"I made all the castings, also machined them, and made the eyepiece," he states. "I spent about four months of spare time on the objective. The images are beautiful." Casey had previously made a reflector.

Figure 2 shows the two ends of this 3 1/8" refractor.

Figure 3 is a 4" Pyrex-mirrored richest-field telescope of unusually attractive design and appearance- clean. Casey cast and machined the aluminum cell, screw-on cover, and screw-on tube bottom (identical with the cover). All optical elements are fully adjustable for collimation. The eyepiece tube has a spiral focus adjustment. The main tube is covered with leather.

Figure 4 caps Casey's climax, a telescoptical invention called the "Flexoscope," which he says is a technicratic aplanat of focal ratio 49.98.

From another amateur telescope maker who finally chose the refracting type of telescope comes the following: "After almost a year's use of my 7 1/2 " refractor I have found its performance, for general, all-around observing, far superior to that of my former 6" and 10" reflectors.', This is from the anonymous "Mr. X," of Mamaroneck, N. Y. (address on request), whose 7 1/2" refractor was described here last July.

Making an objective lens for a refractor is a more tedious but actually less exacting job than making a mirror for a reflector. Chief deterrent is the cost of the blanks, also the need of a flat, a lathe and a spherometer; otherwise nearly every amateur might try one. (It has been suggested by some genius that, in future copies of "ATMA," pouches be inserted at the end of Haviland's chapter on objective lens design and construction, containing the flat, spherometer and a lathe.)

Despite deterrents, refractor making is gradually increasing, as more and more amateurs come to realize the truth of testimonials such as the one above. This is not, however, an attempt to depreciate the reflector. Nevertheless, many an amateur harbors in the back of his mind the hope that, some day, in some way, he can make and use a fair-sized refractor.

CASSEGRAINIAN telescopes appear to have a fatal fascination for the man who has not yet made any telescope, judging from this department's mail, and the main reason evidently is their high magnification. Most aspirants seem quite willing, however, to accept the advice of those who have been there before, to tackle a Cassegrainian only after making two or three less complicated types. A very few others have to experience

the actual feeling of adversity before their self-confidence is tamed. Occasionally, too, a genius turns up who evidently can do difficult things without the preparation that others require. Rummaging among old letters, one was found in which Alan R. Kirkham, no longer active in telescoptics but "tops" while his health lasted, put the matter of lightly tackling a Cassegrainian in a new way. "A Cassegrainian or a Gregorian imposes extremely severe demands on the primary mirror. For illustration, with a secondary which amplifies the image four times, a 1" eyepiece looks the same to the primary mirror as a 1/4" eyepiece, while a 1/4" eyepiece looks to it like a 1/16"-and just show me the hombre who can make a mirror which will take a 1/16" eyepiece and not howl.

"My experience," Kirkham continues, "is that the difficulty of mirror figuring increases with the square of the magnification, right up to the point where the telescope reaches the resolvable minimum. If those who think primarily of high magnification would read what able designers like Conrady, Hastings, and so on, have to say about the wave nature of light and the relation of the instrument to the eye, we would hear less about it. It can be shown that a range of from 5 to 40 magnifications per inch of mirror diameter more than covers all the useful range, and should be obtained, on ordinary Newtonians, with 1/2" to 1" eyepieces. Both theoretically and practically these are the best eyepieces.

"In making a telescope one should put all the fussing into figuring the objective-mirror or lens-so well that it will concentrate as much of the light as possible within the diffraction disk, resulting in a practical telescope whose star images would not break down at 1000 per inch magnification if such could be used. But we will see all there is to be seen at 20 diameters per inch; Martin even says that 10 per inch enables sharp eyes to see all that an objective can show, and that, by doubling this, all the detail the image affords is rendered easily seen."

Unfortunately, until the telescoptician has laboriously plodded through the reasons, which in turn involves doing enough general background study of optics, in books like Valasek's `Elements of Optics," Hardy and Perrin's "Principles of Optics," and Martin's "Introduction to Applied Optics," all of which would be a job to nibble away at for a year or more, such arguments don't usually seem very important and the enthusiastic tyro may deceive himself with the hope, altogether false, that high magnification will dig out the image detail even if the mirror, and therefore the image, isn't very good. But it won't! An exactly parallel experiment would be trying to compensate for poverty by writing a check for $1,000,000 payable to self; all of us could write the check but .... Similarly, the high magnification eyepiece on a less than fine mirror will "bounce." One may safely take this not merely from the armchair optical theorists but from experienced, flea-bitten old telescope users. High magnification requires high mirror perfection and, even then, it isn't good for much.

While it is a practical certainty that all amateur telescope makers will play harps in another world, because of saintliness in this, one amateur who turned professional and wished to make assurance doubly sure recently began taking lessons on a tangible mundane harp in Illinois, and immediately invented a telescope accessory inspired by it.

One of the earlier amateur telescope makers was William A. Calder, then of Beaver Dam, Wisconsin, whose telescopes were described here years ago. Calder was studying to be a physicist at the time, but the hobby swerved him into astronomy. Now he is Professor of Astronomy at Knox College, Galesburg, Ill. The "harp" applied to the telescope is shown in Figure 5, but it isn't musical. Professor Calder explains it thus:

"When a series of equally spaced rods is placed over a telescope objective, the diffraction pattern which constitutes a stellar image becomes flanked with a series of images of decreasing intensity. The relative brightness of the central and auxiliary images, and their separations, may easily be computed according to well-known formulas (King, 'Celestial Photography,' page 138). This device, the objective grating, has been very useful in connection with the problem of the relative brightnesses of double stars, but its use has been photographic. Kuiper and Hertzsprung have estimated the relative brightnesses of visual binaries by placing an objective grating over the telescope and comparing the central image of the fainter component with a side image of the brighter component. It is, however, difficult to estimate the relative brightness of objects which differ by more than a few tenths of a magnitude. By means of a coarse grating, we can divert a known fraction of the light of each star into first order diffraction images and then compare the central image of the fainter star with the effectively weakened image of the brighter star. But, when one makes a grating with the opaque space having a definite, fixed ratio to the clear space, it is found in practice that double stars whose first order image of the brighter equals the central image of the fainter are rare. Nevertheless, the eye is at its best, photometrically, when it matches intensities, but it is unable to judge the amount by which light sources differ. To improve the existing inexact data on the relative brightnesses of binaries, would it not be fine if, therefore, the astronomer could use a variable grating? He then would simply set the grating so that the central image of the fainter matched the first order image of the brighter. Then he would note how the grating was set and, knowing the ratio of clear to opaque, he would have the exact difference of brightness of the stars to an accuracy limited only by the ability of the eye in equalizing intensities.

"It would be mechanically impossible, however, to make a grating whose capacity could be varied, after the manner of the Venetian blind, but the same thing can be accomplished by the simple method of changing the projection of the grating on the telescope objective. Here is where the harp came in. Seated at the harp after my first lesson, I noticed how the clear space between the strings appeared to vary with my position. I therefore hinged the grating on the end of the telescope tube.

"I believe that many amateurs would enjoy making this accessory and could do useful work with it My first results indicate that mar,' of the entries in double star catalogs can be improved. The grating shown in Figure 5, on my 12" reflector, is made of aluminum rods 1/8 " in diameter, with 1/4 " spaces between. When the grating is perpendicular to the axis of the telescope, the first order diffraction image is 1.92 magnitudes fainter than the central image; when the grating is opened out to 40° on the hinges, the magnitude interval is reduced to 1.29."

If any reader of this department listens to Amos n' Andy, on the radio, let him be advised that "de Lodge" has a chapter among amateur telescope nuts, three of whom, though widely separated, pass as Amos (Fred B. Ferson, 414 Reynoir St., Biloxi, Miss.), Mist' van Potah (Russell Porter, of Pasadena, Calif.) and Andy Your scribe, who inflicted that name on himself in a fit of sincerity). Capt. McDowell, who superintended the 200" mounting, is "de Kingfish."

Mist' van Potah found he couldn't sleep one night, so he sat up in bed with three hunks of sugar pine and a jack-knife and whittled the bed and the room full of shavings. What was left proved to be the patterns for the four- or five-inch brass statuettes shown in Figures 6, 7, and 8.

Figure 6 is an amateur telescope 3 maker stopping work polishing to examine his mirror with a watch maker's loupe.

Figure 7 is quite complex: a TN seated on the floor, at right, making a knife-edge test on a mirror at left. Between is his polishing place on the same plank, with a little pot of rouge and a brush sticking up, all very faithfully cast in brass.

On the occasion of a visit to Amos' cabin in Mississippi, Mist' van Potah and he spent several days together, molding, casting, snagging, filing, scouring, chiseling and otherwise finishing these objets d'art. Amos still has the patterns for making more statuettes. He is the author of the chapter on molding and casting, in "ATMA," this work being a sort of side-hobby to his amateur telescope making.

Suppliers and Organizations

The Society for Amateur Scientists
5600 Post Road, #114-341
East Greenwich, RI 02818
Phone: 1-877-527-0382 voice/fax

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

At Surplus Shed, you'll find optical components such as lenses, prisms, mirrors, beamsplitters, achromats, optical flats, lens and mirror blanks, and unique optical pieces. In addition, there are borescopes, boresights, microscopes, telescopes, aerial cameras, filters, electronic test equipment, and other optical and electronic stuff. All available at a fraction of the original cost.

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