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A Telescope in an English School/a Johnsonian Telescope with Fixed Mirrors |
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by Albert G. Ingalls |
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"The preliminary drawings, all patterns and castings and machining were done cooperatively by masters, boys and instructors. The body of the telescope is of duralumin and cast aluminum. The balance weight at the short end is of cast iron. The trunnions forming the declination axis are part of a single casting in the form of a square box. The design had to be simple because all the patterns had to be made by schoolboys. The driving wheel is bronze, 12 inches in diameter, and has 360 teeth which were hobbed by a home-made tool on a milling machine. The three-inch polar axis that carries the fork runs in ball bearings. Four hand screws on this axis permit unlocking it from the driving worm wheel, so that the telescope can be rotated at will. "The drive is taken from an old spring-driven phonograph motor modified in two important ways. The governor weights have been loaded to slow down the motor, so that the single-spring barrel rotates about once a minute; and the spring barrel has been fitted with a shaft which emerges from the frame of the motor and revolves at the same rate. The torque transmitted by this shaft when the spring is fully wound exceeds one pound-foot. The motor will continue to drive for about 15 minutes, and since the foundations are rigid it is in order to wind the motor during a run. The only service demanded of the gears in the motor is to drive the governor. The governor brake is controlled by a screw of fine thread. "The motor is connected to the worm drive by a 1-to-4 bevel gear as shown in the insert drawing, making the gear ratio 1-to-1,440 and so giving ample torque to drive a heavy telescope. A spring-loaded clutch in line with the worm makes it possible to turn the latter either way without interrupting the motor; the resistance of the clutch is overcome by means of the capstan on the end of the shaft." L. C. MARTIN'S Introduction to Applied Optics, a standard reference and textbook which went out of print during the war, has now been revised, enlarged into two volumes and given the new title of Technical Optics. The author is professor of technical optics at the Imperial College of Science and Technology in London, and the book represents the substance of a lecture course given to graduate students at the College. It is not elementary, but many advanced amateur telescope makers add such major works on optics to their libraries. Volume 1, containing 343 pages, deals with geometrical optics, paraxial theory of optical systems, light as wave motion, images and their defects, physiological and physical optics, radiation, optical glass and lens defects. It includes a chapter on spectacles, which most members of the optical fraternity wear without the understanding conveyed in this far from elementary discussion. Volume 2, containing 344 pages, deals with the telescope, microscope, binoculars, photographic lenses, photometry, testing optical instruments, aspheric surfaces. The volumes may be obtained, separately if desired, from the Pitman Publishing Corporation, New York. DIFFRACTION, sometimes defined as the bending of light around obstacles into the shadow, is a process that goes on continuously in all wave fronts. The waves try to spread out sidewise as they advance. When a part of the wave is cut off by an obstacle, there is a special phenomenon, correctly called not "diffraction," but a "diffraction effect," which is what is usually meant in optics by the term diffraction. The misleading definition first stated sometimes creates the idea that the diffracted wave spreads only into the shadow behind the obstacle; actually it also spreads into the illuminated region. Diffraction is a cause of imperfect imagery in optical instruments, but it has never been abolished. Edwin Emil Webb of New York now proposes a trick to defeat it in a reflecting telescope. "Why," he asks, "can't diffraction due to secondary mirrors be totally eliminated by a circular blackened area in the center of the main mirror, the radius of the dark spot being slightly greater than the radius of the secondary, so that the diffracted light would fall upon a nonreflecting area and not be returned to the eyepiece? "To prevent the spot itself from giving rise to diffraction," he continues, "its own edge would have to taper in intensity from full black to full reflectivity as its radius increased from the radius of the secondary to that radius plus, say, 1/8-inch. Optically, such a spot would have no edge and could not give rise to diffraction, though it could, being larger, absorb all the diffraction caused by the secondary.
"Such a spot would be difficult to produce. It could not be painted or airbrushed on, since these processes deposit particles greater in diameter than a wavelength of light, and each particle would then create its own diffraction effect. A molecular process such as tarnishing or anodizing would perhaps be required. Similarly, a narrow nonreflecting ring around the outer edge of the main mirror, tapering inward toward the center, could annul outer-edge diffraction. And finally, an appropriate pattern of tapered black lines could annul spider diffraction. "The result, I think, is a diffractionless reflector!" One optician to whom this proposal was shown commented, "This subject is dear to me. Webb's idea has some merit and-some questionable points." Another said, "I've been thinking about the idea off and on for months since Webb's proposal was shown to me. In my opinion it would work well. Say the spacing of the subwavelength particles went from.... to ...., and then to solid packing at the black portion. As the spaces approached particle size, diffractionless partial reflection should occur. Such partial reflection does take place in fluids when the particle size is less than 1/100-wavelength. How the transition could be attained in practice I don't know. Probably not in 1/4-inch or so. So here, the gift of Webb, is a route to a fortune for some developer-or anyway some fun. IN January this department gave an account of a proposed method to reduce the weight and cost of mountings for very large telescopes to a small fraction of the customary amounts. If the large primary mirror and the heavy mechanism that supports and moves it are removed from a conventional mounting, the only essential parts that remain are the plateholder and a simple mechanism for moving it, or the correcting plate and plateholder of a Schmidt telescope. Mounted as a pivoted single unit, these need not weigh or cost a tenth as much as the original instrument. The primary mirror is attached to the earth without mechanism and derives its necessary motion from the earth. Such a telescope could photograph only one relatively narrow band around the heavens but a chain of them would together cover the sky. Concerning this proposal Lyle T. Johnson, a physicist and amateur astronomer of La Plata, Md., states, "Your article was extremely interesting to me. Väisälä's proposal for a large telescope is the same as one I sent you in 1947, except that he uses a large number of spherical mirrors while I proposed a large single mirror. Russell W. Porter liked the idea when you forwarded it to him, but others at Palomar were against it, mainly on the ground that there would be flexure of the large correcting plate. But this could be reduced by using a spider to give it support at the center." "Later on," Johnson continues, "I figured out ways by which the correcting plate of the monster telescope could be dispensed with by putting the correction on a secondary mirror. [Three methods are shown in the upper drawings in Figure 2.] The first is an adaptation of my modified Gregorian [described in this department in September, 1949]. The second is a Cassegrainian or Gregorian with a flat near the primary. The third has a perforated flat at the primary focus and a Gregorian secondary." Johnson's present proposal is to combine his three unconventional telescopes with the fixed primary-mirror principle. He continues, "All the optical elements except the primary are attached to the mobile tube, which has a declination and a polar axis intersecting at the center of curvature. Elimination of the large correcting plate removes a limit on the aperture and permits use of the huge mirror's full aperture. The aperture would decrease as the telescope was moved." Those who have admired Russell Porter's interpretive drawings in Amateur Telescope Making and in this department may find interest in an extended category of coincidences between Porter and Roger Hayward, the present illustrator of this department. Both men grew up and learned to use machine tools in New England; graduated from the Massachusetts Institute of Technology as architects, Porter in 1896, Hayward in 1922; designed public libraries; painted water colors; moved to Pasadena, Calif., where they became acquainted and friends; built telescopes, spectroscopes, grinding machines and prisms; made large models of the moon; ran toolmakers' lathes; did wartime work in optics, and illustrated technical books. Hayward puts into his drawings the same kind of interpretation of mechanics and optics that Porter did. After illustrating this month's discussion Hayward wrote, "Johnson's fixed-primary telescopes are very interesting. I could not forbear adding a fourth which is a modification of the first. Its advantage lies in the fact that the image is brought back to a point near the trunnion, which is desirable from an engineering point of view. It also uses the tiny flat of Johnson's first scheme, cutting down diffraction. If the instrument is built so that the observing platform is near the ground, the dome and superstructure would be relatively small. I have indicated, by the scale of the observers in the diagrams, instruments with a primary-mirror array 28 feet wide, a radius of curvature of 56 feet and an effective aperture of about 14 feet. The focal ratio at the prime focus would be 2. If the primary-mirror array were arranged to be rolled north and south on suitably curved rails, the instrument could cover most of the visible heavens except the polar region. "The principal defect in this scheme is that the Cassegrainian or Gregorian instruments are by nature long-focus affairs, which means that exposures must also be long, yet the length of exposure with this instrument is limited by the east-west diameter of the primary-mirror array. If the mirrors had the east-west diameter that is shown they would permit exposures of an hour. This is very short, especially for spectrographic work where limiting exposures are about 100 times as long as for direct photography." Continuing on another subject, Johnson writes: "Väisälä's trial telescope with seven 12 1/2-inch mirrors in a cloverleaf (pattern and 33 1/2-inch diameter, described in the January issue, has interesting possibilities for the person who wants to build a fairly large telescope. If five 16-inch mirrors are grouped as shown in the drawing, the clear space between them, about 12 1/2 inches in diameter, might be used as the central 'perforation' of the primary. Thus this secondary would obstruct practically none of the light going to the 16-inch mirrors. The large mirrors are spherical and the correction is on the secondary." A VARIANT of the double yoke mounting with an offset declination axis to permit access to the pole of the heavens, described in this department in the February issue, is proposed by Frank McCown of Holtville, Calif., and is shown in one of the drawings. The purpose of the dog-leg yoke is to keep the eyepiece clear of the A-frame. The drawing in the lower left-hand corner shows how A. G. Chartier of East Hartford, Conn., mounted an Amici roof prism as an image erector, in the usual place of the diagonal mirror. He seated the prism in a right-angle groove in a cube of brass. IF the diameters, focal lengths, and spacing of the two lenses in a Ramsden or a Huyghens eyepiece are given, what is the apparent angular field of the eyepiece? No answer to this problem has been found in the reference books on optics, but one is now supplied by Lieut. Col. Alan E. Gee of the Frankford Arsenal, who is privately an amateur telescope maker. "The kind of answer depends upon how fussy you want to be. To determine it exactly a designer must ray-trace to find the angle of the limiting principal ray. In practice, there are several ways of arriving at an excellent approximation. In the drawing the Greek letter alpha is the apparent field in each case. The sloping dashes show the location and proper size of the eyepiece field stops, if any. Obviously, it would be simple to determine alpha by starting at the objective and using 1/f = 1/u + l/v to determine the intersections with the axis, and then using trigonometry to find the final angle. The clear aperture of the field lens will be the limiting field stop in the absence of metal diaphragms. "Even that method is needlessly precise. A satisfactory and much simpler method is to measure the clear aperture of the field stop (if any) or the field lens and figure thus: ab = diameter of stop or field lens, cd = e.f.l. of eyepiece, alpha = apparent field. Solve for alpha by trigonometry, or scale off and measure with a protractor. "Properly, if this method is used with the Huyghens eyepiece and ab is the stop (not the field lens) diameter, then cd should be the f.l. of the eye lens only. In practice the difference is negligible. "Note that these methods indicate that the diameter of the field lens is the controlling factor for the apparent field. This presupposes that the eye lens is large enough. I have never seen an eyepiece where this was not the case. Anyway, the drawing shows how to determine the minimum size of the eye lens necessary to pass the field that is passed by the field lens. Both eyepiece lenses should be a little larger than the drawing indicates, so that no vignetting of the objective aperture occurs at the edge of the field.
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SHED |
MASTERS
AND STUDENTS IN the Oundle School, the first of England's ancient "public"
schools to make engineering and science its chief educational tools, built
the telescope shown in the drawing on the opposite page. H. C. Palmer,
assistant master in the school's department of mathematics and science,
has supplied the facts concerning it.
