Museo Galileo - In depth - Telescope In-depth Index The first Italian word for "telescope" or, more accurately, "spyglass," was cannocchiale. It is a combination of cannone—a generic term for a tube used by scouts to narrow their field of view—and occhiale (eyeglass lens). The instrument was sometimes called tubo ottico (optical tube). The first telescopes comprised two lenses at either end of the tube: the objective lens (objective), facing the object to be observed, and the ocular (or eyelens) near the eye. In Italian, these instruments, especially if large or fitted with complex optical systems, are also referred to as telescopi, from the Greek tēle ("far") and scopeo ("I see"). The term telescopio—from which the English "telescope" is derived—was coined in 1611 by Prince Federico Cesi (1585-1630), founder of the Accademia dei Lincei. Telescopes are classified into two categories: refracting (or dioptric) and reflecting (or catoptric).

Refracting telescope (refractor) Telescopes are called refracting telescopes (or refractors) when the optical system facing the object to be observed, i.e., the objective, is composed of lenses. The oldest telescopes are of this type. In addition to those of Galileo (1564-1642), they include the instruments devised by Torricelli (1608-1647), Campani (1635-1715), Divini (1610-1685), and others. The refractor was gradually perfected to increase its power and reduce the effects of chromatic aberration. Some extremely long models were built. The use of refractors for astronomical observation ceased with the invention of the reflecting telescope, but they remained in use for terrestrial observations. Refractors have traditionally been classified into Galilean telescopes, astronomical telescopes, and terrestrial telescopes. Galilean telescope Characterized by the use of a negative (i.e., diverging) ocular (eyelens).

In its simplest form, comprises only two lenses: the objective lens, which is plano-convex or biconvex, and the ocular, which is plano-concave or biconcave. The distance between the two lenses is equal to the difference between the absolute values of their focal lengths, and the magnifying power is given by the ratio of the focal lengths of the objective and the ocular. The advantage of the Galilean telescope is that is provides an upright image without requiring an erector-lens system. Its disadvantage is the extremely narrow field of view (moreover, in the Galilean design, the use of a diaphragm on the objective reduces not only brightness but also the field of view); far from being uniform, the brightness of the field of view gradually decreases from the center to the edge. Another drawback that spelled its rapid abandonment by astronomers is the impossibility of fitting the focal plane with a micrometer, i.e., a device for determining the angular size of the objects observed. The Galilean telescope owes its name to Galileo, who, while not its inventor, was the first to build copies powerful enough for him to make his famed astronomical discoveries.

It is also known, particularly in the English-speaking world, as the Dutch telescope—a term reminiscent of Dutch tube, an early expression sometimes used to describe this type of instrument. Because of its serious limitations discussed above, the Galilean optical design—with rare exceptions—was abandoned for astronomical use by the mid-seventeenth century. However, until the advent of prism telescopes, it was applied in the construction of binoculars. Achromatic objectives and compound oculars were used to reduce the heavy aberrations of the two-lens system, which seldom exceeded five magnifications so as to avoid too narrow a field of view. The most sophisticated models had an objective actually consisting of a cemented triplet and an ocular also composed of three cemented elements (a biconvex lens placed between two biconcave lenses), with an overall negative optical power. Today the Galilean telescope is used only for low-cost theater binoculars (with apertures of 30-40 mm and magnifying powers of 3-4), and for viewfinders in some models of still cameras. Keplerian telescope Characterized by the use of a positive (i.e., converging) ocular.

In its simplest form, comprises only two plano-convex or biconvex lenses, one serving as the objective, the other as the ocular. To put it succinctly, the ocular functions as a magnifying lens that enlarges the (real) image formed by the objective. The distance between the two lenses is equal to the sum of their focal lengths, and the magnification is given by the ratio of the focal lengths of the objective and the ocular. The astronomical telescope produces upside-down images. This drawback can be eliminated by inserting an erector-lens system; it is largely offset by a somewhat broader and uniformly brighter field of view than the one offered by the Galilean telescope. Unlike the latter, the Keplerian design also allows the focal plane to be fitted with a micrometer, i.e., a device for determining the angular size of the objects observed. The instrument's designation as "astronomical" is due to its widespread use in astronomical observation, for which image inversion is not a significant drawback. Indeed, the Keplerian telescope totally replaced the Galilean model by the mid-seventeenth century.

The instrument is also known as "Keplerian" because its optical arrangement was first described by Johannes Kepler (1571-1630) in his Dioptrice, published in Prague in 1611. In common use, both terms are actually obsolete. Today, nearly all telescopes are fitted with converging oculars, combined, for terrestrial use, with an erector to provide an upright image. As a rule, therefore, the instrument type is specified only in the rare instances where a Galilean system is used (for example, in certain models of theater binoculars). Terrestrial telescope Astronomical or Keplerian telescope, i.e., with a positive ocular, which, being intended for the observation of terrestrial objects, is fitted with an erector to obtain upright images. Reflecting telescope (reflector) Before the introduction of achromatic objectives, telescopes with a single-lens objective were severely affected by chromatic aberration. After some theoretical attempts—primarily by Marin Mersenne (Harmonie Universelle, Paris, 1636)—several authors, beginning in the 1660s, advocated the use of mirrors to compensate for that drawback.

When reflecting surfaces are used, light rays are not refracted, so the images they produce are effectively free of chromatic aberration. Moreover, by choosing appropriate surface forms, one can even eliminate geometrical aberrations and thus obtain optically correct images. Reflecting telescopes, also called reflectors, use as an objective a concave mirror called primary mirror (to form a "real" image, the objective of any optical instrument must always be converging; convex mirrors are diverging) and one or more smaller, suitably shaped mirrors called secondary mirrors. The most common types of reflector use a parabolic-section primary, which, being free of spherical aberration and other geometrical aberrations, provides chromatically and geometrically perfect images. Newtonian telescope Reflecting telescope named after its inventor Isaac Newton (1643-1727), who built it in 1668 (a second copy, which survives, was presented to the Royal Society in 1672). The Newtonian telescope consists of a parabolic primary mirror, which serves as the objective, and a small plane elliptical mirror located on the optical axis and inclined at 45° to the axis.

As the secondary mirror is plane, it does not alter the parabolic's focal length; its purpose is merely to deviate the image laterally, preventing the observer from blocking the line of sight between the object and the primary mirror. In Newtonian instruments, the eyepiece is therefore placed near the upper end of the tube. The secondary mirror masks the central part of the parabolic, but this has no significant effect on image quality provided that the image size—which increases with the relative aperture—does not exceed about 1/3 of the primary mirror's size: beyond that ratio, the images lose sharpness. Before the introduction of silvering, the extremely low reflectiveness of mirrors (~60%) led some manufacturers to use as a deviating element a small right prism (despite its introducing a slight chromatic aberration) instead of the elliptical secondary mirror. One example of this solution is provided by the Newtonian telescope of Tito Gonnella (1794-?) of Leghorn, exhibited in Room XI. Gregorian telescope Reflecting telescope named after its inventor, the Scotsman James Gregory (1638-75), who described it in Optica Promota (London, 1663).

The Gregorian telescope comprises a parabolic primary mirror, serving as the objective, and a concave, elliptical secondary mirror located on the optical axis beyond the primary focal point (to be exact, at a distance slightly greater than the sum of the focal lengths of the two mirrors). In the Gregorian telescope, as in the Cassegrain telescope, the image forms behind the parabolic mirror, in which a hole has been drilled at the appropriate spot, and the eyepiece is therefore placed on the instrument's breech. The Gregorian telescope has the particular property of supplying an upright image and can therefore also be used for terrestrial observations without the aid of an erector. As a result, this combination, now entirely abandoned, was widely disseminated in the mid-18th C., before the introduction of achromatic telescopes. Famous examples include the Gregorian telescopes made by the English optician James Short (1710-68). Among the several hundred that he manufactured was a model with an 18-inch aperture and a 12-foot focal length—enormous dimensions for the time.

Cassegrain telescope Reflecting telescope named after its inventor, the Frenchman Laurent Cassegrain, who published the design in early 1672. The Cassegrain telescope comprises a parabolic primary mirror, serving as the objective, and a hyperbolic-section convex secondary mirror located on the optical axis just in front of the primary focal point. In the Cassegrain telescope, as in the Gregory telescope, the image forms behind the parabolic mirror, in which a hole has been drilled at the appropriate spot, and the eyepiece is therefore placed on the instrument's breech. The hyperbolic secondary mirror modifies the divergence of the light rays from the primary mirror, thus multiplying the mirror's focal length. For this reason, the Cassegrain combination, still widely used, offers the advantage of providing long equivalent focal lengths—and hence high magnifications—with relatively short, compact tubes. Herschelian telescope Optical combination named after the famous astronomer William Herschel (1738-1822), who, among other achievements, discovered the planet Uranus in 1781. Herschel used the system to construct his telescopes, which were gigantic for his day.

The Herschelian telescope simply consists of a parabolic mirror inclined with respect to the optical axis so as to form the image laterally, near the top of the tube, where the observer is located. In other words, the Herschelian telescope is a Newtonian telescope without a deflecting mirror. Before the introduction of silver coating, the reflectiveness of molten-metal mirrors (specula) was extremely low (~60%), prompting Herschel to remove all further reflecting surfaces. Since the parabolic mirror works off-axis, the image suffers from geometrical aberrations. However, for relatively long focal lengths (1/10 – 1/20), such as those used by Herschel, the aberrations were negligible and largely offset the drastic fall in luminosity that the introduction of a second mirror would have generated. Related objects Telescopes and Microscopes A basic refracting telescope is an optical instrument that has two optical elements, an objective and an eyepiece. We have two thin lenses in air. The objective is a large lens that collects light from a distant object and creates an image in the focal plane, which is a faithful representation of the object. The eyepiece is a sophisticated magnifying glass through which we view this image.

Keplerian telescope Galilean telescope A Keplerian telescope has a converging lens eyepiece and a Galilean telescope has a diverging lens eyepiece. The distance between the image and the eyepiece is the sum of the focal lengths of the two lenses. (Remember that for a diverging lens the focal length is negative.) A telescope by itself is not an image forming system. The eye of the observer or the camera attached to the telescope forms the image. We use a telescope to gather light and to increase the angle that a distant object subtends at the eye. If the eye is relaxed for distant viewing, the telescope simply produces an angular magnification. An incident (approximately) parallel beam from a distant source point, which makes an angle θ with respect to the optical axis, emerges as a parallel beam which makes a larger angle θ' with respect to the axis. The transformation matrix for the Keplerian telescope is . Here f1 is the focal length of the objective and f2 is the focal length of the eyepiece. The telescopic system is characterized by M12 = 0.

The angular magnification is M11 = mθ = -f1/f2, it is the negative ratio of the focal length of the objective to the focal lengths of the eyepiece. The image as viewed through the Keplerian telescope is inverted, and the image formed by the objective lens is in the second focal plane of that lens which is also the first focal plane of the eyepiece lens. The image formed by the eyepiece is at infinity. The telescope is not an image forming system until we add another optical system, such as the lens of an eye or a camera. The angular magnification of a Galilean telescope is positive and the image is upright. A Keplerian (a) and a Galilean (b) telescope with the same angular magnification. For a terrestrial telescope build with only converging lenses, one may insert an erector lens between the objective and eyepiece, such that the image formed by the objective acts as an object for the erector lens which in turn forms an inverted image at the first focal point of the eyepiece lens. The matrix of this system will have M12 = 0, and the angular magnification will be positive.

There are two basic types of telescopes: A refractor telescope that uses a large lens to gather light. A reflector telescope that uses a large mirror to gather light. Newtonian Reflecting Telescope Problem: A Galilean telescope has an objective lens with f1 = 20 cm and the eyepiece lens with f2 = -5 cm. The lenses are separated by 15 cm. Calculate the matrix for this system and find mθ. Solution: The lenses are separated by a distance f1 + f2. Therefore M11 = mθ = +4 is the angular magnification. Gathering as much light as possible is the principal function of astronomical telescopes. The larger the area π(D/2)2 of the objective the more light the telescope can collect. To compare the light gathering power (LGP) of two telescopes we compare the areas of their objectives. For the eye the objective is just the pupil, with a diameter of only ~0.8 cm. The resolving power or resolution of a telescope is the smallest angular separation between two objects that can be seen. The smaller the resolving power of the telescope the better is the telescope.

If two objects in the sky have a smaller angular separation than the resolving power of the telescope, the two objects will appear as one. However, if their angular separation is larger than the resolving power, then they will be resolved and appear as two distinct objects. The resolution depends on the wavelength of the light entering the objective as well as on the diameter of the objective. The bigger the diameter of the objective the smaller the resolving power. For a telescope the theoretical intrinsic minimum angular separation is given by θmin = 1.22 λ/D where λ is the wavelength of the light and D the diameter of the objective. For the human eye and visible light D = 0.8 cm and λ = 500 nm, therefore θmin = 7.62*10-5 rad = 4.37*10-3 degree. Earth-bound telescopes rarely reach their theoretical resolution limit because of the blurring of the images due to atmospheric turbulence. The magnifying power MP of a telescope is the (apparent) increase in the size of an object relative to its size when viewed with the unaided eye. MP is equal to the magnitude of the angular magnification mθ = -f1/f2.

The ratio of the diameter of the entrance pupil to the exit pupil is also equal to MP. The magnifying power should be chosen so that the diameter of the exit pupil is approximately equal to the diameter of the pupil of the eye. Microscopes A compound microscope uses a simple combination of two converging lenses to produce a very effective magnifier. A sketch is shown below. The lens closest to the object is known as the objective, and the second lens is the eyepiece. The object is placed between fo and 2fo. An intermediate image is formed by the objective lens near the object focal plane of the eyepiece. The tube length g is the distance between the secondary focal point of the objective and the primary focal point of the eyepiece. The intermediate image serves as the object for the eyepiece. To have a tube length g an object distance xo satisfying 1/xo = 1/fo - 1/(g + fo) is needed. If the intermediate image is in the object focal plane of the eyepiece then the linear magnification of the objective is -g/f0 and and the angular magnification of the eyepiece is dv/fe.

The total magnifying power of the microscope is given by MP = -g dv/(fofe) = -(g/fo)MPeyepiece. Here dv = 25 cm, the near point of the human eye. The focal lengths of both lenses should be extremely short to generate maximum angular magnification. Making a Galilean Telescope Making a Galilean Telescope Galileo: "In this matter, it behooves all those who wish to make such observations to be forewarned. For it is necessary first that they prepare a most accurate galss that shows objects brightly, distinctly, and not veiled by any obscurity, and second that it multiply them at least four hundred times and show them twenty times closer." Sidereus Nuncius tr. Albert Van Helden, p. 38. What is a Galilean telescope? A Galilean telescope is defined as having one convex lens and one concave lens. The concave lens serves as the ocular lens, or the eyepiece, while the convex lens serves as the objective. The lens are situated on either side of a tube such that the focal point of the ocular lens is the same as the focal point for the objective lens.

How does a Galilean telescope work? The Galilean telescope was innovative in that he was the first to expand the range of magnification of the new spyglasses beyond 3X, using his particular set of lenses. In Sidereus Nuncius, Galileo described how these two lenses served to magnify an object. "When there are no glasses in the tube, the rays proceed to the object FG along the straight lines ECF and EDG, but with the glasses put in they proceed along the refracted lines ECH and EDI. They are indeed squeezed together and where before, free, they were directed to the object FG, now they only grasp the part HI" Galileo, Sidereus Nuncius tr. Albert Van Helden, pp. 38-39. Actually, Galileo could not explain how his telescope magnified precisely. He did not understand, as we now know, that the magnification of his telescope can be computed by F/f (see top figure). Increasing the magnification requires lengthening the telescope. Our 10X telescope is about 4 feet long.

From the above picture, you can see that an image, HI, will be viewed upright, making the Galilean telescope useful for terrestrial purposes as well as astronomical. Keplerian telescopes, in contrast, invert the image. What are the disadvantages of a Galilean telescope? The Galilean telescope's biggest disadvantage is its small field of view. A Galilean telescope typically has a field of view of about 15-18 arc minutes. The moon has a diameter of about 30 arc minutes, so the Galilean telescope only reveals approximately one-fourth of the moon's surface at one time. In the Houston skies, a typical field of view has only one star or no stars at all. This makes it very difficult to map a constellation. Increasing the magnification on the Galilean telescope, like all telescopes, reduces the field of view. Perhaps Galileo built a 30X telescope, but it is doubtful that he used much in his observations. The field of view must have been very tiny. How do you make a Galilean telescope?

Building the Telescope Tube: Last Year's Group's Work Parts List (With approximate cost): Cardboard Telescoping Mailing Tube (1), $3 Diameter = 50mm (or 2"), Length = 1100mm (or 143") Should be comprised of an inner and outer tube with closed ends on the outer tube. Concave Convex Lens (the "objective lens") (1), $16 for this and the next lens as a pair. Focal Length = 1350mm (0.75 diopter) Cut to our specification of 49mm diameter. Plano Concave Lens (the "eyepiece") (1) Focal Length = -152mm (-6.6 diopter), Diameter = 49mm Cut to our specification of 49mm diameter. Suggested Tools: Coping Saw Alternatively, any other instrument that will make a relatively clean cut through the mailing tube. Drill (bit sizes discussed below) Super Glue Alternatively, any other kind of glue that will firmly hold the inner and outer mailing tubes together. It must be of a thin consistency.

Greenlee Punch (optional) Instructions: The basic premise of the telescope tube is to align two lenses the appropriate distance from each other. For this telescope, the lenses are a concave convex (one side curved out and the other curved in) and a plano concave (one flat side and one side curved in). The plano concave lens is used as the "eyepiece" with the plano (flat) side facing the eye. The concave convex is used as the "objective lens" that is aligned with the eyepiece and with the convex side facing the sky. Notice that this lens is actually different than the plano convex lens used in the original Galilean telescope, but still gives the same results. The following design uses pieces of the inner tube of the mailing tube to hold the lenses in place inside the outer tube. This is best illustrated in the following diagram, which shows the cross section of the telescope tube: The outer tube of the mailing tube should have a short end that pulls off, and this can be used for the split in the outer tube shown above. This end will be used to hold the eyepiece.

The inner tube must have two pieces (about 1" to 1.5" each) cut off of it that will be used as spacers to hold the objective lens in place. Make these cuts as straight and clean as possible, which will be difficult since the tube is made out of cardboard. A coping saw works pretty well for this. Take the short piece of the outer tube and cut or drill a hole (from 3/16" to 5/16" should be fine) directly in the center of the metal cap on the end. This will be the eyehole. It is important that this hole be as clean as possible (no metal protrusions) so that the the flat side of the eyepiece will fit snugly against the metal cap. An electrician's hole punch or Greenlee Punch works well for this task. If a drill is used, drill with a light pressure, then smooth out the inside surface as much as possible. Place the eyepiece flush (flat side) against the inside of this eyehole. The large piece of the inner mailing tube left will be used to hold this in place. To do this, drill small holes around the outside of the eyepiece tube.

Then, with the eyepiece properly in place, slide the inner tube into it, put glue into the holes, and turn the tube a little bit to spread the glue inside. Hold the tube snugly against the lens inside the cap until the glue dries. Now, put this aside and take the large outer tube and the two spacers cut from the inner tube. Cut the closed end off of the outer tube, then use the other end to mount the objective lens (since that end already has a clean cut). Again, the "drill holes - put in glue" technique will be used to hold the spacers in place. First, check how far the inner spacer needs to be placed inside the tube so that the lens and other spacer will be able to sit inside the tube comfortably. Then drill holes in the outer tube around this area and glue in the spacer as before. After the first spacer is in place and dry, place the concave side of the objective lens flush against it, and put the other spacer snugly against the lens to hold it in place (again using the drill - glue method). Now there are two pieces, each containing one of the lenses.

Slide the mailing tubes together as shown in the drawing above, and the telescope is done. By leaving these two pieces unglued, the telescope may be focused simply by sliding the eyepiece part inside the objective part. After a desired magnification/focus is found, the two pieces may be permanently attached (or some tape will give a semi-permanent attachment). Two images of a finished tube are shown below. The first picture shows the telescope tube from the objective side while the second image shows the ocular end of the tube: Making the Mounting: This Year's Work For the first half of the semester, we used the mountings built by last year's group. It looked like this: This image was produced by the star jumping in the field of view. We tried to have one person hold the telescope tube steady, but it takes very little movement to cause a star to move across the field of view when the field of view is only about 15 arc minutes. Moreover, our telescope would blow over often and required one person to hold it as still as possible, but this never really worked very well. So, we built a new mounting and stars looked like this, with very little distortion.

The new mounting was constructed on a Saturday morning and afternoon, based on plans by Tom Williams. It looks like this: This telescope mounting is composed of a base a support box for the telescope tubing and its bracket assembly a bracket assembly encasing the telescope tubing with trunnion bearings which fit into the trunnion notch plates attached to the inside of the support box Parts List: Polybutylene Pipe Diameter = 4", Length = approximately 5' These can vary. The length is approximately equal to the height that you desire. Polybutylene pipe for the trunnion (swivel) for the telescope tubing Diameter = 2", Length = approximately 2 pieces about 1" long This is glued to the tube bracket assembly with Wood glue Plastic furniture tacks These are used to stick into the bottom of the legs of chairs to protect the floor. They provide the foundation for the trunnions to turn. Wing Screws These are used for the end plates of the tube bracket assembly. They allow for the different telescopes to be used, just by unscrewing these and inserting a different telescope. Plastic floor flanges which can hold the big tubing and be nailed to the wood Nails, Bolts, Screws A whole lot of plywood!

Suggested Tools: work bench to cut out various shapes of wood Instructions: The base is constructed by attaching the plastic holder to a square piece of plywood and then adding four legs which extend outward. The big plastic tubing can be glued into the holder. At the top of the tubing, another plastic holder is put on, but not glued, so that the entire top of the mounting can be removed for adjustments and travel purposes. The top plastic holder is attached to a round piece of plywood which has a whole cut out of the middle and a screw inserted there. The support box will be attached here and can swivel 360 degrees. The support box is constructed much like a shoe box except that one side is missing. The bottom has a rectangle cut out of end (see where Travis's hand is). This allows the bracket assembly to rotate all the way to a position perpendicular to the ground. Also, on the end of the box opposite Travis, cut out a semicircle to allow the telescope to rotate all the way to a position parallel to the ground. The bracket assembly encases the telescope. It looks like this: It is made up of a rectangular three-sided box with end plates on either end (bottom).

The end plates (at the top) have wing screws which allow the top half of the end plate to be removed from the tube bracket assembly, releasing the telescope. This is useful for making adjustments to the telescope itself or for using a different powered telescope in the same mounting. Notice also the round wooden pieces attached to either side of the support box in the above picture. Wood glue is applied around the circumference of the circle and the smaller plastic tubes, only about an inch in length, are squeezed on. These are the trunnions. The unique part of the mounting is the trunnion. Trunnion notch plates are attached to the insides of the left and right walls (when looking through the telescope) of the support box. Plastic Furniture tacks are stuck into the insides of the triangular cut-out and support the trunnions. These tacks can be moved at any time to increase or decrease friction so that the trunnions will move smoothly, but also not allow the tube bracket assembly (and thus the telescope) to slip. The tube bracket assembly, telescope, and support box combined can be removed from the base. The result is that our telescope moves freely through an entire hemisphere giving us the ability to look at anything in the sky.

It was with this mounting that we were finally able to recreate some of Galileo's observations. If you would like more information on how to build a relatively cheap mounting for your Galilean telescope, just e-mail Tom Williams or Jessica Williams . Galileo's telescope - How it works How it works Home In any telescope system the objective — even when made up of several elements, both converging and diverging — must always be converging as a whole. The Galilean and the Keplerian telescopes thus differ only in the eyepiece, which is diverging in the former, converging in the latter. The Galilean telescope (fig. 1) consists of a converging lens (plano-convex or biconvex) serving as objective, and a diverging lens (plano-concave or biconcave) serving as eyepiece. The eyepiece is situated in front of the focal point of the objective, at a distance from the focal point equal to the focal length of the eyepiece.

Since converging lenses are conventionally positive (or of positive optical power) and diverging ones negative (or of negative optical power), we can also say that the distance between the objective and the eyepiece is equal to the algebraic sum of their focal lengths. The negative eyepiece intercepts the converging rays coming from the objective, rendering them parallel and thus forming, to the infinite (afocal position), a virtual image, magnified and erect. The magnification of the system is determined by the ratio between the focal length of the objective and that of the eyepiece. The Galilean telescope, although it furnishes erect images with the aid of erector devices, has the severe drawback of an extremely narrow field of view (which makes it, in practice, usable only for magnifications up to around thirty). The principle of operation of the Keplerian telescope (fig. 2) is relatively simple. The objective forms a real image, diminished in size and upside-down, of the object observed. The eyepiece — which, consisting of a converging lens with short focal length, is actually a magnifying lens — enlarges the image formed by the objective.

The image observed is however upside-down, so that the Keplerian telescope, at least for terrestrial use, must be fitted with some kind of erector device which, by inverting the image again, erects it. But this disadvantage is amply compensated for by a much greater and more evenly illuminated field of view than that of the Galilean telescopes. Optical Instruments Telescopes A basic refracting telescope is an optical instrument that has two optical elements, an objective and an eyepiece. We have two thin lenses in air. The objective is a large lens that collects light from a distant object and creates an image in the focal plane, which is a faithful representation of the object. The eyepiece is a sophisticated magnifying glass through which we view this image. Keplerian telescope Galilean telescope A Keplerian telescope has a converging lens eyepiece and a Galilean telescope has a diverging lens eyepiece. The distance between the image and the eyepiece is the sum of the focal lengths of the two lenses. (Remember that for a diverging lens the focal length is negative.) A telescope by itself is not an image forming system.

The eye of the observer or the camera attached to the telescope forms the image. We use a telescope to gather light and to increase the angle that a distant object subtends at the eye. The amount of light a telescope can gather increases with the square of the diameter of the objective. So a telescope with a 10 inch objective gathers 4 times as much light from a faint, distant source than a telescope with a 5 inch objective. If the eye is relaxed for distant viewing, the telescope simply produces an angular magnification equal to the ratio of the focal length of the objective to the focal length of the eyepiece, Mq = -fo/fe. An incident (approximately) parallel beam from a distant source point, which makes an angle q with respect to the optical axis, emerges as a parallel beam which makes a larger angle q' = -(fo/f)q with respect to the axis. If Mq is negative, a viewer will see an inverted image. (a) A Keplerian (a) and a Galilean (b) telescope with the same angular magnification. Links There are two basic types of telescopes: A refractor telescope that uses a large lens to gather light.

A reflector telescope that uses a large mirror to gather light. Newtonian Reflecting Telescope The resolving power or resolution of an optical instrument is its ability to separate the images of two objects, which are close together. Some binary stars in the sky look like one single star when viewed with the naked eye, but the images of the two stars are clearly resolved when viewed with a telescope. Many factors such a imperfect optics and atmospheric turbulence can limit the resolving power of a telescope. But diffraction (the spreading of light around the edges of obstacles) determines the intrinsic minimum angular separation of two objects which can just be resolved. It is given by qmin = 1.22l/D, where D is the diameter of the objective and l is the wavelength of the light. Microscopes A compound microscope uses a simple combination of two converging lenses to produce a very effective magnifier . A sketch is shown below. The lens closest to the object is known as the objective, and the second lens is the eyepiece. The object is placed between fo and 2fo. An intermediate image is formed by the objective lens near the object focal plane of the eyepiece.

The tube length g is the distance between the secondary focal point of the objective and the primary focal point of the eyepiece. The intermediate image serves as the object for the eyepiece. The total angular magnification or magnifying power of the microscope is given by MP = -(g/fo)MPeyepiece. A label such as "2X" or "10X" printed on the eyepiece belonging to a microscope or telescope indicates the angular magnification when the object is positioned at the focal length fe of the eyepiece, such that the virtual image is infinitely far away. MPeyepiece is given by MPeyepiece = (25 cm)/fe. The relaxed normal eye can then form an image on the retina. The focal lengths of both lenses should be extremely short to generate maximum angular magnification. Illustration of the angular magnification of a microscope Link: For more information, explore the anatomy of a microscope ! Laboratory Exercise 4 In this laboratory exercise students will build a simple Keplerian telescope, a simple Galilean telescope, and a compound microscope.

The Galileo Project | Science | Telescope Johannes Hevelius observing with one of his telescopes [click for larger image] The Telescope The telescope was one of the central instruments of what has been called the Scientific Revolution of the seventeenth century. It revealed hitherto unsuspected phenomena in the heavens and had a profound influence on the controversy between followers of the traditional geocentric astronomy and cosmology and those who favored the heliocentric system of Copernicus. It was the first extension of one of man's senses, and demonstrated that ordinary observers could see things that the great Aristotle had not dreamed of. It therefore helped shift authority in the observation of nature from men to instruments. In short, it was the prototype of modern scientific instruments. But the telescope was not the invention of scientists; rather, it was the product of craftsmen. For that reason, much of its origin is inaccessible to us since craftsmen were by and large illiterate and therefore historically often invisible. Although the magnifying and diminishing properties of convex and concave transparent objects was known in Antiquity, lenses as we know them were introduced in the West [1] at the end of the thirteenth century.

Glass of reasonable quality had become relatively cheap and in the major glass-making centers of Venice and Florence techniques for grinding and polishing glass had reached a high state of development. Now one of the perennial problems faced by aging scholars could be solved. With age, the eye progressively loses its power to accommodate, that is to change its focus from faraway objects to nearby ones. This condition, known as presbyopia, becomes noticeable for most people in their forties, when they can no longer focus on letters held at a comfortable distance from the eye. Magnifying glasses became common in the thirteenth century, but these are cumbersome, especially when one is writing. Craftsmen in Venice began making small disks of glass, convex on both sides, that could be worn in a frame--spectacles. Because these little disks were shaped like lentils, they became known as "lentils of glass," or (from the Latin) lenses. The earliest illustrations of spectacles date from about 1350, and spectacles soon came to be symbols of learning. The Spectacle Vendor by Johannes Stradanus, engraved by Johannes Collaert, 1582 [click for larger image] These spectacles were, then, reading glasses.

When one had trouble reading, one went to a spectacle-maker's shop or a peddler of spectacles (see figs. 2 and 3) and found a suitable pair by trial and error. They were, by and large, glasses for the old. spectacles for the young, concave lenses [2] that correct the refractive error known as myopia, were first made (again in Italy) in the middle of the fifteenth century. So by about 1450 the ingredients for making a telescope were there. The telescopic effect can be achieved by several combinations of concave and convex mirrors and lenses. Why was the telescope not invented in the fifteenth century? There is no good answer to this question, except perhaps that lenses and mirrors of the appropriate strengths were not available until later. In the literature of white magic, so popular in the sixteenth century, there are several tantalizing references to devices that would allow one to see one's enemies or count coins from a great distance. But these allusions were cast in obscure language and were accompanied by fantastic claims; the telescope, when it came, was a very humble and simple device.

It is possible that in the 1570s Leonard and Thomas Digges in England actually made an instrument consisting of a convex lens and a mirror, but if this proves to be the case, it was an experimental setup that was never translated into a mass-produced device. [3] The earliest known illlustration of a telescope. Giovanpattista della Porta included this sketch in a letter written in August 1609 [click for larger image] The telescope was unveiled in the Netherlands. In October 1608, the States General (the national government) in The Hague discussed the patent applications first of Hans Lipperhey of Middelburg, and then of Jacob Metius of Alkmaar, on a device for "seeing faraway things as though nearby." It consisted of a convex and concave lens in a tube, and the combination magnified three or four times. [4] The gentlemen found the device too easy to copy to award the patent, but it voted a small award to Metius and employed Lipperhey to make several binocular versions, for which he was paid handsomely.

It appears that another citizen of Middelburg, Sacharias Janssen had a telescope at about the same time but was at the Frankfurt Fair where he tried to sell it. Galileo's telescopes [click here for larger image] The news of this new invention spread rapidly through Europe, and the device itself quickly followed. By April 1609 three-powered spyglasses could be bought in spectacle-maker's shops on the Pont Neuf in Paris, and four months later there were several in Italy. (fig. 4) We know that Thomas Harriot observed the Moon with a six-powered instrument early in August 1609. But it was Galileo who made the instrument famous. He constructed his first three-powered spyglass in June or July 1609, presented an eight-powered instrument to the Venetian Senate in August, and turned a twenty-powered instrument to the heavens in October or November. With this instrument (fig. 5) he observed the Moon, discovered four satellites of Jupiter, and resolved nebular patches into stars. He published Sidereus Nuncius in March 1610. Verifying Galileo's discoveries was initially difficult.

In the spring of 1610 no one had telescopes of sufficient quality and power to see the satellites of Jupiter, although many had weaker instruments with which they could see some of the lunar detail Galileo had described in Sidereus Nuncius. Galileo's lead was one of practice, not theory, and it took about six months before others could make or obtain instruments good enough to see Jupiter's moons. With the verification of the phases of Venus by others, in the first half of 1611, Galileo's lead in telescope-making had more or less evaporated. The next discovery, that of sunspots, was made by several observers, including Galileo, independently. ??? A typical Galilean telescope with which Jupiter's moons could be observed was configured as follows. It had a plano-convex objective (the lens toward the object) with a focal length of about 30-40 inches., and a plano-concave ocular with a focal length of about 2 inches. The ocular was in a little tube that could be adjusted for focusing. The objective lens was stopped down to an aperture of 0.5 to 1 inch.

, and the field of view was about 15 arc-minutes (about 15 inches in 100 yards). The instrument's magnification was 15-20. The glass was full of little bubbles and had a greenish tinge (caused by the iron content of the glass); the shape of the lenses was reasonable good near their centers but poor near the periphery (hence the restricted aperture); the polish was rather poor. The limiting factor of this type of instrument was its small field of view--about 15 arc-minutes--which meant that only a quarter of the full Moon could be accommodated in the field. Over the next several decades, lens-grinding and polishing techniques improved gradually, as a specialized craft of telescope makers slowly developed. But although Galilean telescopes of higher magnifications were certainly made, they were almost useless because of the concomitant shrinking of the field. As mentioned above, a the telescopic effect can be achieved with different combinations of lenses and mirrors. As early as 1611, in his Dioptrice, Johannes Kepler had shown that a telescope could also be made by combining a convex objective and a convex ocular.

He pointed out that such a combination would produce an inverted image but showed that the addition of yet a third convex lens would make the image erect again. This suggestion was not immediately taken up by astronomers, however, and it was not until Christoph Scheiner published his Rosa Ursina in 1630 that this form of telescope began to spread. In his study of sunspots, Scheiner had experimented with telescopes with convex oculars in order to make the image of the Sun projected through the telescope erect. [5] But when he happened to view an object directly through such an instrument, he found that, although the image was inverted, it was much brighter and the field of view much larger than in a Galilean telescope. Since for astronomical observations an inverted image is no problem, the advantages of what became known as the astronomical telescope led to its general acceptance in the astronomical community by the middle of the century. The Galilean telescope could be used for terrestrial and celestial purposes interchangeably. This was not true for the astronomical telescope with its inverted image. Astronomers eschewed the third convex lens (the erector lens) necessary for re-inverting the image because the more lenses the more optical defects multiplied.

In the second half of the seventeenth century, therefore, the Galilean telescope was replaced for terrestrial purposes by the "terrestrial telescope," which had four convex lenses: objective, ocular, erector lens, and a field lens (which enlarged the field of view even further). Hevelius's 60- and 140-foot telescopes (Machina Coelestis, 1673) [click for larger image] With the acceptance of the astronomical telescope, the limit on magnification caused by the small field of view of the Galilean telescope was temporarily lifted, and a "telescope race" developed. Because of optical defects, the curvature of lenses had to be minimized, and therefore (since the magnification of a simple telescope is given roughly by the ratio of the focal lengths of the objective and ocular) increased magnification had to be achieved by increasing the focal length of the objective. Beginning in the 1640s, the length of telescopes began to increase. From the typical Galilean telescope of 5 or 6 feet in length, astronomical telescopes rose to lengths of 15 or 20 feet by the middle of the century.

A typical astronomical telescope is the one made by Christiaan Huygens, in 1656. It was 23 feet long; its objective had an aperture of several inches, it magnified about 100 times, and its field of view was 17 arc-minutes. Aerial telescope (Christiaan Huygensm Astroscopium Compendiaria,1684) [click for larger image] Telescopes had now again reached the point where further increases in magnification would restrict the field of view of the instrument too much. This time another optical device, the field lens came to the rescue. Adding a third convex lens--of appropriate focal length, and in the right place--increased the field significantly, thus allowing higher magnifications. The telescope race therefore continued unabated and lengths increased exponentially. By the early 1670s, Johannes Hevelius had built a 140-foot telescope. But such long telescopes were useless for observation: it was almost impossible to keep the lenses aligned and any wind would make the instrument flutter. After about 1675, therefore, astronomers did away with the telescope tube.

The objective was mounted on a building or pole by means of a ball-joint and aimed by means of a string; the image was found by trial and error; and the compound eyepiece (field lens and ocular), on a little stand, was then positioned to receive the image cast by the objective. Such instruments were called "aerial telescopes." Although some discoveries were made with these very long instruments, this form of telescope had reached its limits. By the beginning of the eighteenth century very long telescopes were rarely mounted any more, and further increases of power came, beginning in the 1730s, from a new form of telescope, the reflecting telescope. Since it was known that the telescopic effect could be achieved using a variety of combinations of lenses and mirrors, a number of scientists speculated on combinations involving mirrors. Much of this speculation was fueled by the increasingly refined theoretical study of the telescope. In his Dioptrique, appended to his Discourse on Method of 1637, René Descartes addressed the problem of spherical aberration, already pointed out by others. In a thin spherical lens, not all rays from infinity--incident parallel to the optical axis--are united at one point.

Those farther from the optical axis come to a focus closer to the back of the lens than those nearer the optical axis. Descartes had either learned the sine law of refraction from Willebrord Snell (Snell's Law) [6] or had discovered it independently, and this allowed him to quantify spherical aberration. In order to eliminate it, he showed, lens curvature had to be either plano-hyperboloidal or spherico-ellipsoidal. His demonstration led many to attempt to make plano-hyperboloidal objectives, [7] an effort which was doomed to failure by the state of the art of lens-grinding. Others began considering the virtues of a concave paraboloidal mirror as primary receptor: it had been known since Antiquity that such a mirror would bring parallel incident rays to a focus at one point. Newton's reflecting telescope (1671) [click for larger image] A second theoretical development came in 1672, when Isaac Newton published his celebrated paper on light and colors. Newton showed that white light is a mixture of colored light of different refrangibility: every color had its own degree of refraction.

The result was that any curved lens would decompose white light into the colors of the spectrum, each of which comes to a focus at a different point on the optical axis. This effect, which became known as chromatic aberration, resulted in a central image of, e.g., a planet, being surrounded by circles of different colors. Newton had developed his theory of light several years before publishing his paper, when he had turned his mind to the improvement of the telescope, and he had despaired of ever ridding the objective of this defect. He therefore decided to try a mirror, but unlike his predecessors he was able to put his idea into practice. He cast a two-inch mirror blank of speculum metal (basically copper with some tin) and ground it into spherical curvature. He placed it in the bottom of a tube and caught the reflected rays on a 45ï¿½ secondary mirror which reflected the image into a convex ocular lens outside the tube (see fig. 12). He sent this little instrument to the Royal Society, where it caused a sensation; it was the first working reflecting telescope. But the effort ended there.

Others were unable to grind mirrors of regular curvature, and to add to the problem, the mirror tarnished and had to be repolished every few months, with the attending danger of damage to the curvature. Hevelius's rooftop observatory, (Machina Coelestis, 1673) [click for larger image] The reflecting telescope therefore remained a curiosity for decades. In second and third decades of the eighteenth century, however, the reflecting telescope became a reality in the hands of first James Hadley and then others. By the middle of the century, reflecting telescopes with primary mirrors up to six inches in diameter had been made. It was found that for large aperture ratios (the ratio of focal length of the primary to its aperture, as the f-ratio in modern cameras for instance), f/10 or more, the difference between spherical and paraboloidal mirrors was negligible in the performance of the telescope. In the second half of the eighteenth century, in the hands of James Short and then William Herschel, the reflecting telescope with parabolically ground mirrors came into its own. Notes: [1] They may have developed independently in China.

[2] Note that the word lens was used only to denote convex lenses until the end of the seventeenth century. [3] The claim for an "Elizabethan telescope" has recently been made by Colin Ronin, who has demonstrated an instrument based on the writings of Thomas Digges and William Bourne. [4] Their optical system and magnification was the same as our traditional opera glasses [5] The Galilean telescope produces an erect image of an object viewed directly but an inverted image of a projected object; by substituting a convex for the concave ocular, this situation is reversed. [6] The ratio of the sines of the angles of incidence and refraction is constant. [7] The effect is most apparent for the objective; spherical aberration in the ocular affects the image much less. Sources: For the invention of spectacles, see Edward Rosen, "The Invention of Eyeglasses," Journal for the History of Medicine and Allied Sciences, 11(1956):13-46, 183-218.

The appearance of spectacles with concave lenses is discussed in Vincent Ilardi, "Eyeglasses and Concave Lenses in Fifteenth-Century Florence and Milan: New Documents," Renaissance Quarterly 29(1976):341-360. The entire problem of the invention of the telescope is discussed in Albert van Helden, The Invention of the Telescope, in Transactions of the American Philosophical Society, 67, no. 4 (1977). See also Van Helden, "The `Astronomical Telescope,' 1611-1650," Annali dell'Istituto e Museo di Storia della Scienza di Firenze, 1, no. 2 (1976):13-36; and "The Development of Compound Eyepieces, 1640-1670," Journal for the History of Astronomy, 8(1977):26-37. The most convenient source for information on the general development of the telescope is Henry King, The History of the Telescope (London: Griffin, 1955).

Sources: Top Image: Johannes Hevelius observing with one of his telescopes (Selenographia, 1647). Optical Society of America: Exploring the Science of Light - Terms/Definitions: Optical Instruments and Systems Optical Instruments and Systems A Alignment telescope ï¿½ An optical tooling instrument consisting of an objective lens, a focusing lens, an optical micrometer, a reticle and an eyepiece. The alignment telescope projects a precise line of sight to which a number of targets at varying distances may be aligned accurately. Anomaloscope ï¿½ An optical instrument that uses a yellow light of varying intensity with red and blue lights of fixed intensity to test for colorblindness. Autocollimator ï¿½ A single instrument combining the functions of a telescope and a collimator to detect small angular displacements of a mirror by means of its own collimated light. It consists of a lens with an illuminated reticle. The reticle usually carries several parallel lines subtending known angles at the lens. An electronic autocollimator provides a continuous output voltage, the sign and amplitude of which are related to the angular displacement of the mirror.

Autocollimator Autostigmatic microscope ï¿½ The study of the interactions between sound waves and light in a solid medium. B Barcode scanner ï¿½ An optical scanning device designed to read information printed in the form of bars of different size by detection and processing of the varying reflectivity of light in the bar code. Beamsplitter ï¿½ An optical device for dividing a beam into two or more separate beams. A simple beamsplitter may be a very thin sheet of glass inserted in the beam at an angle to divert a portion of the beam in a different direction. A more sophisticated type consists of two right-angle prisms cemented together at their hypotenuse faces. The cemented face of one prism is coated, before cementing, with a metallic or dielectric layer having the desired reflecting properties, both in the percentage of reflection and the desired color. In a color television camera, for example, a three-way beamsplitting prism is employed in which multilayer films are deposited on the interfaces to divert red and green light to two vidicons, leaving the blue image to pass through to the third vidicon tube.

Beamsplitter Binocular microscope ï¿½ A microscope designed with two eyepieces (oculars), permitting the use of both eyes when viewing through the instrument. Binocular microscope Borescope ï¿½ A device for the internal inspection of hard-to-get-at mechanical parts, such as rifle barrels, sewer pipes, oil wells, or gas mains. The long narrow tube used contains a telescope system with as many relay lenses as necessary. A lamp located in front illuminates the inside of the bore, and the inclusion of a small 45ï¿½ mirror often permits detailed examination of the tube wall. Some small borescopes and most types of medical endoscopes are now equipped with a fiber optic bundle, for image and/or illumination transfer. C Camera ï¿½ A light-tight box that receives light from an object or scene and focuses it to form an image on a light-sensitive material or a detector. The camera generally contains a lens of variable aperture and a shutter of variable speed to precisely control the exposure. In an electronic imaging system, the camera does not use chemical means to store the image, but takes advantage of the sensitivity of various detectors to different bands of the electromagnetic spectrum.

Camera Cassegrain telescope ï¿½ A two-mirror lens design used in astronomical telescopes, the primary being a parabola, the secondary a smaller hyperbola. The image formed is free of spherical aberration and color and is located behind the vertex of the primary. Cassegrain telescope Cathode ray tube ï¿½ A vacuum tube with an electron gun at one end and a fluorescent screen at the other. Electrons emitted from a heated filament are accelerated by a series of annular anodes at progressively higher positive voltages. The electron beam is then deflected by two pairs of electrostatically charged plates between the gun and the screen. Electromagnets are sometimes used in place of the deflector plates. Cathode ray tube Choledochoscope ï¿½ A small fiber optic endoscope used in laser surgery. Chromascope ï¿½ An instrument designed to analyze the optical effects of color. D Detector ï¿½ 1) A device designed to convert the energy of incident radiation into another form for the determination of the presence of the radiation. The device may function by electrical, photographic or visual means.

2) A device that provides an electric output that is a useful measure of the radiation that is incident on the device. Digital camera ï¿½ A camera that converts a collected image into pixels that are black or white digital or shades of gray. The digital data may then be manipulated to enhance or otherwise modify the resulting viewed image. Elbow telescope ï¿½ A refracting telescope that uses a prism to bend the line of sight 90°. Elbow telescope Electron microscope ï¿½ A device utilizing an electron beam for the observation and recording of submicroscopic samples with the aid of photographic emulsions or other short-wavelength sensors. With the electron microscope, the maximum useful magnification is over 300,000. Endoscope - A medical instrument used to view inside the human body by inserting the instrument into a natural or created aperture. The endoscope may use a coherent fiber optic bundle or conventional optics to relay the image to the eye or a television camera. Illumination is provided by a concentric bundle of noncoherent fiber optics. Eye ï¿½ The organ of vision or light sensitivity.

Eye Fabry-Perot interferometer ï¿½ A refracting telescope that yields an erect image by the use of a positive lens for its objective and a negative lens for its eyepiece. Fabry-Perot interferometer Fizeau interferometer ï¿½ A type of interferometer noted for producing narrow multiple-beam interference fringes. As a result, when compared with the Twyman-Green, the Fizeau interferometer has fewer optical components, does not have the large beamsplitter and can be adjusted to a greater accuracy. Fizeau interferometer Galilean telescope ï¿½ A refracting telescope that yields an erect image by the use of a positive lens for its objective and a negative lens for its eyepiece. Galilean telescope Grating ï¿½ A framework or latticework having an even arrangement of rods, or any other long narrow objects with interstices between them, used to disperse light or other radiation by interference between wave trains from the interstices. The ability of a grating to separate wavelengths (chromatic resolving power) is expressed as being equal to the number of lines in the grating.

Grating spectroscope ï¿½ A spectroscope having a diffraction grating for the resolution of light of various wavelengths. Grating spectroscope Gyroscope ï¿½ 1) Ring Laser: A solid block of glass-ceramic material with holes drilled the length of all four sides, and mirrors attached to the corners to form a closed path, in which two independent laser beams oscillate at different frequencies. When the device is stationary, the optical paths traveled by the two beams are identical. Rotation of the cavity causes a difference in path length between the two beams that can be measured. 2) Fiber optic: An instrument to measure angular rotation, based on the principle that the application of force will alter the wavelength of light as it travels in one direction or the other around a coil of optical fiber wound on a drum, thus producing interference fringes at the termination of travel from which measurement can be made. Gyroscope H Hologram ï¿½ An interference pattern that is recorded on a high-resolution plate, the two interfering beams formed by a coherent beam from a laser and light scattered by an object. After formation, if lit and viewed correctly, a 3-D image of the object is seen.

Hologram I Iconometer - An instrument in which an object's image, produced by a lens of known focal length, is used to determine the object's distance from the instrument when its size is known, or the object's size if the object-instrument distance is known. Integrating sphere ï¿½ A hollow sphere coated internally with a white diffusing material and provided with openings for incident beam, specimen and detector used for measuring the diffuse reflectance or transmittance of objects. Interferometer ï¿½ An instrument that employs the interference of lightwaves to measure the accuracy of optical surfaces. It can measure a length in terms of the length of a wave of light. K Keplerian telescope ï¿½ A simple form of astronomical telescope that uses a fixed objective and a focusable eyepiece. The objective forms an intermediate image in the instrument, resulting in an image that appears upside down when viewed through the eyepiece. Keplerian telescope L Laser ï¿½ An acronym of Light Amplification by Stimulated Emission of Radiation. A laser is a cavity, with plane or spherical mirrors at the ends, that is filled with lasable material.

This is any material -- crystal, glass, liquid, dye or gas -- the atoms of which are capable of being excited to a semistable state by light or an electric discharge. The light emitted by an atom as it drops back to the ground state releases other nearby, excited atoms, the light being thus continually increased in intensity as it oscillates between the mirrors. If one mirror is made to transmit 1 or 2 percent of the light, a brilliant beam of highly monochromatic, coherent radiation is emitted through the mirror. If plane mirrors are used, the beam is highly collimated. With concave mirrors, the beam appears to emerge from a point source near one end of the cavity. Laparoscope ï¿½ An endoscopic surgical instrument that includes a channel for the introduction of supplementary instruments. Local area network ï¿½ Data communications network in a clearly defined geographical location, and extending no more than a few miles in length. It generally takes in an office building or group of buildings, a campus or the like, offering many nodes and connecting computers and peripherals with high-capacity links. M Mach-Zender interferometer ï¿½ An interferometer derived from the Twyman-Green interferometer.

Light passing through a sample region in one direction recombines with a second leg without traversing the sample twice. The instrument is used to study transparent objects and is particularly useful in studying wind tunnel gas dynamics. Mach-Zender interferometer Magnifier ï¿½ A lens or lens system that produces an enlarged virtual image of an object placed near its front focal point. Michelson interferometer ï¿½ It produces interference fringes by splitting a beam of monochromatic light so that one beam strikes a fixed mirror and the other a movable mirror. When the reflected beams are brought back together, an interference pattern results. Michelson interferometer Michelson Stellar interferometer ï¿½ An interferometer constructed to be positioned on a telescope to measure the angular separation of the components of double stars. Michelson Stellar interferometer Microscope ï¿½ An instrument consisting of essentially a tube 160mm long, with an objective lens at the distant end and an eyepiece at the near end. The objective forms a real image of the object in focal plane of the eyepiece where it is observed by the eye. The total magnification is equal to the linear magnification of the objective multiplied by the magnifying power of the eyepiece.

Microscope eyepiece ï¿½ An eyepiece located at the near end of the microscope tube. It often is a simple Huygens eyepiece, but compensating and flat-field projection eyepieces are quite efficient. Negative eyepieces are sometimes used as projection relay lenses in photomicrography. Microscope eyepiece Modulator ï¿½ A device that varies the amplitude and phase of a light beam; e.g., from a laser or by sound waves. Also known as a Bragg cell. Monochrometer ï¿½ An instrument for isolating narrow portions of the spectrum by using a prism or grating. Newtonian telescope ï¿½ A telescope with a concave paraboloidal objective mirror and a small plane (diagonal) mirror that reflects rays from the primary mirror laterally outside the tube where the image is viewed with an eyepiece. Newtonian telescope Nuclear Magnetic Resonance (NMR) ï¿½ A phenomenon, exploited for medical imaging, in which the nuclei of material placed in a strong magnetic field will absorb radio waves supplied by a transmitter at particular frequencies.

The energy of the radio-frequency photons is used to promote the nucleus from a low-energy state, in which the nuclear spin is aligned parallel to the strong magnetic field, to a higher-energy state in which the spin is opposed to the field. When the source of the radio waves is turned off, many nuclei will revert to the lower state by emitting photons at the characteristic resonance frequencies, providing information about the sample material. Because the single protons in biologically abundant hydrogen atoms react well to the technique, it can produce excellent cross-sectional images of bone marrow and tissue. O Optical Coherence Testing (CAT scan) ï¿½ A medical diagnosis technique that uses lightwaves to form cross-sectional images of a patient's retina or body. Optical fiber - A thin filament of drawn or extruded glass or plastic having a central core and a cladding of lower index of refraction to promote internal reflection. Optical fiber Otoscope ï¿½ A light-conducting and imaging system used to inspect the inner ear. Otoscope Overhead projector ï¿½ A projector used to project transparencies. A horizontal 8 x 10-in.

stage for writing or laying down preformed transparencies is provided with either an elliptical mirror or a plastic Fresnel lens below the stage to form an image of a lamp filament in the projection lens situated above the stage. A 45ï¿½ mirror above or below the lens turns the vertical beam into a horizontal direction toward the screen behind the lecturer's head. The imagery is such that when the speaker is facing the audience, his normal writing on the stage appears erect and correct on the screen. Overhead projector Periscope ï¿½ An optical instrument designed to displace the line of sight in a vertical direction. Used often in submarines. Periscope Phase-contrast microscope ï¿½ A microscope that has an annular stop in the lower focal plane of the condenser, and a quarter-wave retarding and absorbing ring in the upper focal plane of the objective that just fills the image of the annular substage stop. Direct light passing through a transparent object falls on the ring where it is retarded by a quarter-wave and reduced in intensity.

Light diffracted by the phase differences in the transparent object because of variations in refractive index and/or thickness, misses the retardation ring and interferes with the direct light in the manner of an interference microscope. This leads to an image in which variations of phase in the object appear as variations in brightness. Phase-contrast microscope Radiometer ï¿½ A device used to measure the intensity of radiant energy. Radiometer Refractometer ï¿½ An instrument used to measure the refractive index of solids and liquids. Several types exist, the most common being the Abbe refractometer. S Sagnac interferometer ï¿½ A type of interferometer in which two coils of optical fiber are arranged so that light from a single source travels clockwise in one, and counterclockwise in the other. Rotation of the coils causes a phase shift in the combined output measured by the detector. Sagnac interferometer Scanner ï¿½ 1) A device used to trace out an object and build up an image. One of the most common of these types is video scanning.

The scanning takes place inside the television tube as electrons, guided by electron optics, sweep linearly across a tube face coated on the inside with a phosphorescent material. A scanner can convert a paper drawing or photograph into pixels on a display screen. Scanners are also used to relay information in optical data processing. 2) A device that automatically measures or checks a process or condition and may initiate a desired corrective action by means of switching. 3) A device for sensing recorded data, such as a supermarket bar code. Scanner Scanning electron microscope ï¿½ An electron microscope that uses a beam of electrons — accelerated to high energy and focused on the sample — to scan the sample surface, ejecting secondary electrons that form the picture of the sample. Scanning electron microscope Slide projector ï¿½ An optical projection device designed to project positive color transparencies onto a screen for viewing. Slide projector Spectrograph ï¿½ An optical instrument for forming the spectrum of a light source and recording it on a film. The dispersing medium may be a prism or a diffraction grating.

A concave grating requires no other means to form a sharp image of the slit on the film, but a plane grating or a prism requires auxiliary lenses or concave mirrors to act as image-forming means in addition to the dispersing element. Refracting prisms can be used only in parallel light, so a collimating lens is required before the prism and an imaging lens after it. Some special-purpose spectrographs are equipped with tiny photodetectors situated in the spectrum at the positions corresponding to the lines of elements whose presence is to be determined. Spectrograph Spectrometer ï¿½ A kind of spectrograph in which some form of detector, other than a photographic film, is used to measure the distribution of radiation in a particular wavelength region. Spectrometer Spectrophotometer ï¿½ An instrument for measuring spectral transmittance or reflectance. Spectroscope - In a general sense, any one of a class of instruments used to disperse radiation, visible or invisible, into its component wavelengths and for determining or measuring the resultant spectrum. In a limited sense, the instrument is capable of being used only for visual observation of the spectrum.

Stereoscope - A small instrument containing a picture support and a pair of magnifying lenses so arranged that the left eye sees only the picture taken with the left lens of the stereo camera, and the right eye only the right picture. If the camera lenses are separated by the interocular distance, and if the focal lengths of camera and stereoscope lenses are equal, then a true-to-scale or orthoscopic reproduction of the scene is obtained. Stereoscopes are used extensively in photogrammetry. Stereoscope T Trinoscope ï¿½ A color-television viewing system with three kinescopes, three lenses and three deflection yokes used to form the red, green and blue images required for a tricolor television projection. Twyman-Green interferometer - A testing device that provides a contour map of the emergent wavefront for the observer in terms of the given wavelength of the light. Twyman-Green Interferometer W Waveguide ï¿½ A system or material designed to confine and direct electromagnetic waves in a direction determined by its physical boundaries.

Refractive Telescopes The Astronomical Telescope The astronomical telescope makes use of two positive lenses: the objective, which forms the image of a distant object at its focal length , and the eyepiece, which acts as a simple magnifier with which to view the image formed by the objective. Its length is equal to the sum of the focal lengths of the objective and eyepiece, and its angular magnification is -fo /fe , giving an inverted image. The astronomical telescope can be used for terrestrial viewing, but seeing the image upside down is a definite inconvenience. Viewing stars upside down is no problem. Another inconvenience for terrestrial viewing is the length of the astronomical telescope, equal to the sum of the focal lengths of the objective and eyepiece lenses. A shorter telescope with upright viewing is the Galilean telescope . Go Back Galilean Telescope The Galilean or terrestrial telescope uses a positive objective and a negative eyepiece. It gives erect images and is shorter than the astronomical telescope with the same power. It's angular magnification is -fo/fe .

The image below shows parallel rays from two helium-neon lasers passing through a Galilean telescope made from an objective with f=30cm and an eyepiece with f=-10cm. With the lenses placed 20 cm = fo+fe apart, the parallel input rays are rendered parallel again by the eyepiece lens, giving an image at infinity. This shows one of the uses of Galilean telescopes. It is useful as a collimator that takes a large beam of parallel light and reduces the size of the beam, keeping the rays parallel. The angular magnification of this Galilean telescope is 3. The beams of the helium-neon lasers were made visible with a spray can of artificial smoke.

Question: Galilean, Keplerian, Newtonian and Conde are types of what instrument?
Answer: Telescope