TWO FEATURES of a 12-1/2" reflecting telescope built by Edward Lenard, 654 N. Austin Ave., Chicago 30, Ill., deserve close study by readers who plan second or subsequent telescopes. These are the mirror cell and the focusing arrangement for use in celestial photography.

The mounting (Figure 1) is temporary. Its axes are close pipe nipples pivoting in standard pipe flanges.

The tube is 15-1/8" by 93", for a 12-1/2" Pyrex mirror of 92-1/2" focal length. It was rolled up from sheet aluminum designated as 3S1/2H .065 (3, alloying composition; S, wrought; 1/2H, one-half hard; .065, thickness in inches), a grade which takes welding well. However, the three lengths composing the tube are riveted together. The tube is braced internally by four U-channels formed in a hand brake and then bent to diameter over various round objects. "The reasons I chose aluminum, Lenard writes, "are its very desirable working properties, resistance to weather, heat-conducting qualities reflectivity, and strength-height ratio. It is somewhat expensive. However, it makes a pretty tube." The cell and attachments (Figures 2 and 3) are made of the same sheet stock. Within the cell is a 9-point flotation system carried on a spider of six legs bent from sheet and welded at their common center.

To three of these spider legs are attached short brass blocks through which three bolts (SAE 1/2-20) pass and are tightened up by nuts on the outer sides.

Silver-soldered to the ends of these three bolts are 5/8" ball bearings. These balls are pressed into the 1/8" triangular, aluminum, 3-point plates which carry the mirror, and the depressions serve as the primary pivots. Bolted through the corners of the respective plates are 3/16" brass bolts on which, as secondary pivots, the mirror is carried.

Such a system will float a mirror but the triangles would soon become turned in different directions and destroy the intended distribution of weights. Some kind of preventers therefore must be provided and these must not interfere with the actual balancing. Lenard's system consists of three links, each with a T-head on either end and the ends of the T's pivoted at the central hub of the spider and at one of the triangular plates. This affords them freedom in the up-and-down plane (one degree of freedom) but there must also be a little end play and thus two degrees of freedom are needed. Lenard has provided this by composing the stem of each link of two small brass tubes, one telescoping within the other. He points out, however, that for ideal performance a ball-and-socket joint should be substituted at the triangle end and makes a comparison with a ship, where the motions are theoretically either pitching or rolling but usually are a composition of the two- yawing. Theoretically, he points out the linkages should permit yawing.

As edge supports for the 12-1/2" mirror the ends of the six filister-head machine screws shown in Figure 3 suffice. "The inclined mirror then has two points to rest on in any position and in some positions three," Lenard states "though the lower one in such cases takes most of the weight." To prevent the mirror from falling out of the cell when the tube is dipped below the horizontal, three simple retaining fingers having little leather pads reach around its edge.

Lenard's focusing arrangement is shown in Figures 4 and 5. He states that perfect control is here not easy to attain. "The problem of raising and lowering the barrel without rotation and without sticking was solved," he states, "by using at least two points to which to connect the hinge. This arrangement comes up and down evenly and without sticking, and affords very fine adjustment as well. The single thumbscrew actuates the whole system. The travel of the barrel is not more than 3/8". I use various lengths of brass tubing to suit the focal length of each eyepiece and changing over thus does not require refocusing. [Parfocal. -Ed.] The reason for countersinking the assembly is to permit mounting a Packard shutter directly below, on the inside of the tube, for photographic work. This shutter is operated by a bulb. The entire assembly was made by hand from brass and aluminum."

PUZZLED readers often ask this department why Pyrex telescope blanks have tapered edges and why they have a narrow rim on the back. These features don't just happen and are not mere whims. Corning Glass Works was asked for a blueprint of one of their metal molds and from this a reproduction draftsman redrew Figure 6, which should be studied in detail before reading on. It explains a number of things which make sense once you are shown their reasons.

Now let's cast a blank. The plunger is raised and the ring is lifted off. The workman, known as the "gatherer," pours into the mold a gob of molten Pyrex. This is not like water but viscous, molasses in January, even at 2800 degrees F., its softening point. Now he claps on the heavy ring part and down comes the plunger. It is nice if, when the plunger is all the way down and the mold is filled out, the plunger bottom comes to rest as shown-very near the level of the rim. Making it so depends on the estimative skill of the gatherer and he is very good at this knack but is only human. If it is a bit over or under he can't add more Pyrex with an eyedropper; it's too viscous. Get it hotter? Easier said than done. The softening point of Pyrex is already much above that of common varieties of glass.

The molded disk is removed from the mold very soon and turned over. Its exterior is already relatively cool but its interior, seen through the glass, is still red hot. As this interior cools the center of the wider face of the disk falls a little and this explains the slight concavity usually found on Pyrex blanks.

The 6 degree tapered side is the draft, so that the ring can be slipped off-see the drawing, which also shows a 3 degree draft on the part that lies in the mold.

Visit Corning Glass Works and you may be shown several shelves filled with these heavy metal molds.

The bubbles often found in Pyrex blanks would rise and escape if Pyrex were not so viscous. The bubbles you don't see are the bigger ones that were able to escape. Little ones can't rise so easily. Case of surface-volume ratio. No practicable way to eliminate all the bubbles is known. Corning would be glad to find one but it isn't as if they hadn't given the problem a great deal of thought.

SEQUELAE of the seldom curable telescopical addiction are sometimes serious, and may even include outbursts of light verse. The following is how the hobby affected M. J. Irland, 916 N. Rosevere, Dearborn, Mich. He entitles it "Paradise Lost, or Ignorance is Bliss," and dedicates it to your (blissful) scribe:

There was a man in days gone by

Who loved to watch the skies.

He'd learned the constellations well

With his unaided eyes.

He bought a book called "A.T.M.,"

A first edition slim,

And set to grinding disk on disk

With diligence and vim.

With windshield glass and pocket lens

The optics he completed,

And learned to silver Brashear's way,

Through trials oft repeated.

With joy he gazed at Saturn's rings,

Made lunar observations,

Watched Jupiter's bright satellites,

And noted occultations.

Then "Telescoptics" told of tests

On Lyra's double star;

He drooped with disappointment when

He couldn't split Mizar.

So back to Carbo, glass, and pitch

He turned in wounded pride,

With "A.TM." (edition two)

Well rouge-stained, by his side.

With slits and Ronchi gratings, culled

From Telescoptics' pages,

He strove to get the doughnut shape

Commanded by the sages.

Came "A.T M."-edition three-

With Greg and Cassegrain;

Our hero could not well ignore

This challenge to his brain.

On Hindle sphere, paraboloid,

And convex hyperbolic

He lavished rouge, and sweat, and tea

In frenzy diabolic.

Edition four, with bottlenecks

In declination shafts,

Impelled him in despair to learn

All metal-working crafts.

But when he thought he'd made a 'scope

Without an imperfection,

"A.T.MA." depressed him into

Deep and dark dejection.

For there he found expounded,

As a matter most essential,

New doughnut mathematics

In equations differential.

So, weary but undaunted, he

Performed manipulations

For testing astigmatic curves

And spheric aberrations.

At setting circles, clockwork drive,

And domed observatory

He labored with the fury of

A fiend from Purgatory.

But when he'd run the gamut, in

The hope of satisfaction,

Infinitesimal defects

Still drove him to distraction.

No longer did he scan the skies

To revel in their beauty;

Detecting telescopic faults

Was his fanatic duty.

Instead of watching clusters for

The pleasure of their glitter,

He studied star diffraction rings,

Despondent at their jitter.

The seeing made him grind his teeth

Air currents plagued his vision.

No collimation could attain his

Notion of precision.

And as he sat with head in hands,

Bystanders heard him mutter:

"I'm gonna junk the whole darn' works

And throw it in the gutter."

The moral for beginners is:

Lest knowledge disconcert you,

Recall the aged maxim, viz.,

What you don't know won't hurt you.

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.