The attraction is the sum of a number of things. The AAVSO is mainly an amateur organization. Its members do scientific work that professional astronomers cannot because there are too few of them free to keep close watch on some 600 selected variable stars. That work is described in literature available from the AAVSO Recorder, Harvard College Observatory, Cambridge 38, Mass.

When a candidate applies for membership in the AAVSO (dues $3) and is elected, specific areas of the sky, usually three degrees square and containing a particular variable star, are assigned to him. He receives 10 charts showing these and the background stars, with instructions that tell him how to proceed for telescopes with or without setting circles. Telescopes of three-inch aperture or larger are suitable for this work. It happens, incidentally, that not all variable star observers make their own telescopes. They are generally satisfied with using them.

The candidate studies the charts and the corresponding areas in the sky which, after some observation, become as familiar as the back of his hand. On the charts numerous nonvariable, or constant, stars are designated with numbers that accurately indicate their magnitudes to tenths The observer selects two of these constant stars, one a little brighter and one a little fainter than the variable, and estimates how bright the variable is in comparison with them. As an example, suitable comparison stars are marked on one chart as magnitude 8.8 and magnitude 9.2. By interpolation, the nearby variable is estimated by the observer at, say, 8.9. This is entered in a record book and each month reported to the Recorder on a special blank. Observations for other variables on the several charts assigned the observer are similarly recorded.

Doing this and nothing else would soon become a bore. However, as soon as the beginner has become acquainted with the fields assigned to him, he is encouraged to roam the sky and extend his observing list. He may observe any and all variables for which he has charts. On a single night, as a matter of fact, several observers may work to observe the same variable star.

In the course of time the variable star observer studies the sky in detail. Thus variable star observing provides a good motivation for studying the whole sky. This, many amateur astronomers find a little difficult without the guidance of a program in which other observers are involved.

Observation of the observers at a single meeting suggests several possible reasons for their year-in, year-out loyalty to the AAVSO. It is a purposeful organization in which serious telescope owners may enjoy the dignity and some of the status of the professional astronomer. Some have even become professionals. An additional attraction of AAVSO membership is the scientific and social fellowship of kindred spirits. The tone of the AAVSO is dignified but not stuffy.

The members receive a great deal of individual recognition for their efforts, both from the professionals and fellow amateurs. Rising votes of thanks, with prolonged applause, pleasantly embarrass many successful observers at AAVSO meetings.

The organization is expertly run, which is another satisfaction to its members. Its business sessions are short and to the point. The scientific sessions are conducted like those of other scientific societies, even though no more than 50 AAVSO members constitute the audience. The limited number of papers read, usually about 10, prevents somnolence.

The accompanying social sessions are arranged on the premise that people attend conventions mainly to talk with one another. At the conclusion on a Saturday evening of the autumn meeting there is a banquet marked by sociability and scientific pronouncements. Each year the AAVSO meets twice: once in October at Cambridge, and once in the spring in various other places.

There is a keen, friendly competition among AAVSO members. Everything that any member does in the time he can devote to observing is made known to the other members through the publication of individual observations. There is, however, a great variation in the amount of time that the members can devote to their avocation. In the year ending last September each of 24 members made 50 to 100 observations; 32 members made 100 to 200 observations; 13 members made 200 to 500 observations; 13 members made 500 to 1,000 observations; six members made 1,000 to 2,000 observations; and six members made 2,000 to 7,500 observations.

The heavyweight champions in the 2,000-to-7,500 observation group were Ahrent of Germany with 2,582 observations; Peltier of Ohio with 2.598 observations; Paletsakis of Greece with 3,473 observations; Chessapis of Greece with 3,642 observations, de Kock of South Africa with 4,729 observations; and Fernald of Maine with 7,504 observations. These six crack observers really work at their job. Together they account for 45 per cent of the year's grand total of 54,370 observations. These are made by 154 observers in 18 nations, of whom the U. S. has 105. Greece is second with 11. In the photograph in Figure 1, Cyrus F. Fernald of Maine, the champion variable star observer, stands in the center with hands crossed. The second man at his right, wearing a checked tie, is Leon Campbell of the Harvard College Observatory, the AAVSO's Recorder, one of its organizers, and for more than 30 years its activator. The man second from Dr. Campbell's right, wearing a dark suit, is Harlow Shapley, director of the Harvard College Observatory. In recent years Dr. Campbell has also been responsible for the actual promotion of work among active AAVSO members. In the rear center shows your scribe's left ear-an appropriate representation for a listening, lowly, non-observing freshman member. In addition to variable star observing, the AAVSO sponsors the organized observation of the occultation of stars by the moon, and the systematic observation of sunspots. It also maintains a nightly watch of the sky for the sudden appearance of a bright nova. Cyrus Fernald writes: "The companion ship with the other observers and with the staff of the Harvard College Observatory, not to mention other observatories, combined with the sense of satisfaction that comes from the knowledge that one's observations are doing some good, is a rich reward for the time and effort that is put into the work. "I am," he continues, "one of those who made telescopes from the instructions in Amateur Telescope Making and Amateur Telescope Making-Advanced. From my 16 years' experience with it I am thoroughly sold on the Springfield mounting. My ideal would be an 8- to 10-ineh telescope of about 70- to 80-ineh focal length, with electric drive and slip ring R. A. circle." N the photograph at the right Chester Brown of 1117 Fourteenth St., Spokane, Wash., an active member of the Spokane Amateur Astronomers, may be seen sitting indoors examining the sky through a refracting telescope that points downward through a hole in the wall. This is possible because there is below the telescope a flat mirror (see drawing) which reflects the rays from the particular part of the sky toward which it is turned. The rays then pass upward into a lens. The telescope itself is permanently fixed in position, though it can rotate on its own lengthwise axis. Since the observer is sitting indoors, he cannot find objects simply by looking at the sky Instead he works most accurately by an indirect method. The control mechanism of the auxiliary flat mirror is equipped with a graduated setting circle, and turns on an axis. The telescope, too, may be rotated, turning the mirror with it in a direction at right angles to the first motion; this direction is measured by a circle marked in degrees. From the astronomer's Ephemeris the observer ascertains the coordinates of his star, quickly sets the mirror in both directions, and the star appears in the center of the eyepiece.

Like all good things, indoor telescopes of whatever type have minor drawbacks, but one point outweighs them all. This is plain comfort. In the photograph Brown sits serenely in his shirt sleeves. Outdoors in Spokane, it has been 30 degrees below zero, but Brown has not had to slap his hands to keep them warm. It has also been 108 degrees above zero, at which time he has not had to slap mosquitoes.

The indoor telescope is not, however, a suitable type for the beginner's first instrument. It might well be the second. The beginner should put in his apprenticeship outdoors on the conventional frost-and mosquito-bite telescope, and learn the sky as a whole. It is said that, due largely to their use of the convenient setting circles, some professional astronomers have never learned the constellations as thoroughly as some amateurs who do not use them.

J. M. Holeman of Richland, Wash., who made the accompanying photographs, also coated the lenses and aluminized the flat mirror of Brown's telescope. Holeman has furnished some details concerning the telescope; Brown has furnished others. It is of the Grubb-Gerrish type described in Bell's The Telescope and in Amateur Telescope Making. The objective lens and barrel are parts of a 4 1/2-inch Bardou refractor of good quality. Brown made the flat from a 6-inch Pyrex blank.

Shown in the photograph are some accessories for finding stars rapidly. A wrist watch adjusted to sidereal time hangs above the telescope. Beside it is a chart to convert the positions of certain well-known stars to hour angle for adjusting the setting circles in order to begin the night's observing. Below this is a shallow chart box with a ground-glass cover, containing, for illumination, two six-volt radio lamps that run on current from a transformer. The charts of the heavens are placed on the face of the ground glass. This could be done, for example, with the special charts of the AAVSO. Since these are drawn inverted, and reversed for use with refractors, most observers who own reflectors use them with a mirror. Brown's telescope already has a mirror below its objective lens, which inverts but does not reverse the image of the sky. Hence he inserts the charts in the box face down, so that when he looks through the charts from the back they correspond to what he sees.

The hour scale or right-ascension (celestial longitude) circle, visible as a white belt around the barrel of the telescope tube, is held in place by a tension spring (See drawing below), but may be loosened to slide it around the telescope and correspond to the date of observation. Thereafter it is driven with the telescope by a 110-volt clock motor of one r.p.m., which shows above the observer's thumb in the photograph. Other parts such as worm and segment gears were taken from a war-surplus gyropilot which cost $5.

The flat mirror is tilted in its yoke by means of a worm-driven flexible shaft or cable; its black knob is seen atop the tube of the telescope. This cable passes through the inside of the tube, emerging through a hole halfway down and, with a worm on its lower end, actuates a worm wheel identical with the one at the top. The control knob is geared, through an attached gear train and worm, to a declination circle on the left side of the telescope. Brown has many telescopes, but likes this one best because of its indoor observation feature.

The drawing, made by Russell Porter after a rough sketch by Brown, shows the tube and, around its lower part,

the yoke that is integral with it. Both are rotated 90 degrees; this is to say, the declination control and focusing screw are not in the same positions as in the photograph. The pivoted thrust-bearing below the yoke fits into a hole in a wooden cap on the post. After the telescope was adjusted parallel with the earth's axis this cap was nailed to the post. The yoke is made of one-inch pipe and the telescope is attached to it with strap-iron clamps. The upper bearing of the telescope, just outside the house, is an aircraft-type ring bearing.

THROUGHOUT much of the literature 1 of telescope mirror-making the terms parabola and paraboloid are used interchangeably as if the two were synonymous. Of course nearly everybody understands what is meant, a paraboloid being the two-dimensional surface generated when the one-dimensional curve called a parabola is rotated about its axis. The escape from the alleged crime is that when a mirror is called a parabola its cross section is described.

What more rightfully rubs the mathematician's whiskers backward is calling the paraboloid a curve. Being two-dimensional, it is a surface. In sum, then:

Parabola: one-dimensional, curve.

Paraboloid: two-dimensional, surface.

WALKDEN of London mentions a W wrinkle for observing with his richest-field telescope. In an ordinary straight view where the telescope tube is fixed, as on a mounting, the central 20 degrees of the field may be good and the margins fairly good, until you swivel your eye to get a direct look at them with the central part of the retina. Then they seem strangely dim and also have poor definition.

What you have done, Walkden points out, is to move the crystalline lens of the eye sidewise from the Ramsden circle so only a crescent of the lens and the telescope mirror remain in use. This is because the Ramsden circle is so close to the size of the eye lens; you carefully planned it that way when you designed the telescope.

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