THE LARGE PORTABLE TELESCOPE shown in the drawings below is a long-focus instrument of the type best suited to planetary observation, yet it is small enough to be carried disassembled in the trunk and rear seat of a car. Weighing 215 pounds, it is a careful compromise between full stability and extreme lightness. Some of its features might well be worth including in any telescope.

The builders of this instrument, R. A. Stolle of the Stolle Engineering and Manufacturing Company, Los Angeles, Calif., and his son Mark Allen Stolle, became telescope makers by accident. One evening they started out for a science lecture, but through a mixup of addresses found themselves instead at a meeting of the Los Angeles Astronomical Society. "From then on," Stolle senior writes, "we made telescopes.

"After we had built a 4-inch refractor, and an 8-inch reflector with gas-pipe mounting, we had an opportunity to see George Schmid's 10-inch f/7 portable telescope on a mounting that he had designed. It was operated from an Edison battery in his car with a little d.c. electric motor. Seeing that there could be something better than our mountings, we began work on a mounting like Schmid's for a 10-inch long-focus mirror we procured from Thomas R. Cave.

"Everyone was skeptical about whether such a long telescope could be at once portable and sufficiently steady. So we modified some of the Schmid features to handle our longer tube, hoping that the design would prove satisfactory an still be portable-and I don't mean like a piano.

"We find we can disassemble this instrument and pack it in our car very nicely, with the stepladder that we us for observing attached to ski racks on top. Without our knowledge we were timed, it took us only seven minutes to remove the parts from the car, carry them a short distance, assemble them and have the telescope in operation.

"The pedestal is as steady as if the pipe were set in concrete, and the mounting, while not perfectly steady, is so nearly so that if you slap the tube it will settle after about three oscillations, or as fast as you can count three. It has surprised a good many skeptics who believed that only concrete and massive weight could hold a telescope of this size and length steady.

"The drive movement is a 1-watt synchronous motor. It operates on 110-volt, 60-cycle current when we are at home. When house current is not available we use our car battery, or any small 6-volt battery, with a 6-volt vibrator inverter which changes 6 volts d.c. to 110 volts a.c. This vibrator is similar to those used for radios on cars. We can hold a star in the center of the field for several hours with a half-inch eyepiece giving 225 diameters magnification. We can do even better than this if we vary our frequency with an inverter.

"The long tripod legs are individually adjustable, making it possible to set up the telescope on uneven ground. The upper tips of their diagonal braces latch into sockets on the side of the pedestal and are held there by spring-loaded balls while the screw in the brace is run out.

"Please give much of the credit for this telescope to my son Mark, with supervision did most of the actual work, and to Schmid, whose design was largely used."

The pedestal of the Stolle telescope is made of 3 5/16-inch outside diameter, 3/16-inch wall steel tube. The mounting is carried on a short tube that drops over the pedestal, and carries a level glass for plumbing it. The polar-axis adjustment is a sector, permitting accurate use of the telescope in different latitudes.

The polar axis is a short length of 1-inch Shelby tubing with an adjusting nut. Sliding over this is a length of 1 3/8-inch outside-diameter Shelby tubing with a knurled brake-tension adjusting nut. The braking surface is on the side of the bronze worm wheel, which is lubricated with vaseline. This supplies the desirable slip-ring feature described by Russell Porter in Amateur Telescope Making, fourth edition, pages 145-146, and saves much unnecessary labor in setting the telescope on stars from the atlas or ephemeris.

On the 1-inch declination-axis shaft is a lock for the extension that carries two counterweights. (The second counterweight is used sometimes for balancing a camera and at other times to balance a 4-inch refracting guide telescope that rides pick-a-back on the larger telescope when it is used as a camera.) Also on the declination-axis shaft are a knurled adjusting nut for smooth rotation in declination, a larger knurled nut for brake-tension adjustment, and a fiber washer next to it; this end is arranged much like the lower end of the polar-axis shaft. At the head of the declination axis shaft is a 144-tooth 22-pitch bronze gear, and against its side another lubricated braking surface.

Believing with Stolle that the braking surfaces damp the vibrations and thereby give the telescope its steadiness, illustrator Hayward, himself an amateur telescope maker, suggests experimenting with lanolin for lubrication. Lanolin, he points out, is quite viscous, yet it has a starting friction of nearly zero; it might damp the vibrations critically and increase the stability still further.

The tube saddle consists of two members having a deep, stiff section. The tube rotates in the saddle rings, obviating the neck-wringing otherwise suffered by the observer. Knurled screws on each of three tube-section joints permit quick assembly of the tube.

The eyepiece-diagonal unit slides as a whole on ball bearings against two rods; focusing is accomplished by turning a knurled screw below the eyepiece. This screw turns a rubber friction roller that bears against one of the rods. The adapter tube slips quickly into position, as determined by a spring-loaded ball on the inner wall of the boss or ring that receives it. This boss can receive an eyepiece of full 2-inch diameter to accommodate the wide images given by the long-focus mirror. A camera may be attached in place of the eyepiece for photography either at the prime focus or through the eyepiece. The diagonal support arm is a 5-inch ring for spreading the diffraction around the field, and is made of hard rubber to avoid the temperature effects of metal. The entire telescope has a black crackle finish.

REQUESTS for full instructions to build riflemen's target and spotting 'scopes have reached this department occasionally for many years. Nearly all of these come from riflemen who are not telescope makers. There is a chapter on target 'scopes in Amateur Telescope Making-Advanced, but no pretense is made that it is suited to optical novices. It has therefore been necessary to advise such inquirers to approach their goal by a roundabout route: first make a reflecting telescope as an apprenticeship; then make a small refractor (which might be a spotting 'scope); then tackle the desired target 'scope. This advice has seldom appealed to riflemen.

In a new book entitled Sporting Rifles and Scope Sights, written by Truman Henson and published by David McKay Co., New York, 93 pages are devoted to instructions for building three hunting 'scope sights, one target 'scope sight and two spotting 'scopes. The rest of the book deals with the conversion of discontinued military arms into sporting rifles.. The instructions are full, detailed and specific. Full-dimensioned drawings are included for each 'scope. The author, a lawyer whose hobby is riflemanship, has himself made every scope he describes excepting their optical parts, which he obtained from war-surplus lens dealers. He has included enough basic optics to assist the non-optical builder adequately.

THE basic principles of the lens-erecting system for terrestrial telescopes are described in the following notes Allyn J. Thompson, 1628 Mayflower Ave., New York, N. Y., author of the book Making Your Own Telescope. Anyone who plans and constructs one of these systems with its aid is invited to describe the experience and the result.

The manner in which the terrestrial telescope functions is shown in the upper drawing on this page. Rays from a distant axial object point are brought to an axial focus in the plane y of the objective O. Rays (dashed lines) from a marginal point of the field are brought to a focus on the opposite side of the axis in the same plane y. This gives us an inverted image of the distant object, Now suppose we regard the erecting lens L as a projecting lens. By placing it at some distance p that is greater than its focal length from the image y, an image of the latter will be projected to y'. The image is turned around or inverted in the course of projection, so that the new image is erect. The distance p' to the projected focal plane y' is found from equation 1 where F' is the focal length of L. The relative size of the images y' and y depends on the ratio p'/p. In the diagram, p' is twice the distance p; the image y has therefore been doubled in the projection, and in effect the focal length of the objective has been amplified two times. The equivalent focal length of the telescope is given in equation 2. The new image y' is viewed with the aid of the eyepiece E, and the total magnification M of the telescope is given by equation 3 where F'' is the focal length of E.

A field stop of suitable aperture should be placed in the plane y' to delimit the field to one of even illumination, and to cut off the poorly imaged external parts. The reticle can be used either in the same plane or at y. By mounting the erector lens L in a separate focusing tube so that the distances p and p' can be varied, a similar variation in magnification is effected. This action of course will affect the position of y', and complicates the matter of reticle installation. Also, the use of the telescope for different object distances affects the position of both y and y'. In these circumstances, the best thing to do is to mount the reticle in a unit within which the eyepiece can focus.

Just as the image y is projected by the lens L, an image of the objective O is projected to O'. The distance LO' is 3 found from formula 4, where D_o is the sum of the distances F and p. The relative sizes of O' and O are proportional to their distances from L. O' is the exit pupil of the system OL, and is the entrance pupil of E. As any light passing beyond the boundaries of Q' may serve only to fog up the image y', a stop (called an erector stop) should be placed in the plane O'. An image of O' is projected by the eyepiece to O''; this is the exit pupil of the telescope, and it is in this plane that the lens of the eye should be placed when viewing the image. The distance EO'' is known as the eye relief or eye distance, and from formula 4 it is apparent that this distance is greater than if the erecting lens were absent, a condition that is also found in the astronomical telescope. The diameter of O'' is equal to that of O divided by the total magnification of the telescope, provided of course that there is no curtailment of O' by the erector stop.

The erector eyepiece of the upper drawing is the type that was devised by the German mathematician Christoph Scheiner in 1637. In practice it is rendered nearly useless by overpowering aberrations. An immense improvement was effected in 1645 by the Bohemian astronomer Antonius Maria Schyrleus de Rheita, who substituted for L two equal plano-convex lenses separated by their focal lengths, with the convex surfaces facing each other, and for E a Huygenian eyepiece. This arrangement corrects for chromatic difference of magnification, and at f/8 or higher ratios its performance compares well with that of modern designs. Rheita's erector is shown in the second drawing, although a Kellner eyepiece is used in that illustration instead of a Huygens.

In using a two-lens erecting system, the amplification or relative sizes of apertures at y and y' depend on the proximity of the erectors to y. If this distance is chosen so that m and n are equal, the image sizes are equal, and the magnification is unity. By moving the system closer to y, the projection distance and the image size are increased. Actually projection distance is not represented by n, nor the object distance by m; these measurements are referred to what is known as the principal planes of the system. There is no need, however, to delve further into the study of optics. The distances m and n, which are all that are necessary for construction, can be obtained by using an illuminated artificial image at y, and experimentally positioning the erectors until a sharply focused image of the desired enlargement is picked up on a ground-glass screen.

With the lens separation given for Rheita's erector, the pupil at O' is formed within the second erector lens, so the best place for the erector stop is immediately in front of that lens, as shown. To avoid vignetting in the external parts of the field at y', the erector lenses must be of suitably wide diameter. The clear aperture of the first lens should be as shown in equation 5. The clear aperture of the second lens need be no more than that at O'.

The shorter the focal length of the separate erector elements, the less will be the over-all length of the telescope. Modern terrestrial telescopes employ achromatic eyepieces, seldom of more than three inches focal length. Most achromatic eyepieces usually function well for this purpose, as do projecting systems. A single doublet lens will often be found to perform satisfactorily. An arrangement frequently employed is that shown in the third drawing-two identical achromats placed almost in contact. Hardly anything will be found to excel the performance of a Hastings triplet. This is a magnifier designed by Charles S. Hastings, and manufactured in the U. S. by the Bausch and Lomb Optical Company. It is known as a triple aplanat, and is obtainable at shops dealing in optical goods. The requisite aperture can be determined from formula 5.

In lieu of an optical bench, experimental setups can be made by standing the various lenses edgewise in lumps of artists 'modeling clay mounted on a stick. Care must be taken to maintain a fairly good axial alignment of the elements. In making tests for image quality, magnification, and best lens spacing, the stick should rest securely on some solid object. This allows for painstaking inspection of the image.

Suppliers and Organizations

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Internet: http://www.sas.org/

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