"The mirror is a 12" x l-1/2" plate glass porthole light. Focal length, 93". It was ground by machine ( made by Wanderer and Lennon) and figured by hand and machine by McCarroll.

"Lattice design of tube needs no comment. Tube rotates in saddle on 21 bronze rollers attached to tube. Three long rods (through roller axles) are for adjusting the mirror axis in proper optical line up. Mirror has three-point adjustable flotation pads. (See Hindle 'ATM,' 4th ed., p. 229.) The three short rods with wheel handles are for slow motion in R. A., declination, and clamping in declination. Clamp in declination is a bronze brake band around declination housing operated by gears. Declination slow motion is by gears to declination housing. Rough adjustment in declination overhauls gears. Right ascension control is designed to operate by a flexible shaft-not so good and will be changed.

"Prism is 2-1/4". Is secured by a 3-arm spider and is adjustable in all directions. Three finders make sky scraping easy.

"Mount consists of three heavy castings, a flange casting to take the 1/4" plate saddle and two 6" x 20" castings for the polar and declination axes. The shafts are 2-5/8" cold rolled steel, set in ball bearings at both ends of housings. Job is as solid as the Rock of Gibraltar.

"The bearing on top of the pedestal is a rocking device. It allows adjustment in latitude from 37 to 48 degrees. A threaded adjusting rod with wheel is attached to the underside of the polar axis and a calibrated dial shows the latitude.

"Circles are brass bands 1/8" x 1", etched by hand. Declination reads to 5 minutes of arc and right ascension to 15 seconds of time by means of verniers in each case. "The equatorial system was the brain teaser, with very little simple and available precedent to go on, but the final result has turned out entirely satisfactory. The R. A. circle is adjustable to sidereal time, as suggested by Porter in 'ATM,' but the adaptation is only remotely related in detail. Two verniers are required for the R. A. circle, one for sidereal time and one for setting in R. A. Simply take R. A. out of the Ephemeris and set instrument at once to sidereal time, which can then be read directly like a clock any time while the instrument is in operation.

"Drive is by a 1/40 H.P., 1800 r.p.m. synchronous motor (in pedestal) with reduction gears, including a 1-1/2" worm to a 4" standard spur gear wheel at bottom of worm rod. The latter drives the 15" aluminum worm gear disk on shaft.

"The large gear disk and the R. A. circle are mounted on a collar or hub which can revolve on the polar shaft. The worm disk is permanently secured to this hub, and the R. A. circle is adjustable on this by set screws. R. A. circle is set to sidereal time vernier and is then clamped in place. Pressure screws attached to shaft force a ring up against the hub which, in turn, forces the hub up to engage a friction clutch on shaft just below the end of the polar housing. Rough adjustment can be made at any time, the shaft revolving in hub through the slight friction of clutch.

"The worm rod is a 3/8" ordinary round drill rod with a standard thread, machined and the worm gear on the aluminum disk was cut by the thread on a hardened rod. It works!

"Pedestal is a 'Center Street Model'-of unknown lineage. A lucky break at a very nominal expense. [Center St., New York- second-hand machine equipment hunter's seventh heaven.-Ed.]

"The electric equipment (not fully shown will include a small lamp and hood with switch at R. A. sidereal time vernier. Another lamp will be on the declination vernier. A red pilot light (not visible in photo) is installed to indicate running of motor. Plug for cable is shown under gear bearings on pedestal.

"The whole assembly weighs 1-1/4 tons and is mounted on a truck running on a track. 'Scope is housed in one of the garages shown in the background and when in use it is run out on track to a platform having a floor just under the counterweight. This is a 22-gage sheet metal container filled with concrete and steel plate punchings.

"Since the photograph was made a lining has been installed inside of the tube at eyepiece end to cut off extraneous light, which was found to be very troublesome, from nearby street lamps.

"Mirror was 'axed up' by McAdam's method-with some presumed improvements. We made a 'B' disk ('ATMA,' p. 272) with a 4" dia. hole with cross threads to line up with pin-hole from 'A' disk. We found it impossible to sight through two small holes so far apart-8'.

"The job has taken exactly three years to complete, working at odd times outside of business hours. Performance is entirely satisfactory. Of course we have improvements contemplated for the 'next one.'

ON page 74 of this number Professor Russell describes the Struve spectrograph and mentions a Schmidt camera, shown on that page only as a small bump on the big instrument. Amateur telescope makers will take further interest in this Schmidt camera, especially as it is the same one that was made for Yerkes Observatory by C. H. Nicholson, of DuQuoin, III., a member of the Amateur Telescope Makers of Chicago and described in this department in December, 1936. Therefore we present Figure 2, taken from The Astrophysical Journal, for December, 1937, because it shows the camera in section. Its focal ratio is 1, its mirror is 110 mm (4-1/4") in diameter and; the correcting plate is 3-3/4" in aperture. The mechanical parts of the design are by Dr. G. W. Moffitt, now of Perkin, Elmer and Moffitt, optical designers and consultants, New York.

MARKING mirrors is the subject of the following note which rounds up considerable research done by Fred M. Garland vice-president of the Astronomical Section of the Academy of Science and Art of Pittsburgh (the Pittsburgh club of amateur telescope makers). If a hospital nurse can accidentally give a mother back the wrong baby, so may mirrors be mixed up. He writes:

"Sometimes an amateur sends his mirror to be aluminized, and then can't quite make up his mind whether the mirror he gets back is his own or not. The usual method of identification is knowing your own pet little chips (if any), or a small sleek at say 9 o'clock. If however, your mirror is without chips and sleeks, here are a few suggestions that may help if you desire to identify permanently the ownership or maker's name:

"HF, hydrofluoric acid, is chiefly used for etching designs and markings on glass. The glass is first coated with a film substance impervious to HF, like wax, paraffin, and so on. The desired markings are then cut through the coating with a scriber or sharp instrument, the glass is exposed to the HF or dipped into its solution; and, upon removing the coating, the marks are found etched on the glass. The vapor of HF leaves opaque and white tracings; the solution leaves transparent lines. HF is a strongly penetrating corrosive and care must be used in handling. [It is poisonous, the fumes are dangerous, and good ventilation during its use is recommended.-Ed.]

"Silver ink, black or white glass marking ink, monogram inks, frosting solutions and glass etching materials may all be bought on the market. Precautions are necessary to avoid acid burns or destroyed clothing. Good ventilation must be maintained; properly inspected rubber gloves will help to protect the skin from painful irritation caused by contact with some of the chemicals. The trouble is that, with many solutions of this kind, the glass must be heated to complete the process, and that is too much of a risk for the amateur before or after his mirror is ground. With a glass tumbler or electric light bulb any slight stress, such as is set up by such heating, will make no difference, and marking articles of that kind with chemical preparations is entirely another matter.

"The experience of some amateurs and at least two large glass manufacturers may be of value to the reader. Several methods of marking the back or side are given:

(1) Sharpen an old dentist's tool and use it as a scriber or etching pencil for fine lines.

(2) Diamond point pencils, or carbon points used in a pin vise, may be bought on the market, and leave a well defined mark.

(3) Vibrating electric pencils may be purchased on the market; electric power is required as is careful manipulation, since: the tool is usually heavier than those mentioned in (1) and (2). A small electric hand grinder, employing tools to cut or engrave, may be used.

(4) Sodium silicate (water glass) may be purchased at paint or feed stores. There is enough in a ten-cent can to mark hundreds of mirrors. This may be applied with a steel pen and will last a long time if protected. If the back or side of the disk is ground, the sodium silicate will etch into the glass and leave a permanent mark even after the sodium silicate has dried up. Also red or blue tallow or grease pencil marks will adhere to the glass and may be preserved by covering with a brushing of sodium silicate.

"After making many tests, the writer would recommend to any amateur the process used in (l): just scratch-that word sounds bad-just etch, not too deeply, your initials, date, or any identification mark, on the side of your mirror before you start grinding; or, if the side is to be rough ground, mark it after grinding-the scriber will leave a deep enough mark for the purpose intended."

Pursuing the policy followed in "ATM" and "ATMA," of naming names and giving addresses when publishing data like the above-that is, data about odd materials for which the isolated amateur would find difficulty locating sources-we asked Garland to give us the addresses and he writes:

"Diamond point pencils may be procured from the Arthur H. Thomas Co., 230 7th Street, Philadelphia, Pennsylvania; Carborundum pencils are furnished by the L. S. Starrett Co., Athol, Massachusetts-the body of the pencil is nothing more or less than a small pin vise-and Carborundum points are made by the Carborundum Company of Niagara Falls, New York. Tallow pencils are made by the Joseph Dixon Crucible Co., Newark, New Jersey; the trade name of the pencil is Dixon's Phano. Or one may be obtained from most hardware stores by asking for a pencil to be used on glass surfaces. One type of vibrating pencil is called 'Engrav-Rite' and is furnished by the Quality Merchandise Co., 2306 Lincolnwood Drive, Evanston, Illinois, or an electric engraving 'Handee' tool manufactured by the Chicago Wheel & Mfg. Co., 1101 West Monroe Street, Department E, Chicago, Illinois, may be used.

"Silver monogram ink, black monogram ink, glass frosting solution, and glass etching solution are furnished by the Westinghouse Lamp Division, Westinghouse Electric & Manufacturing Co., Bloomfield, New Jersey."

CLEAN design is a characteristic of equipment shown in Figures 3 and 4, owned by D. C. Wysor, 136 Brookside Avenue, Ridgewood, N. J. The observatory (Figure 3) is 15' x 15', with 7' walls.

Its roof rests on four roller-bearing flanged wheels and by means of a winch and cable it can easily be rolled off on a steel track at one side, giving an unobstructed view. The telescope (Figure 4) is a 12-1/2" f/9.5 with a tube made of Johns-Manville Transite pipe--so far as is known, the first time this material has thus been used. It has .52" walls is and 14" I.D., weighs 220 pounds and is very rigid. The cell was made by J. J. McGuckin of Ridgewood, and has three bayonet joints. The pedestal is of 6" pipe bolted to a 2-ton concrete pier. The mirror was made by Wysor at the astronomical workshop of the Hayden Planetarium, in New York, under the direction of Ramiro Quesada. The mounting proper is one of McCartney's "H.H." types, made for Wysor by E. B. McCartney, of Hempstead, N. Y. The drive is a 12-watt Telechron motor, controlled by a Bodine reversible motor with push-button control, which rotates the motor backward or forward for guiding at about 80 percent of sidereal rate.

TWO high school principals have sent us photographs of their telescopes. Figure 5 shows that owned by Principal J. Russell Smith, Smyer H. S., Smyer, Texas, who made the optics but obtained the mounting from H. L. Armiger of Detroit. Figure 6 shows the telescope made by Principal Theo. Skonnard, Fort Ransom H. S., Fort Ransom, N. Dak. Both are clean, smooth pieces of design.

IF you daub shirts, handkerchiefs, your hair, table linen and bed linen with rouge during your struggles with mirror making, you may be providing somebody with legal grounds for divorce for cruelty. However, as Everest states in a private communication, "if a man is slobbering himself all up with rouge he is probably getting sleeks and scratches also. Anyone who carries rouge from the lap to his clothes will also carry dust and grit from his clothes to the lap. If the necessary care is used during pitch polishing, one can even work in full dress without becoming untidy. In 'ATMA' I mentioned the need of becoming grit conscious, but probably didn't make it strong enough. Almost surgical technic is required, but it becomes second nature after some experience and is absolutely necessary with pitch."

F. C. Gebhardt, 140 East 29 Street, Erie, Pa., states that he has used Bentonite, often called "mineral soap" or "soap clay," for removing old rouge spots on white shirts and, while this took considerable elbow grease, it worked. Your scribe therefore obtained some of this peculiar clay material and similarly found it accomplished much with an old smock that bore many super imposed strata of rouge daubs dating back toward the pre-Cambrian. Bentonite is obtainable from the Eastern Clay Products, Inc., Eifort, Ohio, in one-pound packages, for less than the price of one shirt. Mix it with soap granules, ten parts soap to one part Bentonite. The soap will generally prevent the Bentonite from settling. The cleansing action is one of simple adsorption, due probably to the high surface area of this colloid (particles 1/25,000,000" to 1/50,000" in diameter). Laundries use a great deal of Bentonite, and it is also used in many diverse ways in many other industries not related to cleansing. It is a clay obtained mainly from the Fort Benton (Upper Cretaceous) shales of Wyoming, and is not a "chemical."

RFT fiends should alter d to a, in line 6, column 2, page 50, last month's number; also alter "eyepiece," in analogous position on page 52, to "object glasses," and add "corresponding to the well-illuminated field of view of a Galilean curiously depending on size of objective"-so Walkden writes.

Since last month, John N. Pierce has prepared one of his Hobbygraphs on the RFT, containing much compact working data.

LAST call to Stellafane convention, atop Mt. Porter, near Springfield, Vt., Saturday, August 6. As this goes to press (June 21) Russell W. Porter writes that he is "highly likely" to be present.

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