PORTABILITY is a relative term. Many articles described as waistcoat or "vest-pocket" size are that, especially if the vest pocket is an overcoat pocket, but still they feel so bulky that after the novelty has worn off the acquisition you find yourself leaving them home till some other day.

Horace E. Dall, 166 Stockingstone Road, Luton, Bedfordshire, England, has spent much time seeking such true lightness and portability-actual, not mere assertive portability-in vest-pocket telescopes that you will really carry them daily without a frown. He has built four such telescopes, descriptions of which follow. Of the four, two were successes and two were partial failures. a score that runs about average for TN optical projects. There is almost as much value in describing failures as successes, so these will not be omitted.

The first is a so-called spherical secondary "Cassegrainian" (actually the Dall-Kirkham telescope) of 3-1/4" aperture, a little gem of fine craftsmanship. Figures 1, 2, 3, 4.

"This outfit," Dall writes, "was made to carry about with me on rambles, mainly for terrestrial views but it will resolve stars down to Dawes' limit with a high-power eyepiece. The entire outfit goes into my waistcoat pocket without jutting out more than a fountain pen. It weighs eight ounces including a half-ounce portable stand (Figure 1) with tempered steel woodscrew end for quick attachment to the nearest gate post, window frame, or what you like.

"The power is X35 to X80, being continuously variable (pancratic) by pulling out the eyepiece an inch or so. Other eyepieces may be used to give changes of power but each one gives the same range, about 2:1. The eyepiece is of the comfort type-large eye lens and eye point 3/4" clear of the lens.

"No instrumental scattered light is visible even near the sun.

"The light grasp is decidedly better than that of a 3" refractor and the telescope will do all that a 3-1/4" is capable of. Figuring was carried to the limit and definition is excellent.

"The mirror is supported without strain or shake, and three flush adjusting screws with spring follow-up make adjustment easy and it stays put. The airtight covers for the two mirrors are a part of the framework and don't have to be put back on after use.

"The main ingredients of the mounting are duralumin and aluminum but some steel and brass parts are included where construction and strength demand.

"The spherical secondary Cassegrainian is merely a means of saving figuring time without any improvement in performance but if the spherical secondary Cassegrainian is combined with the intermediate erecting system (Figure 4) adopted on my 15-1/2" telescope described in Scientific American, May 1939, and for all my Cassegrainians since 1931, there is combined with simplification a very important improvement in general performance and an erect image.

"This outfit is many times lighter than anything that has been made, of comparable performance. A fair amount of work was involved, of more than usual precision. Next letter, I'll send a photograph."

All this was written by Dall just before World War II broke out, and the promised photographs were delayed for the duration. Reminded recently of the photographs Dall writes, "The vest-pocket 3-1/4" has been a great source of pleasure and has been carried and used on all holiday journeys since 1939. The Devon and Cornish coasts were our chief holiday centers during the war and I carried this 3-1/4", watching convoys sailing up and down the Channel. Once, in 1941, during an air raid alert, a Czech refugee saw me using it and fetched a policeman who arrived with a screech of car brakes. Fortunately for me, regulations were not so tight in 1941 as they became later and pocket spy glasses were permissible, and I escaped the clutches of the law and did not lose my telescope."

In 1940, the war young, Dall made two more portable telescopes for watching aerial dogfights. One (Figure 5) consisted of a 13/16" two-lens achromatic objective, a four-lens erector giving flat, wide field, and a four-lens, long eye relief eyepiece-the whole a small but carefully designed piece of optics. The eyepiece plus erector alone serve also as an X30 compound microscope. Total weight, 1-1/2 ounces. This telescope is so successful that Dall carries it at all times. It is high grade optics not to be confused with low-cost pocket telescopes of not very dissimilar external appearance. The general public would not pay what such a telescope as this is worth.

The remaining two telescopes that were not complete successes are even more interesting. They came near being successes and their description may challenge others to try to leap the same hurdles. Description also offers free experience to others who plan similar designs, the wise being able to assimilate others' experience and profit by it, the not wise having to re-manufacture their own from scratch.

Figures 6 and 7 show a 1-1/4" Gregorian 25/8" in length, X20, made in 1928. "Disadvantages," Dall states, were: "Although good objective lens, secondary spectrum was very evident at X20 owing to short focus (about 5"). Figuring done well but telescope highly critical to squaring on. Difficult to hold steady owing to shortness. Field only 32degrees. Close eye point and small hole."

Other details: 1-l/4" achromatic objective; hyperboloidal 3/4" primary; 1/2" Tolles eyepiece 1/4" diameter; ellipsoidal secondary 1/3 diameter of objective lens cemented to the latter; screw focus; erect image. In Figure 7, drawn by Dall, F.4, F1.4, F.8 refer to foci these many apertures distant. Partly related observations: It is believed that some American amateurs have worked on reflecting binoculars. Porter tried it as I mentioned last month. Wm. Buchele, former amateur, Toledo-20" reflector described this department 1939, October, 7500 roof prisms in amateur roof prism program in war time, now Modern Optical Co.,-applied, 1943, for patent granted last December (U. S. Pat. 2,413,286) on a Schmidt binocular but in most such designs the mirrors are too hard to anchor in collimation.

The other Dall partial success is the miniature 1-1/2", X 12, 6/10-ounce Gregorian shown in Figures 8, 9,10. This was designed for attachment to the spectacles also shown. To close it the primary mirror (into which the eyepiece is fixedly set) is folded down to right and the secondary mirror to left.

Other details: Eyepiece is an achromatic of 0.90" f.l., 0.57" diameter. Little "handle" seen projecting apparently downward projects actually to right and is a lever of the second class for focusing; it causes the respective halves of the telescope to move apart or together on two rod sliders.

Troubles: "But I couldn't get a proper cutting out of sky flooding," Dall writes "with the system of quick hitching on to my 'specs' that I had in view. (When the photographs were taken I hadn't yet designed hitching-on clip.) The Ramsden disk, at 1/8" diameter, would I first thought, just about fill my daylight pupil and eliminate the necessity for a sky-flood stop. In practice it was too difficult to get the eye sufficiently well located coaxial and coincident with the Ramsden disk."

Score: Two bull's eyes and two on the target.

GEAR TRAIN for telescope drives, supplied by E. B. McCartney, 1205 W. Minnehaha Parkway, Minneapolis 9 Minn., is a simple one. It is: 1 r.p.m. of Telechron motor drive shaft with 8 tooth pinion on it; 52 teeth in gear on end of worm; 221 teeth in worm gear around polar axis. (Make worm gear on lathe with tap. Using 10-pitch sharp V-thread tap, gear will be about 7" diameter.)

Gear reduction is then 1/6.5 X 1/221 = 1/1436.5, and polar axis will turn once in 1436.5 minutes.

To detect pits in fine grinding, McCartney allows the gunk to dry, then wipes off with the hand. The remaining white spots are pits.

RUSTING of cast-iron grinding tools may be inhibited by using water mixed with Oakite, 10 teaspoonfuls per gallon. Says Dr. G. Dallas Hanna, San Francisco.

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