IF YOU ARE AN amateur telescope maker, how many times have you had your whiskers rubbed a little backward by non-astronomical visitors who gratuitously assumed that you must have made your telescope from readymade puts which you had merely assembled, and that "Of course, you didn't make the 'glasses. '" Exercising noble self-restraint-for, after all, it must seem a bit fishy to the uninitiate-you modestly reply that you made the main mirror and most of the rest. Your visitor's eyebrows now go up and so does your stock. You then point out that making the mirror is really the central part of the fun, but that you had about as much fun in concocting the mounting out of various thises and thats which you adopted and adapted. By this time your visitor is on the way to being hooked as another addict of the hobby.

When it comes to using things originally intended for other purposes but kidnapped and re-adapted as parts of a telescope, and doing so neatly, the 10" instrument shown in Figure 1, made by Warner Williams, a Chicago sculptor and designer and connected with the Culver Military Academy at Culver, Indiana, has a high score. Examining the photograph:

The base is of concrete. Williams first modeled it smoothly in clay-to him as a sculptor a familiar medium-then cast a plaster mold of this model, and finally filled that mold with concrete.

The polar axis is a 6" artillery shell casing, and it is both clean and rugged-certainly won't shimmy.

The fork arms are made of the two rear axle housings from a car; such parts are steady enough for a jolting car but require stiffening for a telescope jolted only by light breezes. It's a question of just how stiff you require. In this, a telescope is an extreme case.

The declination axis bearings are connecting-rod bearings from a car motor.

The two setting circles are meridian circles taken from geographical globes.

The finder is built around a mailing tube.

The main tube is sheet metal formed on a tinner's brake, and its open section is made of 1/4" water pipe.

The mirror cell is made from a motorcar brake drum.

The driving mechanism embodies a back gear (1/2 r.p.m.) motor unit such as are used for display tables, gears from a car generator, parts from a player piano and a carburetor. The drive is transmitted to the polar axis drum by means of an endless belt made from a steel tape. "This type of drive," Williams writes, "has the distinct advantage that no special gears need be cut or purchased. The drive wheel can be turned up on a wood lathe to any needed size to give the proper ratio between either standard gears or available junk and the final pulley in the train.

"And thus," he continues, "after a long and fanatically joyful struggle, my telescope is finished and promises years of use and pleasure."

Williams once sculptured a life-sized plaster bas relief plaque of Foucault illustrated and described in this department in December, 1943.

IN THE October, 1945, number of The Journal of the Royal Astronomical Society of Canada, 198 College St., Toronto, Ont., Canada, E. K. White, Chapman's Camp, British Columbia, well known to amateurs in this country, a correspondent of your scribe's, and whose telescope (Figure 2), has a surplus of mechanical, optical, and artistic sex appeal, has so cogently brought together various arguments for the long-focus Newtonian telescope that we reprint his entire article here, with courtesy credit to the periodical named.

Judging from casual comments heard, it is suspected that some Yank amateurs think the Royal Astronomical Society of Canada must be composed at least of professional astronomers who, being "royal," all wear crowns. It is, instead, an organization of amateurs-plain folks, uncrowned, unhaloed-like our own amateurs. Obviously, these Canadians are more enterprising than we Yanks, since they have had their amateur astronomical society for 56 years. It includes 11 local city groups which meet regularly. Our long-projected American association still is a thing on paper, but, now that the war is over, it probably won't be long before Charley Federer will be tearing around the terrain helping organize one as per prewar plan. White's paper:

"This paper deals briefly with some advantages found by the writer in the construction and use of Newtonian reflectors with focal lengths about 12 times the aperture, over the more commonly known f/8 mirrors.

"In most articles dealing with the making of Newtonian reflectors the amateur is advised to choose a focal length about eight times the aperture of his mirror. Rev. W. F. A. Ellison says, 'I recommend f/8 because it combines the easiest figuring with capabilities for excellent performance' (see page 385 in 'Amateur Telescope Making'3 . Just what accuracy of figure is necessary in an f/8 mirror of (say) 8" aperture to give excellent performance? It is an accepted fact that a mirror will give perfect definition if its surface be figured to one quarter wavelength of sodium light, or about five millionths of one inch. It has been shown by Mr. F. B. Wright (page 257 in 'A.T.M.') that an 8" diameter mirror of f/8 must have its surface within 33 percent of a perfect paraboloid to give excellent results.

"In the case of an 8" diameter mirror of f/12, Mr. Wright tells us that it can be finished spherical and still give fine definition, for a sphere and a parabolic surface of this dimension are very nearly coincidental. The allowable tolerance from a perfect paraboloid for an f/12, 8" mirror is 110 percent. Most opticians agree that it is less difficult to figure a spheroid than a paraboloid, and the former is most certainly easier to test by the well known Ronchi method (page 264 in 'A.T.M.'). Also there are no r²/R zones to measure as with the paraboloid.

"The parabolizing of a mirror is necessary to correct the surface for the one aberration found in specula, namely, spherical aberration. In other words a spherical mirror of around f/8 proportions will not reflect the marginal and central rays of light from a star to the same focal plane. However, in small mirrors up to about 10" aperture and of f/12, this is not the case. The old masters, such as Herschel, With, and Lassell, who made their mirrors before Foucault's knife-edge test was known, realized that longer focal lengths enabled them to make better corrections on their mirrors, which also meant better performance. Thus we find that most of their mirrors were made to f/10 and higher, and many-of these specula did give very excellent performance.

"Let us discuss further advantages of long-focus mirrors. It has already been shown that they are at least a little easier to finish, and I have found that an f/12 mirror 'a little off,' will give better images than an f/8 of the same aperture which is about equally imperfect in figure. Turned edge cannot be tolerated in either mirror, but slight zonal errors are not at all serious in the longer focal length.

"Perhaps the most important advantage of long focus is that one can employ a smaller secondary mirror and thereby reduce light loss a little, and diffraction effects a great deal. In my opinion few amateurs realize the bad effects on stellar images a secondary mirror has when its minor axis much exceeds one sixth the diameter of the primary. One has only to place a cardboard disk about one quarter to one third the diameter of the main mirror on the secondary support directly in front of the flat or prism to see the bad effects it will have.

"For finest definition an elliptical secondary mirror that is really flat is superior to anything else.

"The more common types of eyepieces are designed to work best with a slender cone of reflected or refracted rays, and the f/12 mirror will give flatter and better fields with Huygenian and Ramsden oculars than will f/8 mirrors, which should really employ expensive achromatic eyepieces for best results. Also the f/12 mirror will enable one to obtain high magnification, should it be desired now and then, without seeking extremely short-focus oculars at high cost.

"Two further advantages of lesser importance are: first, changes of air temperature actually have little effect on the image quality as a result of temporary changes in the long-focus mirror's shape while in use, but such is hardly true with f/8 mirrors of plate glass. Secondly, many amateurs take photographs of the moon, using their reflectors as the camera. With focal lengths a little over 100" the image of the moon at the focal plane of the primary mirror is about 1" in diameter. This is large enough to record quite an abundance of detail on a plate or film, and bright enough for rapid exposures. With such a sizable image the use of an eyepiece may be dispensed with altogether. The writer has taken some fair lunar photographs with an 8-3/4", f/12 mirror on Panatomic film using exposures as short as one hundreth second. These negatives can enlarged to at least 8", and the resulting prints are quite sharp.

"It can be shown that the disadvantages of the long-focus Newtonian are few and not at all serious. Probably the outstanding one is that a longer tube will be necessary, which will require a well designed and rigidly built mounting, costing little more, however, than a good one for an f/8 mirror. Provided the instrument is permanently mounted out of doors, its shell will also require to be a little larger. The eyepiece position will be somewhat higher, but a good step ladder will bring it within easy reach. These advantages will be found to be of little consequence where the mirror is not greater than eight or ten inches in diameter.

"While mentioning the longer tube, one may wonder if tube currents are increased, particularly when a solid metal tube is used. The writer has not found this to be the case with a 9" f/11 mirror mounted in a closed metal tube. ('Air Currents Within the Reflector Tube,' Journal R.A.S.C., November 1943).

"The field of view will be smaller when using an f/12 mirror, yet with some Ramsden oculars of about 1" equivalent focal length, all of the moon's image can be seen in the field at one time with a mirror of about 100" focus.

"In conclusion, it might be of interest to mention the performance one may expect when using a good, long-focus mirror as compared to that of an f/8 mirror on different classes of celestial objects. With double stars there will be a marked improvement in the images of an f/12 mirror, and there will be less diffraction effect with bright stars. The images will be crisp even with high magnification (in good seeing), and quite comparable to those seen with a refractor. Nebulae and clusters will appear in the f/12 mirror much as through a refractor of equal size; a shorter focal length mirror would of course give brighter images of these objects, provided apertures were the same.

"With planetary and lunar detail the f/12 mirror comes into its own, and it is my opinion that such a good mirror will outperform any f/8 of the same aperture in this field. I have given an account of my observations of Saturn using a 9" f/11 mirror elsewhere ('Saturn with a Nine-inch Reflector" Sky & Telescope, March 1945.)

"The writer hopes that amateurs who are contemplating the construction a mirror will give the long focus some thought before deciding upon the usual f/8 focal ratio, and the tyro who may dread the difficult task of parabolizing cannot go far wrong in trying out a 6" spherical mirror of about 75" focal length."

HERE'S a good-news note from John W. Lovely, Secretary of the Springfield Telescope Makers, 27 Pearl St., Springfield, Vt.: "We at Stellafane planning to have a full-scale convention this year on Saturday and Sunday, August third and fourth." More details when they become available.

The Society for Amateur Scientists (SAS) is a nonprofit research and educational organization dedicated to helping people enrich their lives by following their passion to take part in scientific adventures of all kinds.