KENNETH UITTI, a petroleum engineer of Westmont, III., has several hobbies, among them optics. Like most amateur opticians, he prefers making his own instruments to buying commercial products of higher quality at half the cost. It took him two years to make the microscope shown on the right. Admitting that it leaves something to be desired when compared with the best research microscopes, he says the instrument has proved far more than "just handy" in his primary hobbies of metalworking and gunsmithing.

"I have always done a lot of rifle and pistol shooting," he writes. "That leads t amateur gunsmithing and to dreaming about accuracy and ballistics. If you shoot, you get curious about rifle scopes and spotting scopes. Hence my lathe has turned out several of each. It happens that I have always loved astronomy and accordingly have a three-inch refractor and six-inch reflector. It was while figuring the reflector that I was bitten by the microscope bug.

"About 3:00 a.m. one night I was waiting for the mirror to cool in its cradle in preparation for the knife-edge test. As I sat there puffing a cigarette and thinking things over, I noticed a small scrap of brass tubing on the back of the bench and wondered, somewhat idly, what would happen if I shoved a Ramsden eyepiece into one end of it and a small plano-convex lens into the other. A bit of rolled cardboard served as a cell for the lens. The eyepiece fitted the tube nicely. When I pointed the contraption at a sprinkle of salt on the bench, I was amazed by the quality of the microscope image!

"At the moment I could not remember what a compound- microscope looked like, and I had never really understood how they work. The next day I began scouring the libraries for books on microscope construction written for the amateur. The shelves were empty except for an occasional article telling how to do the job in two hours with spools for eyepieces and cardboard parts fastened together with airplane cement. I wound up by translating as much of the professional literature as possible into terms I could comprehend and working out the rest at the bench. The following description is based on the result. I hope it will encourage others to take up microscopemaking and to pass their experiences along.

"It is only fair to warn the beginner that this is not an easy or inexpensive pastime. The finest microscope objectives stand at the pinnacle of the lens maker's art. The impossibility of duplicating them, however, should not discourage the amateur. His many hours of frustration will be compensated by the increased esthetic delight he will take thereafter in the master craftsman's accomplishment. If you have the patience of Job, a good screw-cutting lathe and a lot of spare time, plunge in. You will. wind up with a useful piece of equipment and a firsthand knowledge of one of the most elaborate and interesting optical instruments known.

"In its essentials microscope theory is not difficult. The apparent size of an object can, in general-, be enlarged in two ways: (1) by bringing the specimen closer to the eye, so that the image covers a larger portion of the retina; and (2) by projection, as in the case of the magic lantern. The compound microscope combines the two methods. A brilliantly lighted specimen, the counterpart of the magic lantern's slide, is brought close to the microscope's objective lens, which acts as a small projection lens. Rays from the object come to focus at a plane near the top of the instrument's tube. If a disk of ground glass is held in the position of this plane, the enlarged image can be seen-just as in a camera. This image, with or without the screen, can now be magnified further by bringing the eye close to it. Another lens, called the eyepiece, is necessary because the normal eye cannot see clearly at distances closer than about 250 millimeters (about 10 inches). The total of the two magnifications will be equal to the actual increase in the size of the projected image multiplied by the apparent increase caused by viewing the projected image at close range.

"Thus it is apparent that the micro scope's objective lens does not differ in principle from the familiar projection lens or the lens of a camera. It projects an enlarged picture of the specimen. It is -not unlike a telescope objective, except for its much smaller size and shorter focal length. The microscope, camera and telescope lenses all bend the rays from an object into focus at an image plane. In the case of the telescope the object is distant and a small image of it is formed relatively close to the lens. In the camera the two distances-from object to lens and from lens to ground-glass screen may approach equality. They can be made equal in many cameras equipped with extension bellows, and the object is then photographed full size. The microscope goes to the opposite extreme. The specimen is brought close to the lens, within a small fraction of an inch in the case of high-power objectives. An enlarged image is then projected on a plane several inches away.

"Although the 'primary' or 'aerial' image is formed near the top of the microscope's tube, it appears to be down near the bottom when viewed through the eyepiece. This 'virtual' image is an optical illusion. Its phantom position is explained by the fact that we are accustomed to seeing objects clearly only when they are at least about 10 inches away. The position of the virtual image is useful in establishing the power of the microscope.

"When the statement is made that microscopes make objects appear larger, it makes sense to ask: 'Larger than what?' Microscopists answer: 'Larger than they would appear if viewed at a distance of 10 inches, the position of the virtual image.' The power of a microscope is arbitrarily defined as the number of apparent diameters an object would have to be increased at 10 inches to equal its apparent diameter as represented by the virtual image. An eightfold magnifier, for example, makes an object which appears to be an eighth of an inch across at 10 inches appear a full inch in diameter.

"From practical considerations, it is desirable to design compound microscopes so that the plane of the primary image is formed at a prescribed point within the tube, just as the screen in a motion picture theatre is at a fixed distance from the projector. I designed my instrument for a projection distance (an 'optical tube length') of 180 millimeters. Most commercial objective lenses are corrected to work well at this distance; hence they can readily be interchanged with those you make yourself. Once this distance is standardized, it becomes a simple matter to calculate the power of an objective lens. The power is equal to 180 divided by the focal length of the objective in millimeters. The power of the eyepiece, however, is found by dividing its focal length in millimeters into 250 millimeters-the standard distance for nearest clear vision. Incidentally, by selecting lenses of proper focal length you can increase the magnification as much as you please, but, as with telescopes, beyond a certain point further power serves no purpose. Each objective lens can resolve only a limited amount of detail. Beyond this limit of resolution the picture becomes fuzzy.

"The optical train and tube are shown in cross section in the accompanying illustration [see left]. A designates the aerial plane where the primary image falls-12 millimeters below the top of the tube and below the point against which the eyepiece shoulders. In use the eyepiece is shoved all the way against the shoulder and is never moved in focusing the instrument. Similarly the objective cell is built to shoulder against the bottom of the tube-at a distance 32 millimeters beyond the 'back focal- plane' (B in the diagram ) of the lens, where a bundle of parallel rays entering the objective comes to focus. The difference between the eyepiece and the objective shouldering distances, when subtracted from the optical tube length, gives the mechanical length of the tube-160 mm.

"Simple lenses for the instrument can be made by the methods described in Amateur Telescope Making-Advanced, or purchased at a few cents each from supply houses such as Edmund Scientific Corporation of Barrington, N. J., or A. Jaegers of Lynbrook, N. Y. High-power objectives can be purchased separately from dealers in microscopes and are available on the secondhand market.

"In designing the cell for simple objective lens combinations it is necessary to calculate several principal dimensions. The distance u between the object and objective-at which the primary image is brought into focus at plane A-is equal:

"If two lenses are used for the objective, the equivalent focal length will be:

where s is the spacing between the lenses and and are the focal lengths of the two lenses. The object-image formula for such a combination will be:

"The illustration shows a Ramsden eyepiece for simplicity. In this eyepiece the image is outside the lens system. The focal length of the lens combination is that of the two-lens system given above. The focal plane will be at this distance in front of the lens system:

where lens is the focal length and s is again the lens spacing. When designed by this formula, eyepieces of the Ramsden type will be focused on plane A when shouldered against the upper end of the tube. For eyepieces where the image is inside the lens system, as in the Huygens type, it is best to treat the field lens of the eyepiece as a unit with the objective system, calculating the equivalent focal length of the whole. The image will be at a plane in front of the eye lens at a distance equal to the focal length of the eye lens. For the kind of microscope that an amateur may wish to build, the positions of planes A and B may not be critical. A fastidious hobbyist, however, may wish to build an instrument which will not preclude the use of good high-power objectives.

"Despite the faint 'rainbows' that surround the image, simple plano-convex lenses give surprisingly good results as objectives of low power-from 10 to 15. The convex side should face the eyepiece. A 10-power plano-convex would have a focal length of 18 millimeters when used in an instrument with an optical tube length of 180 millimeters. For a 20-power objective a single achromat can be used. It would have a focal length of nine millimeters. If two are used, with their surfaces almost touching, the focal length can be longer. Using the equivalent focal length formula for two lenses, where = and s is essentially zero, the focal length of each lens is 18 millimeters.

"The system of illumination comprises the remaining optical element of the instrument. Illumination is critical and its importance must not be underestimated. I use a light source consisting of a bulb in a ventilated housing. This throws a beam of light through opal glass to a small first-surface mirror which is swung from the substage mechanism. The mirror is simple to make by the methods outlined in Amateur Telescope Making, or it may be procured from the supply houses previously mentioned. Light is reflected upward through the stage to the object. Direct lighting is satisfactory for low powers but magnifications above 100 powers require a substage condenser lens capable of focusing a sharp cone on the object. The iris diaphragm from an old camera can be added immediately below the specimen stage for controlling intensity, and means must be provided for raising and lowering the substage assembly. The effect of the substage condenser on resolution is amazing. Martin and Johnson's Practical Microscopy and The Principles of Optics by Hardy and Perrin give excellent treatments on condenser design and a review of them is a must if the amateur is to make real headway.

"Most of the glass elements of my microscope were installed in cells made from machinable (half hard) aluminum rod and secured in place by rolling a thin sliver of metal over their edges with a polished tool-steel burnisher-a practice which I do not recommend for really good objectives. Threaded retaining rings offer obvious advantages. The cells in turn are pushed into brass tubing and secured by threaded rings. The exposed aluminum surfaces were coated with flat black coach paint. (Incidentally, who can tell me about a simple home method of anodizing aluminum for black optical goods?)

"The mechanical parts consist of an arm which supports the microscope tube and which swivels about the inclination joint at the head of a pillar. The pillar is mounted integrally (welded) on the base. The arm also carries the stage and the substage assembly. It is a good idea to make the base out of one-inch plate, either sawed out or cut out with an acetylene torch. The arm should be sawed out of half-inch steel plate or, better, of aluminum, since much machining has to be done on it. The pillar should be of two vertical pieces of half-inch plate welded to the base. The inside surfaces should be parallel and smooth. The arm pivots to the pillar by means of an axle of drill rod half an inch in diameter, threaded on both ends and fitted with large round knurled brass nuts on each end. The stage is simply a four-by-four-inch piece of steel plate a quarter of an inch thick, with a half-inch hole through which the specimen slides are illuminated. If desired, a second plate can be fitted to the stage so that it can be rotated through 360 degrees.

"Care must be taken in aligning the rotating plate with respect to the optical axis, or objects centered on the crosshairs of the eyepiece will arc out of position when the stage is moved. If you really want to be fancy, a mechanical stage-centering device is not impossible to build. This requires a ring mounting with two screws of fine thread at right angles to each other, fitted with calibrated drums. A spring-compression plunger acting at 135 degrees from the screws holds the stage in place. The addition of a mechanical centering device will also pay dividends as a micrometer for measuring small objects.

"Focusing can be accomplished by means of a rack and pinion. The rack is soldered to the tube. The pinion is placed through the arm and rotated by means of knurled brass wheels. In this scheme the moving brass tube fits into a stationary one which is secured to the arm and slotted for passage of the rack. I have had plenty of grief with the rough, 'clunky' motion of my spur-tooth rack and pinion. A helical one, if available, would probably be better. A soft rubber-roller friction drive, using no rack at all, might be smoother, but not as positive.

"The design of an original fine-focus mechanism presents a nice challenge to your inventive genius. Any device that will instantly go into action, regardless of the position of the coarse focus, and yet require many turns of a drum for a short movement of the objective, meets the specification. Many commercial instruments accomplish this by dovetailing the tube-supporting member into the arm. This member is raised and lowered by a lever of large mechanical advantage. A screw of fine thread bears against the long arm of the lever. The short end cams the tube up and down. The system may be spring-loaded.

"In presenting this account I have purposely omitted the fine details of method and dimension. In this business, as in making telescopes and other scientific instruments, most amateurs prefer to 'roll their own. 'Once your microscope reaches the point, as mine now has, where you can use it as a magnifying viewer, you are ready to proceed with refinements and accessories. In one respect the home-built microscope is like an oil painting. In the fond hands of its creator it is never really 'finished.'"

NEARLY all the mechanism of a large telescope of conventional type, such as the 200-inch, exists for moving the massive main mirror. In the unconventional zenith telescope proposed by Guido Horn-D'Arturo and others the main mirror is fixed to the earth at the bottom of a pit, and only auxiliary mirrors or the lightweight photographic plate move. This greatly limits the telescope's range, but it also greatly reduces the cost, since the main mirror no longer need be thick to be rigid. Two designs for zenith telescopes, one of them a concave-secondary combination of Gregorian type proposed by Lyle T. Johnson of La Plata, Md., were described in this department last month. Johnson now adds a zenith Cassegrainian [see illustration right]. Extending upward from the north and south edges of the pit are two piers. One is higher than the other, so that the sloping polar axis which spans them may parallel the earth's axis. The observing unit in the drawing hangs from the center of this span. It can be inclined and set anywhere within a range of 7-3/4 degrees in celestial latitude, that is, to the reader's right or left. Once set, it is slowly driven in longitude (away from the reader) to offset the earth's rotation. The circular field of view of its mirrors is 500 inches in diameter, but the square primary fixed mirror is 700 inches wide. The photographic plate therefore receives full illumination on a given image during 200 inches or about a half-hour of west-to-east travel, and partial illumination for an hour before and after.

This telescope is the rare spherical primary Cassegrainian, not the familiar spherical secondary, or Dall-Kirkham type Bernhard Schmidt used the combination on the Cassegrainians that he built and sold when working as a free lance before he invented his famous Schmidt camera. It was also used on the 20-inch working model of the 200-inch, though only because a finished spherical mirror happened to be on hand; the aim in that case was mainly to test the mounting.

NOT ALL amateur astronomers are convinced that the evil effects of diffraction are as hopelessly incurable in optics as is sin in human beings. In this department for the issues of May and August, 1951, Edwin Emil Webb, William M. Sinton and others wrestled manfully with this supposedly ineradicable form of original sin. Now Arthur S. Leonard of the College of Agriculture at the University of California in Davis, a member of the amateur "Esoteric Order of Ray Tracers," proposes a practical experimental attack on diffraction effects for trial by telescopes owners during the present apparition of Mars. He writes:

"The method might be described as 'diffraction pattern modification.' I did not know about Sinton's article at the time I started experimenting to get rid of the diffraction pattern formed of a star in my telescope. I was using, in front of the telescope objective, an opaque diaphragm containing 19 circular holes arranged in a hexagonal pattern (one hole surrounded by six, and these surrounded by 12). I discovered by calculation, and verified by experiment, that if the diameter of the holes in the outer row of 12 was made one half of that of the other seven, the first and second bright rings of the diffraction pattern were practically eliminated. This suggested that if the transmissivity of the lens (or the reflectivity of the mirror) of a telescope were to be gradually reduced toward the edge of the aperture, the brightness of the rings of the Airy diffraction pattern formed by the objective would be greatly reduced. I reasoned that this might increase the sharpness of the central disk of the pattern. Any planetary image is no more than the mosaic of overlapping diffraction patterns formed on the retina of the observer's eye, each pattern corresponding to a separate point of light on the planet. Therefore anything which will increase the sharpness of an individual diffraction pattern should also increase the sharpness of planetary detail.

"I dreamed up several ways in which this shading off of the objective could be brought about. The first and most obvious solution was to reduce the thickness of the reflective coating gradually toward the edge of the mirror, as proposed by Sinton [filter at left in the group]. It has the objection that the light-gathering power of the telescope would be permanently reduced, with serious loss in ability to show faint objects. Another solution is to add a tiny shading mask behind the eyepiece, at the Ramsden disk. This mask [top of same group] would be made of a thin piece of glass on which had been evaporated a thin reflective coating. The coating would have zero thickness at the center and gradually build up to let through say 25 per cent of the light at the edge of the Ramsden disk. This idea, though it has not yet been tried, has the advantage that the mask could be made removable from the eyepiece, or the eye piece could be changed, but it has the possible disadvantage of mutual interference between it and the observer's eyelashes. (We could use such a mask with only a few of our highest-powered eyepieces.) A variation of this idea would be to alter the design of an eyepiece, such as the Hastings solid ocular, to bring its last surface to the plane of the Ramsden disk, and then apply the shading reflective coat to that surface.

"The other methods that I have worked on might be described as 'shading by small obstructions.' One form of mask placed in front of the telescope has a row of sharp-pointed teeth extending inward for a distance of 40 to 50 per cent of the radius [second filter from the left]. Another consists of an opaque diaphragm having one large hole cut in the center (60 to 70 per cent of the diameter of the objective) and many small holes in the rest of it [third filter from left]. The effect of a gradual shading might be obtained if the small holes were extremely close together near the inner edge of the diaphragm and spaced more and more widely toward the outer edge. A third design, not shown in the drawing, uses a series of concentric rings of various widths and diameters supported on a spider. A fourth type employs wire screen for the obstructions [fourth and fifth filters].

"All of the masks that employ small but finite obstructions have one characteristic in common: they produce halos or line spectra around each point of light in the field. The angular distance from the central diffraction pattern to the closest of these halos is inversely proportional to the linear spacing of the teeth, small holes, rings or screen wires. By making this distance a sixteenth of an inch or less, the unwanted light can be made to fall farther from the center than the angular diameter of the largest planet that is to be observed. To push this light beyond the disk of the Moon, however, would require prohibitively close spacings.

"In the present apparition of Mars what is needed is something simple and easy to make in a short time. For this a wire-screen type of objective shading mask known technically as an apodizing mask, is probably the best that I have thought up thus far. These masks use one, two or three layers of ordinary screen. I am using standard wire cloth with wires 12 thousandths of an inch in diameter and with 14 wires per inch one way and 18 the other. The general effect of a gradual reduction in transmission of light toward the edge is obtained by increasing the number of layers of screen in the light path toward the edge of the aperture. Although this gives a reduction in light which occurs in steps instead of the ideally gradual manner, the harmful effects of the stepwise reduction can be reduced to a rather low value by locating the steps at near-optimum positions. For a single-screen mask to be used on a refractor, a hole between one half and two-thirds the diameter of the objective should be cut in the center. The optimum probably is 60 per cent. With a reflector, the range of really useful diameters is from 55 to a bit over 60 per cent. For a double-screen mask for a refractor the optimum diameters are near 76 and 52 per cent. For a triple-screen combination these diameters should be 88, 76 and 52 per cent. These diameters should be useful with a reflector but somewhat better performance might be obtained from designs in which account is taken of the blind spot in the light path produced by the secondary.

"To make one of these masks, construct a frame of some sort to fit over the front of the telescope, attach the first screen to the frame with nails, screws or solder, cover the screen with paper on which has been laid out a circle the diameter of the largest opening to be cut, center the circle carefully, and attach the paper to the frame with Scotch tape. With a small cold chisel or a sharp sheet metal punch, cut out the central part of the screen and paper along the line of the circle. After removing the paper, attach the second screen to the frame and cut the next smaller hole in the same way, and so on. In a double-screen mask, the mesh of the second screen should be turned 45 degrees with respect to the first, in a triple-screen combination, 80 and 60 degrees.

"These masks produce several different effects, some beneficial, some detrimental. First, the brightness at the center and all other points in the central disk of the diffraction pattern is reduced very appreciably. Second, the effective diameter of the central disk is increased, but to a lesser degree. The net result is a reduction of the total light in the central condensation of the Airy disk. On any kind of object-double star, planet and so forth-the increase in diameter must be regarded as a loss in resolving power. On very bright objects such as Mars, Venus or the Moon, the reduction in total light in the central disk may not result in a loss, and under some circumstances it may even help the observer to see more. On fainter objects, such as Jupiter and Saturn, the reduction in brightness of the image may be somewhat detrimental. The third effect is a very substantial reduction in the brightness and total light in most of the rings of the diffraction pattern relative to the brightness and total light in the central disk. The reduction is greatest in the first ring, and, since it contains almost as much light as the rest together, this effect must be regarded as a very substantial gain in resolving power, or at least in sharpness of detail.

A fourth effect is a reduction in the seriousness of all kinds of aberrations- chromatic, spherical or those due to coma, distortion of the mirror or poor seeing.

"As we go from no mask to a single screen, to the double screen and so on, all these effects increase, but not all at the same rate. Therefore it is virtually impossible to predict by calculation alone which mask will give the greatest net gain in performance. It may well be that on one object one mask will work best, while on some other object another will give a maximum of improvement in detail. Also, since seeing is definitely a factor, and since the effect of seeing varies from night to night, from place to place, and from telescope to telescope (depending primarily on size), we may expect to find the optimum mask a highly variable quantity. This is where calculations based on simple theory leave us high and dry, and where we must look to extensive experiment by many observers. How about some of you other TNs making and trying these masks and reporting your findings?"

LAST October Harvard University withdrew from the American Association of Variable Star Observers the annual income from a fund of $100,000 which has made its work possible for 21 years. This decision, which became effective at the end of the year, left the AAVSO with only $6,356 which had been raised by and mainly among its own members prior to 1931. The organization has new headquarters at 4 Brattle Street, Cambridge 38, Mass. Its goal is a fund of $250,000.To raise this sum the members would each have to contribute $600-an amount far beyond the means of the average amateur astronomer. Contributions of any amount from friends of amateur science will be welcomed by the AAVSO. The Association members keep a regular watch on variable stars year in and year out, each in an assigned portion of the sky. Their work has been invaluable to professional astronomers working on major problems.

Bibliography

THE MICROSCOPE AND ITS USE. Francisco J. Muñoz and Harry A. Charipper. Chemical Publishing Co., Inc., 1943.

PRACTICAL MICROSCOPY. Louis C. Martin and B. K. Johnson. Chemical Publishing Co., Inc., 1951.

AMATEUR TELESCOPE MAKING. Edited by Albert G. Ingalls. Scientific American, Inc., 1952.

AMATEUR TELESCOPE MAKING-ADVANCED. Edited by Albert G. Ingalls. Scientific American, Inc., 1952.

AMATEUR TELESCOPE MAKING-BOOK THREE. Edited by Albert G. Ingalls, Scientific American, Inc., 1958.

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