SO far as is known, the telescope mounting and observatory shown in the drawing at the bottom of the page is an original. It is the design of John H. King of Amityville, Long Island, who also furnished the drawing.
"This mounting," Mr. King writes, "combines some of the features of other fixed eyepiece mountings, with one new feature -it allows the observer to be sheltered, yet does not thereby cut off any of the sky. While other fixed eyepiece mountings accomplish the same thing, they make use of large auxiliary reflecting surfaces."
In Mr. King's design an extra lens system transposes the primary image at such a distance from the tube that it is possible to interpose the observatory shelter, the magnification being either unity or a small factor. "This lens system," Mr. King continues, "involves more than a problem in simple erection, because of the long distances between images and therefore the use of lenses of large aperture (about 1-1/2 inches) and the necessity of preserving the image quality for high-power observation. Two methods suggest themselves: the autocollimation scheme with unity magnification using two similar lenses (see small sketch) which are corrected for focusing parallel light, separated by twice their principal focus, and a special lens designed to meet the sine condition for two finite focal points which would magnify the image by a factor of two or three. Of these methods the first is easily within the means of amateurs, as two good field glass lenses, with their infinity surfaces facing one another, will do the trick."
"Despite some loss of light," Mr. King adds, "I think that this mounting would be more useful for general observation than a conventional exposed telescope. The physical comfort of the observer contributes a lot toward the efficiency of a visual telescope."
Commenting on Mr. King's design, Russell W. Porter says it "is all right, provided the user is willing to pay for the comfort gained by the loss of light through reflection and absorption." A roughly comparable though smaller loss is of course caused by auxiliary reflecting surfaces used in connection with similar telescopes. For example see "Amateur Telescope Making," pages 50 and 51, sketches I to IX. Who will build a telescope of the King type?
By the way, it seems odd that the Hindle enclosed observatory, A.T.M. page 362, 363, 365, has not yet been copied by someone.
SOME months ago we inquired about the reported inaccuracy of synchronized current supply. Gerald E. McCord reports on the letterhead of his "Skylight" Observatory, Oak Park, Illinois:
"I have a four-dollar clock from which I get my time for figuring stellar calculations. I had noticed that it did not always keep what I thought was right time, so, since I saw the note in the SCIENTIFIC AMERICAN, I have been checking up on it.
"My time comes by radio from the clock in the Elgin Watch Co. observatory and is clicked off from the Naval Observatory with a five-second pause before the hour.
I had never found the clock more than 5 seconds off in my calculations before."
Mr. McCord sends a graph (which cannot he reproduced, being on green-lined cross-section paper) which shows twice daily discrepancies of: I second fast, 1, 5. 1, 4, 1, 2, 2, 1, 2 seconds slow, 5 and 5 seconds fast, correct, 1, 1, 1 slow, correct: 1, 1, 2 slow, correct.
Mrs. B. L. Souther, 239 Beverly Road, Mt. Lebanon, Pittsburgh, Pa., also writes concerning the same subject, which she has investigated. "The time signals used were the 9 P.M. signals from the Elgin Watch Co. observatory, through station WJJD, Chicago. According to the announcer, these are accurate to .O1 second. The other standard time signal was the 10 p.m. signal of our Naval Observatory sent out on 690 kc. by station NAA. The clock is a Westinghouse New Haven electric alarm, with subsynchronous motor, running at 200 r.p.m. The second marks on the dial are about 1/4-inch apart, so that the data are correct to approximately l/2-second." Mrs. Souther found the current ran slow on a series of days, as follows: 2, 1-1/2, 1, 1, 3, O, 2, 3, 2,1, 2' 3, 1, 3-1/2 seconds, and she continues: "The above data show that there may be errors as great as 3 seconds an hour, and that, especially at the weekends, the synchronization is inaccurate. The greatest error observed was 5 seconds at a weekend. This apparently indicates an effort to correct it at the first of the week. I have been told by my brother an electrical engineer, that the local system is hand-regulated."
IN the September number, page 132, third column, near the bottom, the figure 2,584,902-the reduction ratio to give true sidereal time-should be 2,584,922.7. So writes Lincoln K. Davis, who says it was incorrect as originally given to us. He gives the following for another set of gears 45:2 x 73:61 x 100:1 x 100:20 x 192:1. Motor 1800 r.p.m. and error 0.156 second per day. But what's the use, for the averag man, if his current is already "off" by 5 seconds? As is the case with hard time something ought to be done about it, but what?
IN spite of Ellison's remonstrance (page 97 of A.T.M.) against fancy dingbats for measuring zones, folks will make the and Frank H. Smith, 505 Prospect Avenue Lima, Ohio, reports his own experiments a follows:
"I enclose photographs of a 'wrinkle' I have been using in my knife-edge testing apparatus. The value is measured with a dial indicator such as is used in machine shops for testing the accuracy of lathe setups and so on, and gives direct readings in thousandths of an inch.
"In using this instrument in the knife-edge test, the mirror is masked in the usual way, and the knife-edge is brought to the correct point where the tones in the two points in the outer zone match. The indicator is then attached so that the indicating post touches the piece carrying the knife-edge, and the dial of the indicator revolved until zero coincides with the pointer. The knife-edge is next moved into the point where the tones in the next zone are matched and the indicator will give the number of thousandths of an inch the knife-edge was moved. By this means one may be able to determine accurately how close he is coming to the calculated value of each zone, to the third decimal place."
We accept this latter-the third decimal place-provided the worker can estimate the comparative darkness of shadows that closely. Beware, however, of fictitious a accuracy. Would not the question of the contrast sensitivity of the eye be involved here? This is given in Hardy and Perrin's "Principles of Optics" as about 0.1 percent though variable. That is, it will vary with the intensity of the illumination, but 0.1 percent is about right.
J. M. VURE, 991 East 163 St., New York City, submits the following note:
"I should also like to call attention to a much simpler formula for calculating the focal length of a refractor than the on used in 'Amateur Telescope Making,' on page 106-110. The formula isXXXINSERT 1933-11-04.jpeg HEREXXX
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"The explanation of this formula is as follows: To produce an achromatic combination from two glasses taken from makers' lists, look to the column headed V, subtract the V of the flint from the crown, multiply the result by the required focal length, divide the result by the V of the crown; the final result is the focal length of the crown. To get the f.l. of the flint, subtract the V of the crown from the V of the flint, multiply by the required f.l., divide by the V of the flint. By using this formula in your example on page 109, 27.9 is the f.l. of the crown and 44.43 is the f.l. of the : flint."
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