Reflecting And Refracting Telescopes

Introduction

The telescope’s invention is often pegged in 1608 with the award of a patent to Lippershey by the States-General, the name for parliament in the Netherlands. However, an Englishman, Thomas Harriott constructed an early, low-power version of the telescope and used it in August 1609 to observe the Moon, at the same time when Galileo presented a similar small instrument to the Venetian Senate. Galileo undertook his own serious observations in October or November of that same year with a larger telescope.

Hans Lippershey , a Dutch eyeglass manufacturer,is most 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 from seeing his device. Despite his reluctance, Metius

was eventually awarded a small sum from parliament, also in 1608, when he applied for a patent on his device a few weeks after Lippershey.However, the Dutch parliament only

allowed 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 DIMENSIONS

Aperture: The diameter of the primary mirror or lens. This determines the limiting magnitude and the angular resolution.

Focal Length: The length it takes the light to converge to a single point. A smaller focal length increases magnification and brightness, whereas a longer focal length has the opposite effect. This makes a difference only for extended objects, not stars.

Magnifying Power: (focal length of eyepiece)/(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 TELESCOPE

The utility of a telescope depends on its ability to collect large quantities of light and to resolve fine details. The brightness of an image is proportional to the area of the light-gathering element, which is proportional to the square of that element’s aperture. The brightness also depends on the area over which the image is spread. This area is inversely proportional to the square of the focal length (f) of the lens. The brightness of the image 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 can be resolved.

TYPES OF TELESCOPES

We will be primarily concerned with optical telescopes which have two basic subdivisions:

Refracting Telescopes:

Refraction works on the principle that light has different “bending” properties in different media (glass,water, air, etc.). Refracting Telescopes use a glass lens to cause the convergence of the light.

Reflecting Telescopes:

Reflecting telescopes use mirrors (concave or convex) to direct incoming light to converge to a point.

REFRACTING TELESCOPES

Small refracting telescopes are used in binoculars, cameras, gunsights, galvanometers, periscopes, surveying instruments, rangefinders, astronomical telescopes, and a great variety of other devices. Parallel or nearly parallel light from the distant object enters from the left, and the objective lens forms an inverted image of it . The inverted image is viewed with the aid of a second lens, called the eyepiece. The eyepiece is adjusted (focused) to form a parallel bundle of rays so that the image of the object may 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 different types, designed 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 the image.

Simplified optical diagram of a refracting telescope.

Refracting optical system used to photograph a star field.

Generally, refracting telescopes are used in applications where great magnification 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 limitations on the size of refracting telescopes.

A refractor lens must be relatively thin to avoid excessive absorption of light in the glass. On the other hand,the lens can be supported only around its edge and thus is subject to sagging distortions that change as the telescope is pointed from the horizon to the zenith; thus its thickness must be great enough to give it mechanical rigidity. An effective compromise between these two demands is extremely difficult, making larger refractors unfeasible. The largest refracting telescope is the 1-m (40-in.) telescope-built over a century ago-at Yerkes

Observatory. This size is about the limit for optical glass lenses.

REFLECTING TELESCOPES

The 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 suffer from the size limitations of refracting telescopes. The mirrors in these telescopes 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 thermal gradients in a mirror, have eliminated the major thermal problems plaguing telescope mirrors. Some advanced reflecting telescopes use segmented mirrors, composed of many separate pieces.

By using a second mirror (and even a third one, in some telescopes), the optical path in a reflector can be folded back on itself, permitting a long focal length to be attained with an instrument housed in a short 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 TELESCOPE

Two fundamentally different types of telescopes exist; both are designed to aid in viewing distant objects, such as the planets in our Solar System. The refracting telescope uses a combination of lenses to form an image, and the reflecting telescope uses a curved mirror and a lens.The lens combination shown in Figure is that of a refracting telescope. Like the compound microscope, this telescope has an objective and an eyepiece. The two lenses are arranged so that the objective forms a real, inverted image of a distant object very near the focal point of the eyepiece. Because the object is essentially at infinity, this point at which I 1 forms is the focal point of the objective. The eyepiece then forms, at I 2, an enlarged, inverted image of the image at I 1. In order to provide the largest possible magnification, the image distance for the eyepiece is infinite. This means that the light rays exit the eyepiece lens parallel to the principal axis, and the 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 of the telescope tube. The angular magnification of the telescope is given by 𝜽/𝜽o, where 𝜽o is the angle subtended by the object at the objective and 𝜽 is the angle subtended by the final image at the viewer’s eye. Consider Figure, in which the object is a very great distance to the left of the figure. The angle 𝜽o (to the left of the objective) subtended by the 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 𝜽o= 𝜽o= -h’/f o where the negative sign indicates that the image is inverted. The angle 𝜽 subtended by the final image at the eye is the same as the angle that a ray coming from the tip of I1 and traveling parallel to the principal axis makes with the principal axis after it passes through the lens. Thus, tan 𝜽=𝜽=h’/fe We have not used a negative sign in this equation because the final image is not inverted; the object creating this final image I2 is I1, and both it and I2 point in the same direction. Hence, the angular magnification of the telescope can be expressed as m= 𝜽/𝜽o=h’/fe /-h’/fo=-fo/fe and we see that the angular magnification of a telescope equals the ratio of the objective focal length to the eyepiece focal length. The negative sign indicates that the image 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 are so far away that they always appear as small points of light no matter how great the magnification. A large research telescope that is used to study very distant objects must have a great diameter to gather as much light as possible. It is difficult and expensive to manufacture large lenses for refracting telescopes. Another difficulty with large lenses is that their weight leads to sagging, which is an additional source of aberration. These problems can be partially overcome by replacing the objective with a concave mirror, which results in a reflecting telescope. Because light is reflected from the mirror and does not pass through a lens, the mirror can have rigid supports on the back side. Such supports eliminate the problem of sagging. Figure shows the design for a typical reflecting telescope. Incoming light rays pass down the barrel of the telescope and are reflected by a parabolic mirror at the base. These rays converge toward point A in the figure, where an image would be formed. However, before this image is formed, a small, flat mirror M reflects the light toward an opening in the side of the tube that passes into an eyepiece. This particular design is said to have a Newtonian focus because Newton developed it. Above figure shows such a telescope. Note that in the reflecting telescope the light never passes through glass (except through the small eyepiece). As a result, problems associated with chromatic aberration are virtually eliminated. The reflecting telescope can be made even shorter by orienting the flat mirror so that it reflects the light back toward the objective mirror and the light enters an eyepiece in a hole in the middle of the mirror.

LIMITATIONS

For many applications, the Earth’s atmosphere limits the effectiveness of larger telescopes. The most obvious deleterious effect is image scintillation and motion, collectively known as poor seeing. Atmospheric turbulence 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 atmosphere glows faintly because of the constant influx of charged particles from the Sun. This airglow adds a background exposure or fog to photographic plates that depends on the length of the exposure and the speed (f/ratio) of the telescope. The combination of the finite size of the seeing disk of stars and the presence of airglow limits the telescope’s 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.