"I fully agree that Porter is entitled to a place on the moon and am very glad indeed that it lies within my power to further this. One of the finest of all the lunar formations is Clavius. On the walls of Clavius we find two prominent craters, one on the south wall and the other on the north. They used to be known as Clavius A and B. The one on the south is now called Rutherford after the famous American lunar photographer but, until the present time, the crater on the north wall is still unnamed. It is about 25 miles in diameter. I therefore propose to name the crater Clavius B, Russell W. Porter.

"To this end I have already inserted it on my copy of the 800-inch map and on the tracings from which prints are taken, and this name will appear on all future copies of the map. I do not think I could have selected a better object.

"I will also see that appropriate action is taken by the various learned societies, so far as lies in my power. The chief step has been taken: the name is now on the map." No doubt this action will be sanctified by the Commission de la Lune of the International Astronomical Union. The conviction that this kind of memorial would have been appreciated by Porter is based on some comments he penciled on the back of a letter in 1944. His attention had been called to the writer's attempts to rename Breezy Hill on which his home "Stellafane" rests, "Mount Porter." His comments were:

"Well, I cleaned up 3,000 square miles of unknown blank5 in Franz Josef Land and 3,000 more in Alaska but my name is not on one island, point, bay or lake. Oh yes, there is a lake in Baffin Land that my party discovered and put my name on, so I suppose I ought to be satisfied. Anyway it was a sizable lake, six miles long and a lot bigger than Breezy Hill (which I did not discover)."

Where is the crater now named Porter? It is indicated by the white rectangle on the Mount Wilson photograph of the moon which appears in Figure 1. Above it lies the great mountain-walled plain Clavius.

Amateurs will now eagerly aim their telescopes at the moon, at Clavius, and at Porter, and will study their minute features with special interest. No equally detailed description of these exists other than that which appeared in the classic work The Moon, by Walter Goodacre today out of print, obtainable even at second hand with great difficulty and at a cost of about $25 (originally $7.50).

In his book Goodacre, who until his death in 1938 was the Director of the Lunar Section of the British Astronomical Association, says this about the crater:

"Clavius–A noble object at all times being a vast walled depression 140 miles in diameter. The inner terraced slopes rise upward to the crest, which is 12,000 feet above the floor. The aspect which this object presents when seen under a low sun is one of remarkable grandeur and absorbing interest. The crest of the wall is nowhere circular but is composed of a number of linear and irregularly curved sections presenting strongly marked polygonal features. The height of the crest generally above the surrounding country is insignificant and in many places it does not exist, the inner slopes dropping sheer from the surrounding surface. At several places along the inner slopes are evidences of landslips on a large scale.

"A is about 28 miles in diameter and, with B on the north wall are evidently much older formations than the regular circular craters found on the floor. A contains a fine peak; its floor is very rough. From the north wall of A radiate ridges which Elger compares to the ribbed flanks of some of the Java volcanoes. Some of these reach nearly to B. Professor W. H. Pickering has also noticed similar ridges on the outer slopes of some of the Hawaiian volcanoes.

"B is similar in type to A and probably contemporary in point of age; it contains a fine double-peaked mountain and a crater under the north-east slope. Between the central mountain and the south wall are three minute craters in a row east to west, quite good test objects.

"The floor of Clavius is very smooth generally, but sprinkled over with many craters, craterlets and crater pits. Among these there is between A and B the fine clear-cut crater D, with walls rising 3,000 feet above its interior, having a small central mountain. Outside its wall on the east is a small isolated peak with a minute crater a little to the south.

"In addition to D there are four other large craters of the same type running in a curve convex to the north, decreasing in size as they go. The mountains in A and B have very smooth, gently rising sides very much resembling large sand dunes.

"Between D and the west wall are seven or eight craterlets arranged in an elliptical curve. I find these are good test objects for a 10-inch object glass; they are faintly shown on the Mount Wilson photograph."

The walled plain Clavius was named in 16S1 by Joannes Baptista Riccioli for Christopher Klau, a German Jesuit mathematical teacher, according to Who's Who in the Moon, published in 1938 by the Historical Section of the British Astronomical Association. After examining this 130-page catalogue, R. S. Richardson of the Mount Wilson Observatory listed in Leaflet 193 of the Astronomical Society of the Pacific (March, 1945) the following Americans whose names have thus far been given to features of the moon: Bruce, Burnham, Mitchell, Hall, Newcomb, Ritchey, Rutherford, Lick, Yerkes. To this short list the name of Porter may now be added.

THE reflecting telescope shown in the photograph at the left was designed and built by Captain William C. Bryson, U. S. N., of the U. S. Naval Proving Ground, Dahlgren, Va., assisted by Donald L. Winchell and Walter N. Larsh. Captain Bryson states that its mounting was patterned after the prototype sketch of the English yoke mounting shown in Amateur Telescope Making, page 139, and says "I have found this to be a very satisfactory type of mounting." Oddly, however, this mounting, so easy and inexpensive to build, so portable, so attractive, so rock-rigid if well built, has not often been made by amateurs. It is the same type as the mounting of the 72-inch reflector at the Dominion Astrophysical Observatory, Victoria, B. C.

The buried south pier of the Bryson telescope is made of welded steel. Pivoted on a ball bearing is the thick polar axis, a length of 4.5-inch steel pipe. At its top this pipe is similarly carried on a ball bearing at the apex of a bipod or A-frame of welded pipe.

The lower legs of the bipod slide within the upper parts and may be adjusted to the desired height and held by means of hand screws. "Like most Navy men I never know what the latitude of my next duty station will be," Captain Bryson states. At whatever latitude, the bipod legs need only be adjusted until the polar axis is parallel to the earth's axis.

"The tube," he continues, "is made of wood and its cross section is a regular polygon of nine sides, a convenient multiple of three to accommodate the three-legged spider that holds the diagonal prism. For added strength every third stave is of oak rather than pine and there are six internal reinforcing rings.

"The tube is clamped removably to a saddle attached to one end of the declination axis. When it becomes necessary to rotate the tube to obtain more convenient access to the eyepiece as the telescope is swung in following the stars, I have to swing the tube back across the polar axis, unclamp the tube, and bodily roll it over. I'm not very happy about this feature. I want external concentric tube rings so I can rotate the tube without wrestling. My advice to other amateurs would be to get a cylindrical tube at the outset.

"The white 'corset' around the tube is a canvas chafing gear to protect its corners. I store the tube in our dining room (not unopposed) and when I added this corset my wife irreverently draped female garments on it.

"The declination setting circle and vernier is made of brass and pivoted on the declination axis and, by means of the knurled knob visible between vernier and polar axis, it can be cast loose and readjusted to read correctly when the telescope is on a star of known declination, a principle explained in Amateur Telescope Making. The declination circle is divided in degrees, 10 graduations on the vernier spanning 9 on the circle, and thus the vernier can be used to set the telescope in standard one-tenth degree increments of declination."

DETAILS of a support for the secondary mirror of a Newtonian reflecting telescope, as designed by E. B. McCartney of Minneapolis, Minn., are shown in the first two of his three drawings reproduced above. "This support," he states, "has been used in three telescopes and is the easiest to make and adjust of any I've seen. It is shown with a homemade flat but can be used with a prism. Tilting the flat does not throw it far off laterally, as in some designs. It should be made 'tight' and if this is done no locks will be needed."

The first drawing is a side and end elevation. The second drawing is an exploded plan and elevation of the spider hub into which the main rod of the secondary support is adjustably fixed by means of the thumbscrew shown.

The third drawing represents an adjustable support for the secondary of a compound telescope. The spherical nut and, at the opposite end of its stud, the spherical washer, explain the three-dimensional adjustment afforded by this support. McCartney is the designer and builder of the "Hempstead Hydrant" mounting shown in Amateur Telescope Making-Advanced on page 380.

INCLUSION in Amateur Telescope Making-Advanced of the chapter entitled "Dealing with Spider Diffraction" has led to a considerable development of forms of curved supports for secondary mirrors because these abolish the spikes that seem to project from bright stars. Franklin B. Wright of Berkeley, Calif., in a recent letter takes a similar point of view. "I think," he says, "the idea is much overrated. There are occasions when it would be handy to put something over the straight ribs to spread the diffraction streak from a nearby bright star. But it is usually possible to rotate the telescope tube to move the diffraction streak out of the way so that it will not obscure an object.

"The objection to the curved ribs," he continues, "is that the total light spread outside of the central image of a point source is bound to be greater than for thin, straight ribs because the curved ribs must be longer and thicker in order to be sufficiently rigid. This difference would not be important except when the moon, large and bright planets, or the sun were being viewed. In such an event every bright point on the object would appear to be surrounded by diffraction either in the form of straight streaks or an even spread all around the point, depending on the form of the supporting ribs whether straight or curved, the diffraction always being at right angles to the part of the rib which causes it and K extending both ways from the image of the bright point.

"These diffraction images, no matter what form they have, would be duplicated all over and around a bright extended object such, for example, as the moon. This is because of the infinite number of bright-points on the surface of the moon, each one acting as a source and producing one of the diffraction patterns at the image plane.

"Therefore, no matter what the diffraction pattern might be for an individual object, the combined effect of these overlapping patterns would be one big blur of scattered light overlying and extending beyond the image of the moon. Since the total amount of diffracted light is greater with curved than with straight ribs, it follows that the scattered light would be brighter and would interfere more seriously with the clearness of faint markings when curved ribs are employed on extended objects such as the moon.

"The only thing that could afford a worthwhile improvement on thin straight ribs under tension would be to support the diagonal on an optically figured, nearly flat glass plate or correcting lens near the focus, or to take the diagonal entirely outside the cylinder of light as in the Herschel telescope or some other of similar type."

The curved secondary support gets rid of the spikes on stars but does not get rid of the diffraction. This fact was pointed out in Amateur Telescope Making-Advanced on page 620.

PHOTOGRAPHS of terrestrial objects, the sun and moon, says Lyle T. Johnson of La Plata, Md., may easily be taken with any camera and any telescope. Johnson's method is merely to open the camera diaphragm, focus the K camera at infinity, hold it almost in contact with the telescope eyepiece, and snap the camera shutter. "I have been doing this for years," he writes, "but found sometimes that the photographs were out of focus, especially when using a short-focus mirror in the telescope. I then stumbled on the idea of looking through the telescope with a small telescope such as one side of a binocular, previously focused independently on a distant object, and adjusting the eyepiece of the large telescope."

Explaining the principle involved, Johnson states: "The large telescope must in some way be brought to such a focus that the light emerges from its eyepiece in parallel rays, that is, at infinity. Unfortunately the eye, because of its power of accommodation and defects if any, does not afford close estimate of the correct infinity focus of the telescope. But when a telescope has been focused on a distant object the light rays entering it from a point source must be parallel or very nearly so to give a sharp image. Now if this small telescope, previously focused, is placed between the eye and the eyepiece of the large telescope, the indefinite accommodation factor of the eye is eliminated."

When taking the picture the iris diaphragm of the camera should be opened wide so that no part of the Ramsden disk of the telescope will be cut off. Theoretically the Ramsden disk should be in the center of the camera lens, but this is not important.

Of course, if the camera has a ground-glass focusing screen the use of the auxiliary telescope as described above may be dispensed with and focusing done directly, since the accommodation of the eye will no longer be a factor.

Suppliers and Organizations

The Society for Amateur Scientists
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Internet: http://www.sas.org/

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