George Herbig, one of the more prominent amateur telescope makers in Los Angeles and now connected with the Griffith Observatory (planetarium) there, writes that he visited Mt. Palomar Observatory before it was closed up, and took the cover picture.

This is no ordinary photograph. As anyone knows, who has tried to photograph a telescope within its observatory dome, it is most difficult to obtain points of view that take in the whole instrument-usually one gets only various parts. Herbig did not knock out one wall of the building, but luckily found the yoke and tube so positioned that he could shoot partly upward from below and get an excellent general view of the whole. The ladders propped against the mounting give the scale. The object in the lower left foreground is the aluminizing tank-really a gigantic affair, Herbig states.

There is no use arguing that the 200" is beautiful. It's a plug-ugly, like a big square-toed, hobnailed boot. But it wasn't designed for sex appeal. Functionability was the true governing principle in the design and no real sacrifices were made to beauty, as this would be criminal. So it's simply a he telescope, unashamed.

Figure 1 shows the main worm wheel for the R. A. drive of the 200" telescope. This is 11' in diameter and the spacing error of the teeth is one second of arc, or one part in about a million and a half. The man shown worshipping is M. K. Baughman, technician of the Griffith Observatory. George Herbig took this photograph also.

GENERALLY, in precision optics, surfaces are worked to a tolerance of one eighth of a wavelength, or around 1/400,000". But not all of them. Suppose light is to be reflected from two surfaces, serially; or three, serially. The tolerance then becomes smaller. The 200" mirror for example, is to be worked to a tolerance of 1/20 wavelength-just about a millionth of an inch. This is to be done because in certain combinations of its auxiliary mirrors there will be six reflections, serially.

This 1/20 wavelength is simply an attenuated case of the famous Rayleigh tolerance, described in Conrady's "Applied Optics and Optical Design." For one reflecting surface the tolerance is 1/8 wavelength; For n surfaces in series it becomes 1/(8n^1/2) and where there are six reflections this is approximately 1/20. Summarizing the matter, H. H. Selby says in a letter:

"Maximum phase difference at ultimate focus for all rays should be less than 1/4 wavelength, 1/8 for perfect definition.

"Assuming a smooth surface, a zone one wavelength out of position will introduce two wavelengths' path difference at focus, for one reflection.

"If (n - 1) surfaces are interposed between the first surface and the focus, least squares treatment indicates that a total of n reflections will have an average of the square root of n effect.

"Assuming that each surface adds an error in a common direction (either plus or minus), the summation of path differences must be less than 1/8 wavelength, and the first surface must contribute no more than 1/(8n^1/2). For six reflections this is approximately 1/20 wavelength."

All of this being the case, but often lost sight of, it seems that where two surfaces are involved, the tolerance should be pushed up a bit, to about 1/11 wavelength-at least where perfection is the high aim. It is one argument against tackling Cassegrainians lightly.

OWNERS of the book "Amateur Telescope Making-Advanced" will recall, in that book, page 319, a description of a drive for a fine 12 l/2" telescope made by Alan Gee and C. Carvel Diller, the former then a cadet at the West Point Military Academy. Gee, now a Captain in the 18th Engineers at Vancover Barracks, Washington, where he is working on a 20 l/2" Cassegrainian in conjunction with two Portland amateurs, sends in the following, with the statement that by the time it is in print he may be elsewhere.

"Here is my contribution to the Cassegrainian secondary problem.

"It is impracticable to test the hyper" boloidal convex through the back, so as to consider it a concave. However, it is practical to test a concave hyperboloidal test mirror and then use this mirror to test the convex by means of interference fringes.

"What I propose is this. Determine radius of curvature of the secondary by the usual method. Grind and fine grind to this curve. Then set aside the convex mirror and polish the tool, which of course is of the same curve as the secondary except it is a concave. The tool need not be polished to the perfect degree required of the finished secondary, but should be fairly well polished.

"Then figure the concave tool to a hyperboloidal curve, using the good old Foucault test and the following formula to determine the radii of the various zones.

[The symbols are the standard ones of Ritchey-see "A. T. M.," page 62.-Ed]

"Obviously, the same procedure would work for testing the spherical convex of a modified Cass. The test mirror would of course be figured spherical.

"This method sounds long and difficult but remember that you already have the concave test mirror fine ground to the correct radius, and that only a few hours of polishing will be sufficient to permit figuring and testing by interference fringes. Also the test mirror need not be wasted after the job. It can readily be refigured into a good paraboloid primary.

"I inclose my derivation of the above formula in case some of your math nuts would like to try to pick it apart." [Available to math maniacs, on request.-Ed.]

Gee says he and his co-workers are using this test on the 6" secondary of the 20 1/2" mirror, which is an old Tinsley replica of the 200" mirror with prime focus 80". Secondary focus is to be 240", hence overall f ratio is about 12. English type mounting. Split ring outer bearing, allowing the Pole to be reached. It will be extremely heavy and rugged, he adds.

ON a bluff just outside Moline, Illinois overlooking a valley and with an excellent horizon, stands the

observatory shown in Figure 2, built by Carl H. Gamble, R.F.D. 1, that city. It contains a 5 1/2" Zeiss refractor on a Clarke mounting

Lower story of the observatory houses the owner's science library in a room 18' by 20', which also will accommodate the members of the local club of amateur astronomers. Above is a deck 22' by 32' and on this is the spacious dome, 16' 8" diameter-just the size, by the way, of the 200" mirror in California!

Details of this neatly designed dome and slot cover are of especial interest Figure 3). The former is a steel silo top made by Lamneck Products, Inc., Middletown, Ohio similar to domed silo tops made by a number of manufacturers. For example, The Independent Silo Co., St. Paul Minn. Entire dome rotates on eight rollers near the floor level, the conventional fixed wall thus being eliminated. In this respect it is similar to the type shown in "Amateur Telescope Making-Advanced," page 493, Figure 13.

DURING the soul-trying work of making a first telescope mirror, when it is altogether natural to entertain misgivings whether the claim that an amateur can make his own telescope mirror is true, something is needed to bolster or shore up the morale of the poor, lonely, and perhaps wavering worker.

A mirror-maker who has "been there" in his own time but wishes to remain anonymous, writes the following: "It is practicable to use a mirror in your mounting before it is aluminized, before it is figured, before it is even fully polished.

The famous Brashear, in his Autobiography (page 49), tells of doing this. I, too,

have done it. Sunspots can be studied (always using two pieces of smoked glass between eyepiece and diagonal), the Moon's major features can be seen, likewise the phases of Venus. I saw all those fairly well when the figure still was a rank hyperbola. It renewed my courage."

The tyro may, however, encounter one lesser discouragement in connection with doing this-collimation of the mounting to hold the mirror. This usually is a tough, cussed job. Perhaps, if told in advance that it usually is such, and thus led to expect it-for most owners have been through that same mean stretch and few have found it otherwise-this setback to morale won't seem so big.

AFTER completing an 8", f/8 Newtonian, a 6" Richest-field telescope, and a 10" Gregorian (Figure 4), Ernest L. Coates, 6432 Perry Ave., St. Louis, Mo., built the attractive backyard observatory shown in Figure 5.

He made the patterns for the castings used in the mounting of the Gregorian and had the castings made and

The dome is mounted (Figure 6) on 18 rubber rollers and is noiseless when moving. There are six guide rollers to hold it in position. The slot opening is 30" wide. The drive (Figure 6) is of the friction type.

The dome is painted dull black inside (lampblack, alcohol, enough shellac for binder), and with aluminum outside.

PROFANITY is traditionally the chief by-product of making a pitch lap-at least during the first 10,000 attempts. New stunt (so far as is known) is offered by A. Rowland, 909 Easton Bldg., Oakland, Calif.

The two disks are warmed, the face of the mirror disk is given a thin coating of common vaseline, and the two disks are stood on edge, side by side, separated by the thickness of the desired lap, on a strip of heavy adhesive paper. Next, the paper is rolled around the disks and is stuck fast to them. Some kind of props can be used to hold the disks in place for the pouring, if desired. A pouring cup or funnel, of modeling clay, is set on top, over a hole punched in the paper, and the melted pitch is poured in till it reaches the top and ceases to settle. The whole unit is then set in a cool place for the pitch to settle fully.

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.

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Phone/fax toll free: 877-7SURPLUS (877-778-7758)
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