AT first glance the telescope shown in Figure 1 might appear to have a Springfield mounting, a type invented by the late Russell W. Porter and named after his native Springfield, Vt. A closer inspection of the illustrations will reveal that the mounting is a simplification of the Springfield. To gain portability the Springfield's fixed eyepiece with its second reflection feature is omitted. The base casting is also-reduced to about four inches in diameter and made to overhang the pedestal.

This compromise mounting was devised and built by A. D. Johnson of the Minneapolis Astronomy Club, who explains that "the base overhangs the north side of the pedestal in order to permit following a star through the zenith without the tube's encountering the post. This avoids the wearisome necessity of shifting it to the other side, it is the old and familiar bent pier.

"Much of this mounting," Johnson continues, "has been copied from the Springfield, but the declination screw rides on the tube saddle where it is always handy. The castings were made in my basement. With a crucible, a bucket of molding sand, a little parting sand from a foundry supplier and a book from the public library, I went to work on the adjunct hobby of making patterns and molds and melting scrap aluminum in the furnace. Casting parts, rather than sawing and filing them out of solid stock, proved to be not only very convenient but also a lot of fun."

The technique of molding and casting telescope parts is described in Amateur Telescope Making–Advanced and in a chapter by Roger Hayward, the Pasadena architect, amateur telescope maker and illustrator of this department, in Procedures in Experimental Physics, by John Strong and others.

"The saddle that fits the telescope tube was cast hollow," says Johnson, "with the ends straight across, and these ends were then cut to fit the tube. The counterweight arm has seven-inch horns that were welded to each other and to their extension by a blacksmith."

Asked whether his patterns are available for loan to other amateurs, Johnson replied that "they are pretty well beat up, since at least 50 amateurs in and around the Twin Cities have had castings made from them.''

AN UNCOATED piece of flat glass could be used as the diagonal mirror in a telescope, but only five per cent of the light would reach the eyepiece. However, in observing one star–the sun –it becomes necessary to throw out nearly all of a great excess of light. The piece of glass would accomplish this purpose, but it would produce a second image of the sun, not so bright as the main image and displaced from it due to back reflection from the inside of the rear surface of the glass. To solve this problem the younger Herschel, Sir John, tilted the rear face about 10 degrees to throw this back reflection out of the visual field. The result is called the Herschel wedge.

"My Herschel wedge," Johnson writes, "has proved to be a useful instrument for observing sunspots, the moon ( except in its less dazzling first quarter) and Venus. The reduced light is easy on the eye, yet the reduction is accomplished without masking the outer parts of the mirror and reducing its resolving power or masking all except an outer ring and increasing diffraction. I altered the eyepiece adapter of my telescope to include a lateral outlet for insertion of the wedge unit (cutaway drawing in Figure 2 ) but found that on a refractor it was difficult in some positions to place the eye at the eyepiece." A better arrangement proved to be the one in the drawing in Figure 3.

Johnson has equipped both diagonals and wedges with holders having bayonet fastenings, and states with enthusiasm that these always go back to perfect adjustment when charged.

PROBABLY all amateur telescope users carry about with them, often for years, unsolved puzzles in the bottomless science of optics. They read articles and optical books. They ponder, study, experiment, discuss their problems with a fellow-sufferer, if they are fortunate enough to have one available. They read the same book 20 times. They pin down its language word by word and compare it with related fragments in other books. They wish the books would not merely state facts but would also point out which "facts" are false.

Yet persistently from time to time new insights arrive, often suddenly. Blank walls fall down, and one day that particular puzzle is put down as solved. Then is the time for the worker to write about it for the benefit of others-not years later when he has become expert and his recollection of the struggle bas blurred. This is what David Rosebrugh of Waterbury, Conn., past president of the American Association of Variable Star Observers, has done by request after three years of groping with the language of standard treatises–specifically Louis Bell's The Telescope and H. Dennis Taylor's The Adjustment and Testing of Telescope Objectives–on the subject of testing refractor objectives while testing three such lenses. If what Rosebrugh has written fails to impress the expert as containing anything not already in these books, it may be that the expert Unconsciously projects what he already knows into what he. reads. Anyway, in commenting on his own note Rosebrugh rightly states that "if I had had it when I started studying in preparation for testing my objectives I would have found the whole subject vastly easier than I did." The note follows:

"Full instructions for testing a refractor objective assembled in a telescope can be found in standard works and, for those who are used to testing mirrors and complete reflecting telescopes, the reasons for most of the standard tests made on refracting telescopes are immediately apparent. For example, the necessity of aligning the optical axis of the objective with the mechanical axis of the telescope is obvious. Beyond a certain point, however, the testing of an objective diverges from that of a mirror. Once the reasons for this divergence are clear the reflector man will have no trouble in understanding the testing of an objective. This note will therefore be devoted to clarifying the one fundamental difference between testing a refractor and a reflector.

"A natural question that arises in the mind of the reflector expert is, 'Why cannot one test-an objective by means of the Foucault knife-edge test?' The answer is that one could if one could secure parallel, monochromatic light (light of a single hue or wavelength).

"It is easy enough to secure parallel light, either from a reflector set up to act as a searchlight, or from a star, or by autocollimation with an optical flat. But it is difficult to secure monochromatic light, whether from a light source or from high-quality photographic filters. Light filters pass quite a mixture of light, as spectroscopic examination will show.

"Moreover, as an objective cannot be made fully achromatic like a mirror, even a test of an objective with parallel monochromatic light will not tell the full story, as it would with a paraboloidal mirror. In practice, for visual use, objectives are designed to focus the greenish-yellow light of about 5,500-6,000 Angstroms, which is what the eye sees best, but to throw away the red and blue rays, to which the eye is less sensitive, and which in any case cannot be focused if the greenish-yellow light is in focus.

"The method of throwing away the red and blue rays is to have them come to a focus farther away from the objective than the greenish-yellow rays. Thus if the telescope is correctly focused for greenish-yellow rays, the red and blue rays are thrown into the discard by being out of focus. In a well-designed f/15 refractor they will not come to a focus until the eyepiece is moved away from the objective a distance usually about 1/2,000 the local length of the objective. At this point the greenish-yellow rays in turn are entirely out of focus.

"If the blue light is thrown away to the same distance as the red light, the objective is considered fully corrected. However, the skilled makers have found that it is better to overcorrect an objective, which means focusing the blue light at a point even farther than the red light.

"The fact that a mirror brings rays of all colors to the same focus, i.e., is achromatic, while a visual objective brings only the greenish-yellow light to a usable focus, is the key to the one fundamental divergence between testing a mirror and an objective.

"In practice it is hardly worth while for the amateur to attempt to use the knife-edge test on an objective lens, though if a yellow-green filter is placed near the focus and Jupiter or a yellowish-white star is used as a source of parallel rays, one can make a knife-edge test on an objective which will show fairly well whether it dims equally all over, like a spherical mirror at center of curvature or a paraboloidal mirror when tested with parallel light. If the objective dims equally all over it is reasonably free from zones and has good spherical correction; that is, the rays that pass through its center focus at the same point as those that pass through the outer zone.

"However, a better test for zones and spherical correction is to look at a suitable star such as Polaris and use a high-power eyepiece covered with a yellow-green filter. Simply throw the image out of focus in both directions, and examine the resulting concentric rings by eye for irregularities that might indicate zones or spherical aberration. The interpretation is fully covered in standard works.

"This method of testing appears to be about as delicate as the knife-edge test is for a mirror, and on an objective it is more delicate because of the mixture of colors of varying focal length passed by even the best yellow-green photographic filter.

"This simple test still leaves the problem of whether the objective is properly corrected for color. The test for color correction can be made in the same manner for spherical correction and zones, but omitting the filter. Focus on the star and then rack the eyepiece away from the objective, that is, outside the greenish yellow focus. As the eyepiece is moved outward it is possible to detect the point at which the red rays come to a focus. This appears as a little red spot in the center of the out-of-focus image of Polaris. With less certainty. the point still farther outside the yellow-green focus at which the blue rays come to a focus may be detected. If it is not detectable its location can be deduced from the appearance of the blended blue and red light in the middle zone of the star's out of-focus image.

"The nature of these tests and the interpretation of what is seen are given in standard works, but the reasons why the out-of-focus eyepiece tests are better in general than any attempt to make knife-edge tests on an objective are .15 stated above.

"The tests suggested in standard works for detecting striae and possible internal strains in the glass of an objective are easily understood and require no elaboration here."

The comprehensive testing of an objective lens would require tests for spherical aberration, both axial and lateral, axial and lateral achromatism, astigmatism, coma and perfection of workmanship, and would best be done on an optical bench. Statement of criteria would be included, and the tester would need enough experience to judge what he sees in comparison with a nearly perfect objective. However, in the preceding note by Rosebrugh the practical aim has been to help the average isolated worker to make a fairly good test, using the resources that he has. It is also aimed at destroying the widespread belief that a flat shadowless cutoff in the autocollimation test is sufficient to attach a seal of quality to an objective lens, since this criterion applies to spherical aberration a]one.

FEW faults of a telescope mirror are so 1 injurious to image definition as the one called turned-down edge. Somehow during grinding or polishing a ring-shaped zone extending inward from the edge for a distance between a hair-width and almost an inch acquires a longer radius of curvature than the rest of the mirror. Enough has been written about detecting turned-down edge, or "TDE," but not enough about different methods of dealing with it.

An isolated amateur who appealed to this department for help in dealing with a compound case of TDE and overdeepened center was told that both ills might disappear if a hard lap were substituted for pitch that was possibly too soft. The appeal was then passed along to Albert H. Johns of Larchmont, N. Y., an advanced amateur and part-time professional, who added the following comments to the reply:

"Over a period of years I've seen hundreds of mirrors ultimately become first-class jobs simply through trial and error. There are so many variables that it is impossible to evaluate them one by one. For example you can not precisely describe the quality of your lap under the exact conditions of operation. Your trouble most probably is too soft a lap, but maybe we couldn't be more wrong. The best I can do is to give you an outline of the way I would go about correcting your troubles.

"The most important factor is the hardness of the lap. Remember that the correct hardness for one day may be way off on a colder or hotter day. I do not use fancy mixtures of pitch with oil, wax and turpentine but only reasonably soft pitch mixed with rosin to bring it to the desired hardness for a 70- to 75-degree temperature, two parts pitch to one part rosin. Extra rosin gives greater hardness. Use rouge or. cerium sparingly.

"Always keep the lap beveled to 1/8- to 3/8-inch less diameter than the mirror. In any rare instances when turned-up edge appears, allow the pitch to flow out to full diameter—but watch out lest the edge become turned down. Avoid short strokes such as you describe.

"All the above is for polishing. To correct the TDE lay waxed paper around the edges of the lap. Press for a few minutes in width slightly less than the TDE with mirror plus weights, remove the paper and calmly proceed to apply one-third strokes. Under no circumstances dunk or wash the mirror or tool after pressing and before working.

"Instead of using paper strips for depressing the edge of the lap one might scarify the edge with scratches, but these are too permanent and may cause turned-up edge through not filling in soon enough.

"Another way to arrange the paper for depressing the lap temporarily is to cut out a ring having the same internal diameter as the turned-down edge zone and serrate its inner edge with broad notches that leave teeth projecting from the mirror's edge into and just across the turned-down zone. Do not try to make these teeth uniform, since repetition may cause zones. Press this notched ring into the lap as described, remove it and go to work.

"The overdeepened center may be brought up with a paper star pressed into the center of the lap. In fact, the two zones, edge and center, may be treated as above simultaneously. After the zones have been raised, press for a longer period to eliminate the depressions in the pitch before resuming work.

"The most powerful method I have found for mirror correction in general is the zigzag stroke described for grinding by Everest in Amateur Telescope Making—Advanced, page 35, figure 30. This stroke blends out zones like nothing else on earth. By suitable distribution of the zigzag stroke one may continue right on up to full paraboloid, move the crest in or out as desired, and never be bothered with rings."

The editor of this department once learned a point by getting mad. After working interminably with tiny, fussy strokes to eliminate a hyperbola plus a TDE, a long deep scratch occurred. Since precision work would now not matter for many hours, the mirror was given an angry spell of longer-than-one-third strokes to teach it a lesson for getting itself scratched. Lo, the TDE and hyperbola both vanished. (P.S. The scratch didn't.)

Suppliers and Organizations

The Society for Amateur Scientists
5600 Post Road, #114-341
East Greenwich, RI 02818
Phone: 1-877-527-0382 voice/fax

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.

SURPLUS SHED
407 U.S. Route 222
Blandon, PA 19510 USA
Phone/fax : 610-926-9226
Phone/fax toll free: 877-7SURPLUS (877-778-7758)
E-Mail: surplushed@aol.com
Web Site: http://www.SurplusShed.com