An amateur telescope maker, E. S. Ensign of the Ensign Foundry Co., 2100 Hendon St., Toledo, Ohio, has devised another kind of extension "rod," an electrical one. On the mounting of his telescope-a four-inch refractor made by himself-is a tiny electric motor (145 r.p.m ). In the observer's hand is a spring switch on the end of a flexible cord. Whenever the observer presses the switch the motor sets and keeps the telescope in motion until he releases it.

Actually, there are two switches. These are soldered together as shown in Russell Porter's drawing made from the original.

The over-all circuit is shown in the diagram. There are eight cells on either side of a neutral wire. Thus the motor can be reversed.

Better still, the plunger-type radio switches used combine rheostats. This permits micro control of the telescope.

It all looks fine in the illustrations, but it proved to have one "bug." Ensign writes, "I have found that no matter how tightly I set up the tripod screws the telescope shakes when the motor is started and stopped." If a prize were offered for a torsion balance the familiar structure called the tripod might win it. Vertically a tripod is as stiff as a post. Laterally it is stable in any situation short of a hurricane. Due, however, to an inherent diagonal weakness it is rotationally unstable. If a long object, say a telescope, having a large moment of inertia and a slow period of oscillation, is placed on top of it, this instability is accentuated. Add a breeze and your prize is won. Despite this, tripods have good points-at least show us some thing better. One of these points, however, is not their use as engine beds or motor pillows. Ensign is substituting an iron pedestal for the tripod and retaining the electrical feature.

One experimenter who reads this department recently remarked that the lot of the experimenter would be a happier one if other experimenters would similarly record not alone their successes but their mistakes. This department likes to publish valuable mistakes.

Ensign's refractor mounting was cast in aluminum. The polar axis has at top and bottom, respectively, 1 1/4 inch and 5/8-inch double-race ball bearings. The declination axis shaft turns in a plain bearing.

SINCE Ensign runs a foundry he was well situated to make a Springfield mounting more closely similar to Porter's revised Springfield described in Amateur Telescope Making-Advanced than any this department has seen. Perhaps because the patterns look formidable the revised Porter Springfield has seldom been tackled. After examining the original of the photograph reproduced here, Porter remarked, "A fine-looking job." This more than casual remark-for Porter doesn't effuse-carried more than common weight to one who has heard him loudly say nothing when shown photographs of compromise Springfield jobs.

Smelling a possible chance to make this rare Porter Springfield available to amateurs, this department sounded out Ensign about castings for others. Reply: "I have the patterns but we do not pour aluminum. I would have to take it to another foundry. If some now and then would like castings out of iron I could probably accommodate them, but the prices might seem steep " Ensign specializes in a low carbon iron with nickel and chromium called Tensloy.

In the month after the above reply was received more inquiries about Springfield castings reached this department than during the previous three years. Do you believe in telepathy?

THE FOURTH illustration shows a trick Springfield telescope ("Very ingenious."-Porter) proposed by Daniel Langpoop of Los Angeles. The diagonal mirror is on a pivoted support and is aluminized on both faces. In A the observer is guiding a photographic plate. In B two persons are observing simultaneously. The smaller telescope is also a finder, the best aspect of this rather stunty proposal.

ADVANCED amateur telescope mirror makers examine and study in the minutest detail the shapes and outlines of the Foucault shadows on mirrors and thus come to know their finer subtleties, many of which are not even perceived by the tyro. The same painstaking study has not yet been given the map of the bands in the useful Ronchi test. George P. Arnold of State College, Pa., and Joseph Vrabel of Boalsburg, Pa., argued heftily about the curvature seen on the Ronchi bands in testing a short-focus mirror they were making. Should these bands neck in as much as they did at the top, or should they have the same curvature everywhere along their length?

This started Arnold, a graduate student in physics, on a hunt for a general formula by means of which the exact shape of the bands for a mirror of given specifications may be worked out in advance for the particular Ronchi grating used. He found one which he says will do the trick with fewer pains than may at first appear. "0f interest," he comments, "is the fact that one formula can give so many shapes." On its use he has prepared what follows:

The correct appearance of the bands for any point inside or outside focus may be plotted. The lines are drawn in for the separations between the light and dark shadows thrown by each wire or edge of the grating, and the areas within these boundaries are blacked in solid. The formula is:

L is the distance of an edge of the actual grating wire measured horizontally from the optical axis;

R is the radius of curvature of the mirror;

N is the fractional correction (for a parabola, N = 1 );

r is the distance of a chosen point on a band from the optical axis;

s is the radius of the zone, at the focus of which the grating lies. (s is related to the distance of the grating from the center focus by d = Ns²/R.)

The plus sign is used in the denominator if the grating is outside the center focus, and the minus sign if it is inside.

The formula is easy to apply, using cross-section paper and a compass. Take, for example, a six-inch f8 mirror and a grating having 200 lines per inch. Suppose we want the correct appearance of the bands when the grating is 0.15 inch inside center focus. In this case L₁ is 0.00125 - inch; L₂ is three times that amount or 0.00375 inch; L₃ is 0.00625 inch; R is 96 inches, N is 1; and, since s²/R is to be 0.15 inch, s² is 14.4.

Confining our attention to one side of the first band we have

The compass is set at values of r and swung to the corresponding x's calculated from this equation, where points are marked. Enough points are plotted to enable a smooth curve to be drawn joining them. This is done for the L's on both sides of the center until the bands no longer fall within the circle which represents the mirror. Any values of x that turn out larger than the corresponding r are discarded.

One of the illustrations shows the results of applying the formula to four 6-inch mirrors; once I started this I couldn't stop-the drawings looked so pretty. They were done with some care.

Drawings 1 to 9, reading across from left to right, show the appearance of an f8 paraboloid, with a grating having 200 lines per inch, located at various positions ranging from 0.15 inch inside to 0.30 inch outside the center focus, 5 being the appearance at focus.

Ten is an f3, 11 an f5, both having been made with the grating at the focus of the center zone. Compare with 5.

Twelve is the f8 with a grating having 100 lines per inch, adjusted so that the band width is the same as in 2. Note the decreased curvature.

Since the formula gives only the lines separating light and dark areas, the black and white bands may be interchanged and the effect will be that obtained by moving the grating laterally half a line.

Because of diffraction the bands will never appear exactly as shown. If, however, the grating is not too fine and is kept fairly close to focus the figures will agree closely with the actual patterns, the main difference being a slight widening of the bands. Very fine grating wires give bands that may be several times the geometrical band widths.

The formula was developed independently of the method and related formula that are alluded to in Amateur Telescope Making-Advanced, page 108. That approach was developed in 1932 by Franklin B. Wright.

Suppliers and Organizations

The Society for Amateur Scientists
5600 Post Road, #114-341
East Greenwich, RI 02818
Phone: 1-401-823-7800

Internet: http://www.sas.org/

At Surplus Shed, you'll find optical components such as lenses, prisms, mirrors, beamsplitters, achromats, optical flats, lens and mirror blanks, and unique optical pieces. In addition, there are borescopes, boresights, microscopes, telescopes, aerial cameras, filters, electronic test equipment, and other optical and electronic stuff. All available at a fraction of the original cost.

SURPLUS SHED
407 U.S. Route 222
Blandon, PA 19510 USA
Phone/fax : 610-926-9226
Phone/fax toll free: 877-7SURPLUS (877-778-7758)
E-Mail: surplushed@aol.com
Web Site: http://www.SurplusShed.com