"Protection against temperature effects is not necessary to those who use a new telescope at the Roslyn House Observatory, Wynnewood, Pennsylvania. This unique institution, the private observatory of Mr. Gustavus Wynne Cook, Philadelphia banker and manufacturer, has been described in previous articles in SCIENTIFIC AMERICAN (August 1932, p. 74, January 1934, P. 17). To own a well-equipped 28-1/2 inch reflecting telescope, provided with a large spectograph so that it is used regularly on a research program for measuring the radial velocities of the stars-not to mention the 40-foot focal length sun camera, the spectrohelioscope the astronomical transit instrument, a 7-inch aperture and meridian circle and other items that would be creditable in any college observatory-might satisfy many persons. But Mr. Cook is not happy unless he is putting up a new telescope, and the latest addition has now been in use only a few months. With it he sits in a nicely furnished, and heated, room. At one end is an assortment of dials that reminds one of the control room of a submarine, and an eyepiece tube that might represent the end of a periscope. Without moving his head, the observer can press buttons and view almost any part of the sky. The dials automatically show exactly the position of the region at which he is looking.

"This is the latest form of siderostat telescope. Such instruments are not new in principle, as the first was designed by the French astronomer Leon Foucault, who died in 1868, and a huge one was displayed at the Paris Exposition in 1900. But now, probably for the first time, such a telescope has been constructed which makes full use of modern electrical equipment so that the operation is not as much trouble as driving an automobile. The flat mirror is mounted in a vertical fork, so that it can turn freely around either a vertical or a horizontal axis. Attached to the back of the mirror, projecting out at right angles to its plane, is a rod. The driving mechanism is placed immediately to the north, and is equipped with a polar axis which turns once in 24 hours. Attached to this is an arm which moves north and south, and at the end of this arm is a sleeve on which slides the rod attached to the mirror. This mechanism is shown in one of the photographs. By means of it, the mirror is automatically moved at exactly the right speed for each declination. The instrument is adjusted in declination by moving the arm attached to the polar axis. The tube of such telescopes is usually placed to the south of the mirror, so that the eastern and western halves of the sky may be reached with equal ease.

"The objective of the Roslyn House instrument is of 15 inches aperture and 225 inches focal length. It was made in 1907 by John A. Brashear, and remained unused until purchased by Mr. Cook. It was tested by J. W. Fecker, of Pittsburgh, successor to the Brashear firm, who pronounced it excellent.

"Mr. Cook was undecided as to how he should mount this objective. The conventional mounting, with the dome, would have been as large as a two-story house, and out of keeping with the residence and other buildings nearby, in addition to lacking the advantages of a warm observing room. A polar telescope would have required the erection of a somewhat unsightly tower, so a horizontal telescope was definitely indicated.

"Accordingly, Mr. Fecker was given the contract for the instrument, and the buildings were designed by Mr. Cook and built under his direction The Pyrex glass plane mirror, 25 inches diameter, the driving mechanism, and the lens, are mounted on concrete piers in a small square building, provided with a circular, rotating roof, in which a wide slit can be opened to expose the mirror to the sky. At present the mirror is coated with silver, but equipment is being built to give it an aluminum surface. The tube of the telescope extends to the south, across a short open stretch and into the other building which, in turn, connects with the transit room, a clock room, and the room for the 2-1/2 inch reflector. The observing room is heated from a heating plant some yards away. The breech of the telescope, and the various dials, are mounted on a third concrete pier to give the greatest possible stability against vibration.

"A total of 12 motors is used in the operation of the instrument. One is a synchronous motor for the main drive of the polar axis. Two motors are used for the motions in right ascension and declination, the same motors serving for the fast and slow motions by shifting an electrically operated clutch. A motor generator supplies direct current to operate the numerous relays. A second synchronous motor turns the eyepiece, when used for photography, as there is a rotation of the image, and a third operates a dial to show sidereal time so that the instrument can be set directly to a star's R. A. without stopping to figure out the hour angle, which is normally necessary. A pair of Selsyn motors connects this dial with the polar axis, and another pair connects another dial with the declination axis. Selsyn motors are "self-synchronizing," and when two are connected to the same power source the shaft of one turns in exact step with that of the other, as if they were mechanically coupled, even though miles of wire might intervene between them. Other motors operate an iris diaphragm over the lens, and the rotating roof.

"Mr. Cook intends to use the instrument to a great extent for photography, and the support of the objective is now being slightly modified to simplify the attachment of the photographic compensating lens, which shortens the focal length. Guiding for photography may be done in two ways, one by using a double-slide plateholder, with two eyepieces on each side of the plate, through which stars just out of the field being photographed are kept in view on cross hairs. The other way is with a separate 6-1/2 inch lens (see photograph), of the same focal length as the large objective, which is fed by a small flat mirror, attached to the side of the big one, and moving with it. This lens supplies a separate eyepiece, at the left on the observing panel. With this, however, the rotation of the image cannot be checked, as with the guiding stars on each side of the plate.

"The telescope cannot reach the pole, but can come within about 20 degrees of it, and thus can reach the most interesting parts of the sky. Mr. Cook points out that it has one advantage over a polar telescope, for when such an instrument is directed to the southern sky, the plane mirror is used at a grazing angle, the least desirable position, as this emphasizes any of its defects. With the siderostat instrument, on the other hand, the light from an object in the south is reflected back almost at right angles to the surface.

"For comfort in use, and easy, convenient operation, the new Roslyn House telescope is to be commended, as anyone who has had experience with it, also with usual types of telescopes, can amply testify. But the fixed eyepiece has other advantages beside mere comfort for the observer. There are many problems of astronomy, such as the measurement of star brightness with photo-electric cells, where a considerable amount of delicately adjusted apparatus must be attached to the telescope. When this also has to swing around at all sorts of angles, the mechanical problem is a difficult one. At Roslyn House, a whole roomful of apparatus might be placed at the focus, and left there as long as needed. In fact, it is so effective that it will be rather surprising to the writer if, as it becomes better known, it is not duplicated again and again."

THUS ends Mr. Stokley's description. Mr. Cook, however fortunate in having so much fine equipment, is not reserving it for his own pleasure, but has also invited in and is maintaining two professional astronomers, Dr. Orren J. Mohler and Mr. I. M. Levitt, in order to keep it in steady use for routine scientific programs in connection with astronomy as a whole.

In his description Mr. Stokley refers to a siderostat, and so this is a good time to harp a bit on siderostats, coelostats, and heliostats-just precisely what are they? For years we have tried to find out. Few persons know just what each of these three is, though it is easy to jump at conclusions or discover that one really doesn't know. We once asked a professional to tell us. He started off in high gear, and then choked and discovered he didn't know just what each of the three was. The dictionary is as clear as mud about them. The other day, however, we blundered across the following, from an article in Astrophysical Journal, March 1900, by M. A. Cornu of Paris. Can anybody find any flaws in these? A siderostat, Cornu says, is especially constructed to send a reflected beam toward the southern horizon; a coelostat (we don't want to seem high-brow, but that word is pretty often mispronounced. It's seé-lo-stat) has a mirror that turns about an axis parallel to the earth's axis with an angular velocity half that of the diurnal; revolution in the same direction; a heliostat sends the reflected beam in the direction of the northern horizon, rarely beyond NE or NW.

THE slanting telescope shown on this 1 page is an old Herschelian now at the Smithsonian Institution, U. S. National Museum, Washington, D. C., and the following comment was sent in by Frank A. Taylor, Division of Engineering, that institution:

"The telescope illustrated is a professional job of 1835. It was made by Amasa Holcomb(e) of Southwick, Mass., who was probably the first man in the United States to make telescopes in any number for sale to astronomers. This telescope was recently presented to the United States National Museum at Washington by his descendents.

"Holcomb began the construction of instruments for students whom he instructed in astronomy and surveying about 1820. From the manufacture of small refracting telescopes he progressed to the construction of reflectors on the pattern of Sir William Herschel, only a few of which had been seen in the United States at that time. About 1833 Holcomb took two of his telescopes to Philadelphia, where they were examined by a committee of the Franklin Institute, which was very favorably impressed with their performance.

"The instrument at the Museum has a 9-inch Russia iron tube approximately 9 feet long, closed at the lower end with a slip-on cover within which is attached a tin-alloy speculum. On the inside of the upper end is a roughly-made rack which carries the eyepiece and which meshes with a small pinion attached to a focusing knob on the outside of the telescope. The lower end of the tube is supported on a brass bar which terminates in a spike at one end and a wheel at the other, designed to permit the tube to pivot easily about the spike as a center. The upper end of the tube is supported on a simple bipod [Similar bipod mounting in SCIENTIFIC AMERICAN, Apr., 1933, p. 241.-Ed.], each limb of which is readily adjustable by means of a cord wound about a winch and running through small blocks in combination with the two sliding parts of each limb. By working the two small winches properly, the upper end of the tube can be made to describe practically any motion required in sighting or following a star."

The speculum of the telescope is in good condition, and it has been used with some success since it was presented to the Museum.

TYPICAL of the wide variety of occupations represented by the followers of the amateur telescope making hobby is a compilation sent us by Leo. J. Scanlon of Pittsburgh, at our request, showing a cross-section of the membership of the Astronomical Section of the Academy of Science and Art of Pittsburgh, essentially a club of amateur telescope makers.

In this club are: electrical engineers 7, Westinghouse employees 8, electrical installation repairmen 2, cigar rollers 1, chemists 6, insurance salesmen 2, millwrights 3, clerks 11, sales executives 5, high school students 14, restaurant managers 2, salesmen 6, news photographers 1, stenographers 2, community house executives 2, draughtsmen 6, radio technicians 2, attorneys 1, vice-presidents of railroad 1, railroad engineers 3, baggagemen on railroad 1, grocerymen 1, news editorial writers 1, printers 2, chauffeurs 1, physicians 4, physicists 2, science teachers 1, commercial photographers 2, electricians 1, dentists 2, skilled mechanics 7, sign painters 1, Boy Scout executives 1, welders 1, research workers (Gulf) 2, projection machine operators 1, plumbers 1, college students 2, machinists 2, farm wives 1, refrigerating engineers 1, machine designers 1, credit men ~ 1, editors farm journal 1, nuns 2.

So this is what amateur telescope makers are made of. A pretty solid cross-section of America, is it not?

THE test for approximate radius of a mirror, described on page 78 of "Amateur Telescope Making," involves wetting the surface with water, which is difficult to control, since the water soon runs off or dries. Dr. S. H. Sheib, a testing engineer and chemist, Box 737, Richmond, Virginia, states that he has found that the substitution of oil for the water affords a better opportunity to measure the radius. A mixture of ordinary machine oil and kerosene, 50-50, worked well.

WHAT, exactly, is rouge? We asked Dr. Sheib and he replied: "I understand: that rouge is Fe₂O₃, and that black rouge is Fe₃O₄. If iron sulfate is precipitated with ammonia you can't get anything but Fe(OH)₃ or Fe₂(OH)₆ which is merely a multiple of the former. When you heat this you drive off the chemically combined water and get Fe₂O₃." Has anyone else any other light to throw on this question?

Another little matter: Dr. Sheib and your scribe have been trying to work out the depth-not the diameter but the depth-of pits for different abrasives. Is any amateur equipped with a microscope having a vertical illuminator and micrometer, who can and will help us measure, if possible, the distance to their bottoms, possibly by focusing on them and using the flat surface as datum; that is, focusing the flat surface and making a reading, and then focusing the bottom and making another reading This is theoretical and maybe it won't work. Good quantitative data on depths of pits for each size of abrasive ought to provide a basis for working out optimum grinding time for each stage of abrasive. Is depth strictly proportional to grains?

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