When a worker has completed a six-inch telescope, though it should magnify from 50 to 200 diameters and be the equal of a refracting telescope four inches in diameter and costing 300 dollars or more-a real astronomical telescope-he is ready to tackle something larger. A 10-inch is about the ideal size for the average amateur ultimately to possess, representing a compromise between skill, pocketbook, and pipe dreams.

MR. RICHARD NETTELL, 155 North Boylston Street, Los Angeles, California, sends us a description and photograph of his 10-inch reflector. "I have made four or five telescopes before," he writes, "but this last effort of mine was made since I got the SCIENTIFIC AMERICAN instruction book 'Amateur Telescope Making.'

"The point I wanted to stress in my design was convenience and ease of observation and operation. The observer stands on the ground level, which is a great convenience for elderly or nervous people. The telescope is a tripod, take-down affair, as I have no room to keep it set up. The mirror is 10 inches in diameter, with a focal length of 130 inches. The tube is 11 inches in diameter and is eight feet long. At the upper end of the tube there is a three-inch flat, reflecting the rays back to the diagonal located opposite the eyepiece: making three reflecting surfaces in all. I appreciate the fact that by this design I lose some of the light, besides the loss through adding another reflecting surface, but I thought the convenience of standing on the ground more than compensated for that loss.

"The grinding and polishing were all done in the evenings, within three feet of our eook stove. The curve of the mirror when I got through with polishing it was as perfectly spherical as I could determine by the knife-edge shadow test; this with a room temperature drop of two degrees per hour. It showed in a slight degree the oblate spheroid form when the room temperature was even. All my previous mirrors had been more or less hyperbolical, and I wanted to avoid this if possible, so I estimated that the drop in temperature out-of-doors would give it the necessary parabolic form if I left it spherical.

"In performance, my mirror, when testing it on distant bright terrestrial objects gives fairly sharp definition when using a 1/5-inch focus eyepiece. That means a magnification of 650 diameters.

"The mounting is an equatorial, on a collapsible tripod, with a clock and weight drive. The pipe handle to carry it by, is also the finder."

Mr. Nettell evidently has succeeded in producing a good mirror despite proximity to a stove. Usually mirror making is done in the cellar, because there the temperature is most nearly uniform. Yet if the kitchen were also held uniform, and were about the same each night, the difficulty might not be great, for it is changing, not changed, temperature that causes trouble, as Ellison states (page 95, "Amateur Telescope Making"). One point to be considered is that ordinary pitch is too soft when used in a warm room. However, it may be boiled down to greater hardness, or resin may be mixed with it. At any rate, nothing succeeds like success, an adage which Mr. Nettell seems to have proved. He gets results from his telescope, it suits him, and that's that.

MR. ARTHUR W. LAMBERT, JR., Arcadia, Missouri, started on a four-inch mirror which he made from a disk of half-inch plate glass. His telescope is shown in one of the illustrations. Later he made a six-inch. He suggests that two mirrors should be started at the same time and "leap-frogged." He writes, "Let the worker get the first one ground and polished. Then mount it, silver it and begin to observe. He ought to get good results, at least intriguing and inspiring. Then let him try his hand on glass No. 2 and bring this to some kind of figure. He will improve on his first, then can switch mirrors, and can begin again on No. 1. By the time he has it again in the mount he will know something more than he did and can begin again on No. 2. This can go on indefinitely, until perfection is attained.

"Another thing really surprised me. That was the real pleasures you can get from a poor glass. My first mirror, a four inch made of windshield glass, is a very poor mirror. It brings a bright star to a sort of cock-eyed focus. Yet it splits double stars, sees the moon very well, and performs in a surprisingly good manner. I have it mounted in the telescope shown in the enclosed picture. With this little fellow I easily find the ring nebula in Lyra."

COMMERCIAL polished plate glass IS the material almost universally used lor the mirrors of reflectors less than 12 inches in aperture, but a finer material, though a more costly one, is Pyrex. It has the advantage of a low coefficient of expansion-about one third that of glass. Where the amateur is skilled enough to produce a nearly perfect figure he may use this material to good advantage. Mr. George Staffa, 32 Front Street, Sehenectady, N. Y., used it, and reports as follows:

"The mirror was ground according to instructions in 'Amateur Telescope Making', from two 71/2-inch by 3/4-inch disks of Pyrex, to a focal length of 54 inches. The polishing was done on a lap made of resin, beeswax, and paraffin which, while hot, was impregnated with rouge. This took only a short time, as the grinding had been done very thoroughly; in fact the whole process of polishing and figuring took only about four hours. The mirror was polished to the very edge, as is show by a microscope. I experienced none the troubles described in 'Amateur Tel scope Making.'

"The figure appears just the same after standing for half an hour as it was rig after removing iron the lap. Very goo results were obtained on Saturn; also Jupiter, on which the markings can be see quite in detail. There are a number other telescope enthusiasts in Schenectady who have made mirrors of Pyrex."

MR. GEORGE H. CHASE, 28 Washington Square, Newport, Rhode Island, has erected an observatory on the roof of his shop. He writes:

"The observatory shown in the photograph was built by Milton H. Chase and myself. The tower is built on top of a concrete workshop and is 14 feet in diameter. It rests on steel channels, and rolls around on eight wheels, being moved by an electric motor through a chain drive which passes around the outside of the tower.

"The electric motor control is on the post near the observer's seat. The framework of the tower is built of two-by-four joists and is covered with corrugated galvanized iron-on the outside. The inside is covered with wall board. The present telescope is a six-inch, but provision has been made to take a 12-inch. This observatory is located at my home in Portsmouth, Rhode Island."

Speaking of amateurs' private observatories, Popular Astronomy (Northfield, Minnesota) began in January publishing an interesting series of descriptions of these which is to run through the present year.

HOW about making a spectrohelioscope? Since Dr. Hale contributed instructions for the job, ("A. T. M.," pages 180-202) a disappointingly small number have tackled it. One big obstacle is undoubtedly the cost of the grating, 125 dollars, but the remainder of the materials car, be made by the amateur. In a recent letter Dr. Hale states, "You will be glad to know that spectrohelioscopes of the type shown in my article are already in use at Pomona College, California, the University of South Dakota; the University of Michigan; Yerkes Observatory; Ohio State University; Vassar College; the Royal Observatory at Greenwich; the Astrophysical Observatory at Arcetri, Florence; the American College at Beirut. Syria; the Government Observatory at Canberra, Australia; and the Observatory at Samoa. Others have been ordered for use in Chicago, England (2), and China, and two native Indians in Madras (one an amateur) are building outfits for themselves." Isn't it time more amateurs jumped into this interesting game? Thus far, five American amateurs have nibbled at it, and of these some are believed definitely to be "hooked." The blueprints cost only a dollar and a half. Reread Dr. Hale's chapters, also the Astrophysical Journal for December, 1929, and see whether these do not make you want a spectrohelioscope.- A. G. I., Tel. Ed.

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