objective and viewed with the eye. To construct a photographic refractor or simply a camera, a photographic plate is placed at the position of theimage.Simplified optical diagram of a refracting telescope.Refracting optical remains used to photograph a star field.Generally, refracting telescopes are used in applications where not bad(p) exaggeration is required, namely, in planetary studies and in astrometry, the measurement of star positions and motions. However, this practice is changing, and the traditional roles of refractors are being carried out effectively by a few reflecting telescopes, in part because of effective adjustations on the surface of refracting telescopes.A refractor lens must be relatively thin to exclude excessive absorption of light in the glass. On the other hand,the lens backside be support ed only around its edge and thereof is subject to sag distortions that change as the telescope is pointed from the horizon to the zenith and past its thickness must be great enough to give it robotic rigidity. An effective compromise between these two demands is extremely onerous, making larger refractors unfeasible. The largest refracting telescope is the 1-m (40-in.) telescope-built over a century ago-at YerkesObservatory. This size is about the limit for optical glass lenses.REFLECTING TELESCOPESThe principal optical element, or objective, of a reflecting telescope is a mirror. The mirror forms an image of a celestial object (Fig. 3) which is then examined with an eyepiece, photographed, or studied in some other manner.Viewing a star with a reflecting telescope. In this configuration, the observer may block the mirror unless it is a very large telescope.Reflecting telescopes generally do not induce from the size limitations of refracting telescopes. The mirrors in these te lescopes can be as thick as necessary and can be supported by mechanisms that prevent sagging and thus inhibit excessive distortion. In addition, mirror materials having vanishingly small expansion coefficients, together with ribbing techniques that allow rapid equalization of thermic gradients in a mirror, have eliminated the major thermal worrys plaguing telescope mirrors. or so advanced reflecting telescopes use segmented mirrors, composed of many separate pieces.By using a second mirror (and even a trine one, in some telescopes), the optical path in a reflecting telescope can be folded back on itself, permitting a long focal length to be attained with an instrument housed in a small tube. A short tube can be held by a smaller mounting system and can be housed in a smaller dome than a long-tube refractor.DERIVATIONS IN TELESCOPETwo essentially different types of telescopes exist both are designed to aid inviewing distant objects, such as the planets in our Solar System. The refracting telescopeuses a combination of lenses to form an image, and the reflecting telescopeuses a curved mirror and a lens.The lens combination shown in Figure is that of a refracting telescope. same the compound microscope, this telescope has an objective and an eyepiece. Thetwo lenses are arranged so that the objective forms a real, inverted image of a distantobject very near the focal point of the eyepiece. Because the object is essentially atinfinity, this point at which I 1 forms is the focal point of the objective. The eyepiecethen forms, at I 2, an enlarged, inverted image of the image at I 1. In order to providethe largest possible exaggeration, the image outperform for the eyepiece is infinite. Thismeans that the light rays exit the eyepiece lens parallel to the principal axis of rotation, andthe image of the objective lens must form at the focal point of the eyepiece. Hence,the two lenses are separated by a distance fo + fe , which corresponds to the length ofthe tele scope tube.The angular elaboration of the telescope is given by /o, where o is the anglesubtended by the object at the objective and is the angle subtended by the netimage at the viewers eye. Consider Figure, in which the object is a very greatdistance to the left of the figure. The angle o (to the left of the objective) subtended bythe object at the objective is the same as the angle (to the right of the objective)subtended by the first image at the objective. Thus,tan o= o= -h/f owhere the negative sign indicates that the image is inverted.The angle subtended by the final image at the eye is the same as the angle that aray coming from the tip of I1 and traveling parallel to the principal axis makes with theprincipal axis after it passes through the lens. Thus,tan ==h/feWe have not used a negative sign in this equation because the final image is notinverted the object creating this final image I2 is I1, and both it and I2 point in thesame direction. Hence, the angular magnific ation of the telescope can be expressed asm= /o=h/fe /-h/fo=-fo/feand we see that the angular magnification of a telescope equals the ratio of the objectivefocal length to the eyepiece focal length. The negative sign indicates that theimage is inverted.When we look through a telescope at such relatively nearby objects as the Moon and the planets, magnification is important. However, individual stars in our galaxy areso removed away that they always appear as small points of light no matter how great themagnification. A large research telescope that is used to study very distant objects musthave a great diameter to gather as much light as possible. It is difficult and expensiveto manufacture large lenses for refracting telescopes. Another difficulty with largelenses is that their saddle leads to sagging, which is an additional source of aberration.These problems can be partially overcome by replacing the objective with a concavemirror, which results in a reflecting telescope. Becau se light is reflected from themirror and does not pass through a lens, the mirror can have rigid supports on theback side. Such supports eliminate the problem of sagging.Figure shows the design for a typical reflecting telescope. Incoming lightrays pass down the barrel of the telescope and are reflected by a parabolic mirror atthe base. These rays converge toward point A in the figure, where an image would beformed. However, before this image is formed, a small, flat mirror M reflects the lighttoward an opening in the side of the tube that passes into an eyepiece. This particulardesign is said to have a Newtonian focus because Newton unquestionable it.Above figure shows such a telescope. Note that in the reflecting telescope the light never passesthrough glass (except through the small eyepiece). As a result, problems associatedwith chromatic aberration are virtually eliminated. The reflecting telescope can bemade even shorter by orienting the flat mirror so that it reflects the li ght back towardthe objective mirror and the light enters an eyepiece in a hole in the middle of themirror.LIMITATIONSFor many applications, the Earths ambience limits the effectiveness of larger telescopes. The most obvious deleterious effect is image scintillation and motion, collectively known as scummy seeing. Atmospheric upthrust produces an extremely rapid motion of the image resulting in a smearing. On the very best nights at ideal observing sites, the image of a star will be spread out over a 0.25-arcsecond seeing disk on an average night, the seeing disk may be between 0.5 and 2.0 arcseconds. It has been demonstrated that most of the air currents that cause poor seeing occur within the observatory buildings themselves. Substantial improvements in seeing have been achieved by modern design of observatory structures.The upper aura glows faintly because of the constant influx of charged particles from the Sun. This airglow adds a background word picture or fog to photograph ic plates that depends on the length of the exposure and the reanimate (f/ratio) of the telescope. The combination of the finite size of the seeing disk of stars and the presence of airglow limits the telescopes ability to see faint objects. One solution is placing a large telescope in orbit above the atmosphere. In practice, the effects of air and light pollution outweigh those of airglow at most observatories in the United States.