In the number for last February we opened up the subject of automatic guiding of telescopes, pointing out the great need for it among astronomers, and in the May number we hinted at it again. It now appears that several have had this idea on their mind for some time, and one of these is Wilbur Silvertooth of 273 Ximeno Avenue, Long Beach, California. He writes:

"My interest in astronomy has led me to make several telescopes, among which were a six-inch reflector, made when I was 14, and 11-inch Cassegrainian, completed last fall.

"Your column in the February issue now prompts me to send a sketch of a device I experimented with some time ago. Referring to this sketch, the photographic plate was fastened to a circular aluminum plate, which was in turn suspended by three heavy springs. The 'electric eye' was the same diameter as the eyepiece, and replaced it when a suitable guide star was located. The photo-cell used contained copper nitrate, sealed in. The surface of the triangular glass prism was coated with cupric oxide. This, with the carbon, was sensitive to light. However, except on bright objects (2nd mag.) the action was poor even with '210 amplifiers. Since then I have tried selenium, which seems more sensitive.

"The advantages of this device are obvious, in that it requires no partitions between the elements, no mechanical relays t0 make the motion jerky, and has freedom of motion on an infinite number of diameters. The correction is made almost before the shift occurs."

WE invited Mr. Silvertooth to expand the above note, and the following is what he then wrote:

"The cupric oxide in conjunction with the copper nitrate is photo-sensitive, the carbon having no effect other than as the second terminal, or anode, to complete the circuit. Any other electrical conductor would serve equally as well. Since some photo-cells require a series resistance, the carbon acted in this capacity.

"The plate (see drawing) is fastened to a circular aluminum (or other non-magnetic material) disk, which is in turn suspended by three heavy springs, leaving it free to swing on any lateral diameter. The photocell is located in one corner of the plate, according to conventional practice. An ordinary eyepiece is first used to locate a suitable guide star, later being replaced by the photo-cell. All connections from the cell and magnets are contained in one flexible cable. The magnets which move the plate are similar in principle to the dynamic speaker in a radio, viz: the large outer magnets are rigidly attached to the telescope proper. They create a strong magnetic field. The smaller magnets on the movable plate vary in strength and thus cause the plate to shift. In this manner there are no moving parts other than the plate itself. This eliminates friction and reduces failure in operation to a minimum. The anode leads from the three segments must be shielded to avoid mutual conductance.

"In reference to the drawing, the solid rays represent the position of no movement. However, if the star image should shift (see dotted line), the resistance of the cell would be reduced in that direction, causing a current to pass from the central cone to one or two of the segments, depending upon the direction of the shift. It is evident that the greater the shift, the shorter the distance will be between the cathode and anode, causing a greater decrease in resistance and a consequent increase in the amount of current passing through the particular magnet, with a proportional effect.

"The cone, or cathode, is first silver plated and a lead is then fused on. The anodes are set up around the cone, as pictured. A small capsule containing calcium and caesium oxide is placed on one side, and the device is evacuated and sealed. It is then placed in a high frequency furnace which beats the capsule and the anodes. The caesium formed condenses on the cool cone and glass container. After that, with the aid of a small gas flame, in conjunction with the furnace, the caesium is persuaded to condense entirely upon the cone. I believe the introduction of three anodes in one cell is novel.

"The resistance type amplifier was used to minimize distortion, even though more stages were required. No attempt was made to employ alternating current, as this would be fatal to successful operation.

"A few minutes study will show that a single star is not necessary as a guide. A planet, a cluster, or any other such object would serve equally as well, as it is the relative intensity of light in each of the three segments that is the determining factor, and not the sudden presence or absence of light. As long as some object is present in the field the plate will immediately adjust itself so as to divide the light equally in three parts, and will then maintain this position throughout the exposure."

NEXT month we hope to publish a longer contribution on the same subject, sent in by James T. Barkelew of Los Angeles.

Several astronomers with whom we recently discussed automatic guiding shook their heads. One stated that he would like to see the idea tried out, while another seemed to think that nothing could replace the human factor in following a star image around when it was moving rapidly and irregularly. A third emphasized the extremely high criterion for his own work on parallax plates: while most of the experimenters have aimed at one 10,000th of an inch as their criterion for precision, this astronomer demanded one 50,000th of an inch.

This matter of automatic guiding provides an excellent opportunity for the inventive amateur telescope maker to donate something really worth while to astronomy -since it is fairly evident now that he will not contribute real astronomical work, preferring to center his interest on the mechanical aspect of the hobby. A solution of this practical problem would constitute a major contribution to science.

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