"The most difficult part was cutting and mounting the slits. By fixing the curved one on the end of a long piece of wood pivoted at the other end, I got a good and true edge by rubbing it against a piece of fine Carborundum stone for the male half and reversing them for the female.

"It was all intensely interesting and well worth the trouble. My first photographs were terrible tragedies. The solar spectrum, though clear, was so over-exposed that it came out positive. But if there was nothing to work for it would not be half the fun. I have found no need for the long focusing rods or for a shed, a galvanized cover giving sufficient protection."

Mountfort, at the time he wrote the above, was painting a portrait of R. W. Porter to be contributed to a gallery of astronomers and telescope designers of our time, "in order that those who will be working the 200 inch in 1000 years," as Mountfort puts it, "may know something of the pioneers of this development of today."

FIRST big hurdle to jump when undertaking a Schmidt astronomical camera has been the deep curve; otherwise a lot more Schmidts would be made by amateurs. The Lowers used 25 pounds of Carbo and played wet nurse to their grinding machine for 98 hours, in 1935, when roughing a deep f/1 curve out of a 12-inch Pyrex disk for a Schmidt. Today they use a Borium tool on a large lathe and do the job (to within 1/4 inch of ultimate depth-as far as is safe) in one day. This is excellent-provided you have a Borium tool and a large lathe. The alternative-hand work-would require "less than a month," working union hours, Harold Lower states.

Fred Ferson, Biloxi, Mississippi, therefore suggested some months ago that an attempt Corning, N. Y., makers of Pyrex baby bottles and 200-inch mirrors, into casting a batch of Schmidt blanks with approximate curve. Approached, Corning showed a co-operative spirit. "What size, what curve, would you like?" they asked. Offhand, one might reply casually, "Oh, all sizes, all curves." But this would make the disks cost real money, for the sale will not-cannot-be large. Ferson therefore suggested 12-1/2 inches, 32-inch radius, as a mean-or, rather, mode. Lower agreed. So did others.

Advised, Corning stated that the most economical way for them to make these would be to rough grind the cavity rather than cast it in, as the molds are expensive; also, for some technical reason, to make the radius 36 inches. They found they could supply these at a little more than double the price of ordinary Pyrex disks of the same size.

Before announcing this fact it was thought best, however, to put these disks to actual test, so Corning sent Ferson a disk, 12-1/2 inches, 36-inch radius, and he spit on his hands and went to bat with it. Final report: He spent 12 hours and used two pounds of Carbo in deepening the curve from 36 inches to 32 inches and truing it. Used convex tool of full diameter, which he, cast from type metal. The soft metal ground away as fast as the glass: hence, no concave mate was needed. Metal tool also gave shallower pits, smoother surface, he states. In sum, he reported, it is now possible to grind and polish a primary for an f/2 Schmidt, or an f/3 Wright, by hand, in size but common focal length. For this, we may thank Ferson and Corning.

LAST month in this department it was hinted that James G. Baker, of the Harvard College Observatory, had designed a family of flat-field cameras (Figure 2) equivalent in performance to the Schmidt. The following is his description of them:

"Many years ago Schwarzschild set out to find the most useful system of two mirrors possible. He found that, although there exists a system that is aplanatic and anastigmatic on a flat field, it is of no practical use because of the total silhouetting of one mirror by another. As a consequence Schwarzschild reintroduced astigmatism and solved for the parametric equations of a family of aplanats with flat field. The best known member of the family is a system of two concave mirrors, the secondary being half the diameter of the primary. Astigmatism in the system limits the available field of this reflector.

"Improvements in the image quality of reflectors were of a minor nature until Schmidt introduced the idea that small deformations in an otherwise weak lens could be most effective in helping to eliminate the image defects of coma and spherical aberration. Wright has generalized the original Schmidt camera into an entire family of Schmidt: cameras and has shown that astigmatism cannot be eliminated on a flat field by correcting plate and one mirror alone. Wright's work in turn can be generalized to show that astigmatism cannot be eliminated on a flat field, even when several correcting plates are employed, all separated, so long as the total powers of these correcting plates are of the second order with respect to the mirror.

"Three mirrors with aspherical surfaces can be designed to give excellent performance mathematically, but the silhouetting in such a system is hopeless. Thus, only one more possibility remains, and that is a system of one correcting plate and two mirrors. This combination produces a two parameter family of telescopes with flat field. The two parameters remaining after all third order aberrations have been satisfied, are the distance of the correcting plate from the primary mirror and the distance of the photographic plate from the secondary mirror. When the correcting plate is placed immediately before the secondary, the tube length becomes extremely short compared with the focal length. For example, one combination produces a tube one sixth the length of the tube for a Schmidt of the same focal length and performance. For such a short tube length all three of the curved optical surfaces are aspherical. The two aspherical mirror surfaces fortunately possess no inflection points.

"If the tube length be of no serious consideration, the two-parameter family includes a remarkable one-parameter family, with tube length about 3/4 that of the Schmidt of the same focal length, such that the two mirror surfaces depart inappreciably from spherical surfaces. The curve of the correcting plate becomes much more shallow, and is not much deeper than for the Schmidt of same focal length and aperture.

"The reasons for the existence of the two parameter family can be seen from consideration of the Schmidt and Schwarzschild systems. Then, too, one can deduce the family from the Schmidt system. One starts by considering the usual Schmidt camera with an additional plane mirror secondary, untilted, so that the curved focal surface is inverted and near the primary. By keeping the radial and tangential astigmatic surfaces coincident, and by variation of all the curves and separations, the curvature of field can be continuously reduced to the flat field here considered.

"Figure 2 shows an f/3 camera of short tube length with an 8 degree unvignetted field, also the corresponding camera of longer tube length and spherical mirror surfaces. Moreover, for comparison purposes a Schmidt camera of exactly the same light-gathering power, focal length, and unvignetted field is shown. The apertures of the two-mirror systems have been adjusted to compensate for the additional silhouetting and extra surface, absent in the Schmidt.

"The advantages that can be pointed out for the two-mirror system are the extremely short tube length possible, with consequent saving in space and material, convenience in loading the plate holder, and the existence of a large, flat, unvignetted field. To give an idea of the importance of the field, if the customary large paraboloidal reflector were converted into this type in the same tube, the number of square degrees photographed per night would equal the number of square degrees photographed by the paraboloidal form per month, the definition at the edge of the useful field being, the same, from a very conservative calculation. Fortunately, the two mirrors and plate-holder are close together in what is usually the stiffest part of the tube."

In Figure 2 the spherical surfaces, tangent to the true surfaces of the mirrors in the shortest type, are shown dashed. The mirror curves are exaggerated to twice their true

depths, the correcting plate surface many times, and the focal surface of the Schmidt twice.

IN the Journal of Scientific Instruments (London), August 1939, H. W. Cox, 47 Upper Green, Mitcham, Surrey, England, an amateur whose first Schmidts were described in the February number, describes in a six-page article his technic of making Schmidts. He sent us an extra copy of the article, which will be lent to those seriously doing Schmidt work who wish to ask for it, provided they will promise to return it promptly so that it may be passed on to others. Cox will also send copies to genuine Schmidt workers, if asked. The paper would mean little or nothing to others, as the methods are in no way applicable to work on ordinary telescopes; Schmidts being advanced work.

ONCE a year a "nova" breaks out among telescope makers: verses. Most recent poet is the proprietor of the Apex Monument Works (gravestones! ), 6815 West 27th Ave., Edgewater (Denver suburb), Colo., who offers the following outburst (novae are due to an overproduction of energy):

The Amateur Mirror Maker By Anton Bohm

He walks at night, around a keg: on top a disk of glass.

He pushes forth and pulls It back-another piece of glass.

For months and months he's kept it up, his patience still intact.

One hundred miles he's surely walked around that circular track.

He counts the minutes and the strokes; elliptical and straight.

He fights against a turned down edge and spheroids much oblate.

Paraboloid, hyperboloid A.T.M.A., T.E.-

Are words and terms that he can use with ultrafluency.

And every now and then he takes his pretty, shiny disk-

With loving care he handles it, indeed he takes no risk-

And puts it on a testing stand to see what he can see

And squints along a razor blade and then he shouts with glee:

''At last! I see what seems to be a doughnut fat and round ! "

But though the shape is there alright, the depth too profound.

The distance from the inner zone. compared with from the rim

Is much too far away as yet. So It appears to him.

Again the weary grind begins. It surely is no cinch

The surface must be right within one millionth of an inch.

One millionth of an inch is small, and you will promptly ask:

"How does he know when he is done with his gigantic task?"

His pinhole lamp and razor blade-they will reveal to him

The mirror in its Perfect shape, correct from rim to rim.

But if you knew the grief he has with rings depressed and raised,

With turned-down edge and edge turned up, you'd surely be amazed.

But that is not the only grief-experience he must gain,

It's finding scratches, sleeks and pits that causes him much pain.

Dust is a thing that he abhors he flies into a tantrum

When finding on his polished disk a grain of Carborundum.

"Oh Wifey! Did you sweep the floor?" he hollers from his shop.

''If you insist on cleaning up, I'll surely buy a lock."

"Indeed you'll not," replies his spouse, ''I never yet have seen

A place so gory, It's a fright-I'll never get it clean.

"Rouge over every thing I see. It surely is a mess.

''Rouge on my towels, yes on the walls, It's even on my dress. "

But all hard tasks come to an end. Some night the Foucault test.

Will throw upon his speculum the shadow loves best.

'The shape is there-the depth is right, at last the thing is done.

He heaves a sigh of deep relief-some people call it fun.

And so will you, I dare predict, some day you'll see it-but

You never will, till you've become a Telescoptic Nut

FORKS, if made by casting from carefully shaped patterns, may be made very pleasing to the eye but, if not, they some times fail to satisfy the telescope maker's feeling for beauty.

A kind of fork that can be made with a lathe alone, yet which is pleasing to the eye, is shown in Figure 3. Otto C. Rasmussen, 57 Brunswick Ave., Troy, N. Y., designed this fork, in which he used 1-1/2" steel shafting, a circular plate of 1/4" steel and underneath it, two rollers. The telescope, a 6" of 50" f. l. is equipped with n motor drive mounted on its triangular baseplate. The latter is of angle iron with a 1/8'' steel deck. On this deck are two double glasses and a the apices three leveling screws with lock nuts. The hour circle is of the floating type. The square tube is made of l/2" x 1/2"x 1/8" angle iron with diagonals 3/8" x 1/8" strip steel.

Rasmussen also made a nocturnal or horometer (Figure 4) for giving sidereal time. The handle is held perpendicularly, Polaris is sighted through the central hole and the straightedge is turned to line up on Beta Ursa Minoris. The pointer then shows sidereal time within about 5 minutes.

BOTH "ATM'' and "ATMA'' recently underwent minor operations when exhaustion of stocks led to reprintings (note: not new editions). Misprints were corrected and in "ATM" a few changes made. Old note on p. 333 discouraged refractor work, new one encourages it--things have progressed. Note on p. 458 was replaced by one telling why photographic lens making is highly advanced work, because numerous novices seem to get the idea that making a photographic lens would be a "simple introduction" to telescoptics! Hallucination.

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