TERRESTIAL TELESCOPES for ordinary landscape gazing do not require the same top optical standards as astronomical telescopes and thus the one shown in the upper part of Figure 1, made by H. Clifford Allen, Princeton, N. J. proved good enough for that purpose. It contains an erecting system made of selected pieces of ordinary plate glass.

Not certain his plan for a three-mirror erecting system was correct, Allen collected a few compact mirrors, feminine gender, arranged them as shown and proved that his idea would work. Then he cut some pieces from 3/16" plate glass, tested them by interference against a standard flat, and, selected three which were thus proved to be not more than two fringes (1/50,000") uneven. These he front-surface aluminized and mounted as shown.

Using an eyepiece of 1-1/4" focal length he obtained a magnification of 26 diameters-the focal length of the main telescope mirror divided by that of the eyepiece-and he reports that definition, light-gathering power, and contrast proved very good in terrestrial observing.

Plate glass diagonals, given some sanction in "A.T.M.," may satisfy the tyro when used with his first mirror, if selected as described there-as some have not been. No matter how nearly flat they may be over their area as a whole they still are not flat on a minute basis because plate glass is felt-polished and has a minute "lemon-peel" finish. This diffuses some of the light. They get by easily on terrestrial telescopes (except target spotters) but less so for astronomical uses.

A better erecting system is described by Doncaster G. Humm, 1219 West Twelfth St., Los Angeles 15, California. He writes:

"Reflectors make excellent terrestrial telescopes. They are light and easily transported. Their light-gathering power is relatively high. The chief obstacle preventing their more frequent use in this field is difficulty in securing a good erecting system.

"The Ramsden eyepiece, used somewhat differently than is customary, offers a solution for the difficulty. Bell notes in "The Telescope" that the Ramsden performs best when the faces are exactly their focal distance apart, but notes also that this brings every bit of dust into sharp focus, thus interfering with good vision. This objectionable feature is not, however, noticeable when the Ramsden is used as an erector.

"The Ramsden eyepiece consists of two plano-convex lenses arranged with their convex sides facing each other. Usually, the two lenses are spaced two thirds or three fourths their focal distances apart.

"The formula for the combination is:

"In this formula, F is the focus of the combination; f₁, the focus of one lens; f₂, the focus of the other; and d the spacing.

"The best chromatic compensation is obtained with

"In the Ramsden suggested as an erector, f_l = f₂. Hence, both equal d So, the formula becomes: F=f (1 or 2)

"Two such Ramsden eyepieces may be arranged in tandem (Figure 1, below) in such a way that the distance between them may be varied. The closer they are placed, the lower the magnification, since the combination is in effect a low-power compound microscope.

"I have such a combination mounted on a 6.5" reflector of 50" focal length. Each Ramsden is mounted in a brass slip-tube 3" long. One Ramsden has a 2" focus; the other, a 1-1/2". Their slip-tubes have a sliding fit in a 6" focusing tube. When the two Ramsdens are at closest position, the power of the telescope is approximately 40; at their farthest distance apart, the magnification is nearly 100.

"Used at these low powers, the light-gathering capability of a 6" telescope is very great. Flowers viewed at a little distance take on a richness of color not apparent to the naked eye. Twilight and evening viewing of animal life becomes possible, since the setup is in effect a very powerful night glass.

"Astronomical viewing suffers little from the light loss caused by the erecting lenses, since seeing at from 50 to75 power is more frequently possible than at high powers. The reflector has been used to view the cloud bands of Jupiter, the globular cluster in Hercules, the great nebulae, and many other spectacles-all very effectively."

AMATEUR ASTRONOMERS who have had to run numbers of non-astronomical uninitiates quickly through observations well know how hard it is to manage the manipulations and keep the visitors happy. For such uses the special mounting shown in Figure 2, made by John Holeman, 305 Thayer Drive Richland, Wash., seems especially well adapted. The telescope runs easily up and down its post on two roller-skate wheels which don't stick, and has a clamping screw. It has a simple pointing handle which even your old Aunt Sally can work without a course in mechanics. Holeman designed it for use in teaching large classes of totally unsophisticated observers who, of course, would be of all heights.

The tripod at the bottom is made of 3/4" pipe and the post is a piece of 1-3/4" seamless steel conduit. The telescope is mounted on an arm which Figure 3 describes better than words. The arm was built up of 1/4" and 1/2" dural pinned together with taper pins and screws: Holeman didn't have a casting.

The alt-az mounting at the outer end of the arm is a Royal camera tripod head.

"The RFT telescope," Holeman states, "contains a 3", f/6 Navy coated spyglass objective. I made the mounting, including the toric cell ring and push-pull adjustments, from scrap. The chart table shown in Figure 2 beside the observer (Mrs. Holeman) is a folding photographic light stand (tripod) with a plywood table and spring clothespin clips to hold charts, as I also use this telescope for looking for telescopic meteors of which we see quite a few in the clear dry desert air of southeastern Washington. Over the table is a length of BX conduit carrying a flashlight bulb behind a red filter, the batteries being clipped under the table."

TO POOL among amateurs, through publication in this department, as many specifications for good objective lenses as can be found by its readers is an aim of its editor, expressed some time ago. From Colonel Troy W. Lewis, 1812 Schiller Avenue, Little Rock, Arkansas, come the specifications given in Figure 4. He writes:

"This lens was computed and constructed with Parra-Mantois et cie crown and flint glass prior to the outbreak in Europe of World War II. It gave excellent results on either nebulae or clusters, and was quite effective in terrestrial work when viewing objects against deep shade or in poor light. Because of its large light-gathering power and short focal length it should make an exceptionably desirable objective for a spotting telescope or, with some modifications, a night-glass for rich field observation.

"When French glass became unobtainable the design was recomputed for Bausch and Lomb optical glass with the thought of making the objective available for amateur construction. That computation, although performed with patience, was made during odd moments using log-tables and pencil under trying conditions. The probability of error in the calculations became at once a sobering factor making it seem imprudent to release the specifications without further study. Since the time necessarily involved to re-check the work could not then be taken from missions of greater importance it was thought advisable to withdraw the design from the publisher.

"Recently it was my good fortune to acquire a large computing machine that is capable of the highest class of work. Now that it appears probable English optical glass soon will be on the American market again, and since Chance Brothers Company, Smethwick, England, makes a glass-pair highly suited to this air-spaced design, I have recomputed my Clark-type objective to be made with Chance Brothers' glass specified in Figure 4. The glass is obtainable through the American Agents, The Ednal Company, Inc., 95 Madison Avenue, New York.

"This objective design was subjected to a careful trigonometrical trace of two rays through both the axial and rim regions. Spherical and chromatic aberrations have been reduced to zero, or practically so. I was unable to satisfy fully both sine and Gauss conditions simultaneously, due in all probability to the form of the objective rather than to the glass selected, but the residuals, as shown in the specifications, are considerably smaller than the amount that can be tolerated in this type of lens when used for the purposes indicated. A slight increase in separation would satisfy entirely the Gauss condition but other more desirable corrections would be slightly upset. The chromatic difference of magnification on the axis, now roughly -0.0033", would be slightly enlarged, while the error against the sine condition, now + 0.0023", would be increased. The corrections as they stand are probably of astronomical grade although not so claimed. The highest class of terrestrial objective is claimed.

"I have not actually constructed this particular Clark-type objective with these specified English glasses although a similar objective has been made with French glass to specifications not so perfect as those given here. Splendid results were achieved, as previously stated, and no reason is seen why this recomputation for English glass should not prove equally good. Judging from past experience, and relying on the accuracy of the computing machine, I am convinced this objective will fully satisfy those amateurs who may choose to make for themselves a fine spotting telescope from those specifications.

"After long and careful study of this design I am forced to conclude that the perfection attained in the original Clark air-spaced objectives was due more to the magic touch of Lundin, the elder, than to the exact science of pure mathematics. My air-spaced objective, when made to specifications, will prove to be highly acceptable. If we have the necessary skills we may hope to satisfy both the sine and Gauss conditions by figuring the fourth surface slightly. Perhaps, with diligence and patience, we, too, may attain the high degree of perfection seen in the masterly work of Lundin."

Doctor D. Everett Taylor; Willimantic, Connecticut, has made the valued suggestion that the telescope be equipped with a roof prism inverter for terrestrial use, also that the surfaces be given a coating of fluoride, which should add much to resolving power.

Any reader who undertakes this objective is invited to keep in touch with Colonel Lewis or with this department, or both-though progress reports to either one are likely to be seen by the other since this department similarly keeps in direct-mail touch with numerous readers everywhere.

The Society for Amateur Scientists (SAS) is a nonprofit research and educational organization dedicated to helping people enrich their lives by following their passion to take part in scientific adventures of all kinds.