Mr. Barkelew writes:

SOME time ago. George Mitchell and I did a little arm-chair work on automatic telescope guiding. George is a telescope nut. In his spare moments he is the guiding genius of the Mitchell Camera Corporation. In the course of evolving several different physical schemes for automatic guiding of telescopes, some shown in the sketches, we have found several difficulties that any successful automatic guide apparently must overcome.

The guiding star image is very small and of very low total illumination. In the form of I the square shim S between four totally reflecting prisms P (two shown) can be located out of focus, so that the enlarged image A will just occupy the shim end or somewhat overlap all of its edges. If the image moves, one or two of the light-sensitive cells receives more light, the opposite one or ones receiving less. A magnifier or condenser at M may concentrate the light on the cell.

In II a four-sided pyramidal prism has a small flat on top, and the image lies on or overlaps the flat, the light from the overlapping portions of the image being surface reflected to the four cells C. The smaller this point flat can be made, the better, as more light then goes to each cell. A major problem seems to lie in getting as much light as possible to each cell; the average guiding star that one can count on is so faint that the reliable sensitivity limit of the cell is closely approached.

The scheme of III was gotten up to offset small image size and normally to throw half the light into each of two cells. Here H is a half-transmissive mirror and EE are sharp edges adjusted to pass half images. This form, however, requires duplication for the other two directions. Other physical forms we have worked on involve oscillating edge plates and prisms which will periodically throw light from an image portion on the cells. Those are probably too complicated mechanically, and on the whole the single prism set in out-of-focus arrangement appears best, particularly as the small prism element may be put on a star image in the focal field of the main objective, along with the photographic plate.

One can readily visualize, mounted on the plate holder, a small element including the prism set and the cells. Microscopic observation of the image through the prism would facilitate setting. Russell W. Porter, who has made many suggestions [Nowhere print, but orally; the two are neighbors.--Ed.] has suggested that the prism might be a cone rather than a pyramid, and that the cone, like a pyramid, could be set base-on or point-on to the light. Condensers again could be used to gather the light spread by reflection from the cone's surface. He also suggests additional reflecting surfaces around the prism to reflect the beams up to the cells supported above the prism, in order to minimize the lateral space taken up at the plane of the plate holder.

By mounting the guiding element at the plate holder, and applying the corrective movements directly to the holder, the inertia lag of the whole system may be reduced to a minimum. The plate holder may be made as light as possible, mounted on anti-friction bearings; and the final driving motors, utilizing the amplified currents from the cells, connected directly to the plate adjusting screws whose pitch would depend on the load, the requisite acceleration, and the motor characteristics. A motor of small inertia seems indicated.

The inertial lag may thus be cut down to compare favorably with manual guiding. There is left, however, the sensitivity lag which is inherent in any such system and which appears to constitute the major difficulty.

In the sketch IV, lag a may represent the distance beyond edge E, or beyond normal which the image must move in order to actuate the cell sufficiently to obtain a corrective result from the system. Suppose that b and c similarly represent time or inertia lags in the system. These latter may be reduced to much less than the corresponding lags in the average personal element in manual guiding; but the reduction of sensitivity lag a seems to present major difficulties.

Assume the simple movement of the image from a normal position, on an excursion out and back. Sensitivity lag not only puts the plate movement behind the image on the way out and back, but, most important, brings the plate back to a stopping position short by an amount primarily equal to the sensitivity lag. Now, if inertia lag is equal to sensitivity lag, then the over-run of the mechanical parts may make up for the sensitivity lag, and finally bring the plate to proper position. That, however, depends on the assumption that the image is going to move back to normal in a straight line, an assumption by no means justified. Furthermore, the simple case of the image having a normal position from which it makes temporary excursions is probably not the fact, although the image undoubtedly does predominantly occupy a definite small circle. The actual fact is probably a constantly moving image; but, whatever the conditions may be, the sensitivity lag has the effect at all times of putting the plate position behind the image position by an amount dependent on the lag, even though the image may come to a position of effective rest.

Sensitivity lag allows the image to move outwardly a certain distance without any corresponding control movement at all; the image can thus move around in a circle having a radius equal to the image radius plus the lag, without the control operating. An to attempt to correct, the sensitivity lag by an equal inertia lag (over-run) thus mean that the combined lag would be twice the sensitivity lag, and the resultant star image very large.

The problem consequently seems to be one of eliminating the inertia lags and reducing the sensitivity lag to a very small amount. If the star image is of the order of 0.0001 of an inch, the variation in manual positioning by any one eye, while the image is still, is probably only a fraction of that amount. The sensitivity lag of an automatic control must thus be less than such fraction, to be worth while in improving photography.

Magnification of the image movement might help, but we have to remember that the guiding star image is now viewed through a magnifier, so that the magnified movements are availed of for following. The problem thus seems to resolve itself into one of providing an extremely sensitive system. One thought we have had in that direction is that the star image may be allowed to overlap all four edges in its normal position thus normally affecting each of the four sensitive cells equally. Approximately one half overlap would give the greatest instant difference between increasing and decreasing overlaps. Then it might be possible to develop a balanced electrical system, such that a very minute unbalance of the opposing light-sensitive elements might result in a relatively very large current unbalance which would then be used for motive control.

Another manner of operating in the same way might be as follows. Instead of having the star image normally overlapping in all directions, let it normally overlap in two directions, say north and east. In other words, the constant tendency of the image could be to over-run in those directions. Then the two sensitive cell controlled currents might be balanced against two constant currents, with unbalance producing a large resultant.

All of this is about as far as we have gotten on the general subject, except to conclude that in any case the automatic system should have some kind of automatic occulter to cover the plate whenever the star image becomes so active as to run away from the control, or expands to a blur, or, what amounts to both those things, shimmers around rapidly.

MR. BARKELEW'S contribution, presented above, ends the discussion of this subject for the time being; the next stage being logically when somebody sends in something indicating results. In the meantime, we sound the last call for the annual convention of telescope nuts to be held Saturday, July 21, at Stellafane, Springfield, Vermont. Everybody come.

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