SCHMIDT wide-field camera-telescopes heretofore could not be used visually. Moreover, their adjustment has been a bugbear, requiring the patience of Job, because it had to be done photographically and piecemeal. In the following item, contributed by Russell W. Porter, these bugbears disappear: it describes a new Schmidt that may be used either visually or photographically.

"Dr. John Anderson," the executive officer in charge of the 200" telescope project, "has suggested a modification of the Schmidt camera," Porter writes, "which is shown in Figure 1. He thinks that this type, which includes the Schmidt principle, may appeal to amateur telescope makers, in that, by the additional reflecting surface, the focus of the camera is brought out into the open where it can be more easily collimated. Moreover, with the focus outside the camera box, the instrument may be used visually as a telescope.

"If an f/3 ratio is adopted, and the aperture is 8", then the two mirrors will be of 12" diameter, and the focal length of the instrument 24". Eyepieces of any e.f.l. may be used.

"By jack-knifing a Schmidt, as shown, it is set at 60°, which is about the angle required to prevent light that enters the correcting plate from striking the photographic film.

"The camera box could be put together with 1/4" and 1/2" plywood, the corners reinforced with small angle irons. It may be mounted in a fork and used as an alt-azimuth or equatorial.

"The cells of all the optical parts should have push-pull screws for adjustment."

FIRST to make the aplanatic reflector proposed in "Amateur Telescope Making-Advanced" by Franklin B. Wright, and by him modestly called the "short" telescope, though the name "Wright" telescope is suggested now, instead, is Robert T. Smith, night assistant at the Lick Observatory, Mt. Hamilton, California. Smith took astronomy at the University of California, made telescopes as an amateur (6" Newtonian, 6" Cassegrainian, 4" semi-RFT), then lost his pure amateur status by working at the Tinsley Laboratories where he made 200 eyepieces. For the past year he has been at Lick, where he wrestles with the large telescopes at night, and on his own time has worked on his Wright telescope in the basement of the dormitory where the astronomers sleep. "My little telescopes look insignificant," he says, "beside the big instruments I work with every night, but I still have TN blood in my veins and intend to push pieces of glass for some time to come." Smith's description follows:

"I have just completed an 8", f/4, flatfield 'short' telescope (Figure 2), as described by Mr. Franklin B. Wright in 'ATMA'. As far as Mr. Wright and I know, it is the first one to be completed to his specifications. Mr. Wright himself has one of a slightly modified form very nearly completed, the optical parts of which were made by Mr. Carl Wells, of Roseville, California. Mr. Wells had completed the oblate spheroidal mirror before I started mine, and I am indebted to him for several hints on the figuring of such a curve.

"The oblate spheroid is a very difficult figure to achieve. Since it is fourth degree curve, the slope be comes rapidly steeper as you go from center to edge, and such a curve is extremely difficult to figure with full-sized solid (pitch) lap. One first thought is that an inverted, rose convexing lap would produce the required curve, but, although it does produce the necessary high center the slope is greatest at the center stead of at the edge. In figuring the oblate spheroid I first obtained high center with a full diameter inverted rose with six petals removed. l then proceeded with laps ranging from 1" to 4" in diameter, all circular (that is, no star or rose-petaled shapes) These were all worked with tangent strokes with the mirror face up, with care that an even number of revolutions were made around the mirror If zones began to appear they were smoothed off with the 4" lap stroked radially, or at right angles to zone. Although this local figuring has a tendency to produce ring zones, great trouble is experienced if one proceeds slowly and cautiously. T mirror was figured until all zones approximated the error of observation.

"The correcting lens (Figure 3) was made of Pittsburgh's Crystal or 'Water Clear Plate,' 17/64" thick and 9" in diameter (the 9" being diaphramed to 8" in the camera, which is the usual practice with Schmidt correcting lenses). The index of refraction was determined to be 1.515 by measuring the difference in focus of a microscope with and without the glass. Both surfaces were tested against a sphere for flatness, and one surface was found to be flat within one wave, so I decided to work only one surface. It might be argued that one wave is not flat enough, but I may explain that the deviation from flat, curiously, was concentric with the edge and amounted to a uniform concavity of one wave or less, so that, as long as the correction on the other side was figured to take care of this, no harmful results could be expected. The side on which the correction was put had ridges several waves high, slightly curved, about 1/2 " wide and about 1/2" apart. These were apparent as dark bands across the lens when the lens was viewed in front of the mirror under test, when the knife-edge was parallel to the ridges. They undoubtedly were due to the large circular polisher used at the factory for polishing the glass after it is rolled, and they might well be looked out for. The glass was checked for striae and strains under polarized light and none was found, so work proceeded.

"Grinding and polishing were both done on mobile laps made of sponge rubber cut from ordinary kneeling pads. The grinding facets were cut from projection slide cover glass 1/16" thick, and were fastened to the rubber base with Goodrich Running Board Matting Cement, which proved very satisfactory and is recommended. The polishing laps went merely one step further, in that HCF squares were cemented to the glass facets. I have now given myself away in the mention of HCF, and must admit that it was used entirely for polishing and figuring both the mirror and the correcting lens. I realize that HCF is not approved of in the best circles [for final figuring.-Ed.], and I freely admit that it does not produce the satin-smooth surface that pitch does. But it does work glass faster and is much easier to handle. Therefore, since the tolerances in a photographic instrument are greater than in a visual, and because I am fundamentally lazy, I was induced to use HCF.

"The grinding of the correcting lens was done with American Optical's emery, a finer grind than usual being desirable in order to facilitate testing while still in the fine-ground stage. I found that the ring lap usually recommended for correcting lenses produced one deep zone rather than the necessary curvature. So the ring, which was of 1" square facets, was modified with 1/2" square facets placed by trial and error until the correction moved in the right direction. The first attempt at correction was made with M302, but this took away far too much glass and the surface had to be brought back to flat by grinding on a flat piece of plate glass. The correct depth of curve was roughly reached with M303 1/2 and was smoothed off with M304 and M305. With a surface finished with M305 a Ronchi test can be made through the lens placed in front of the mirror. Five minutes' polishing was sufficient to make a rough zonal knife-edge test to determine the approximate extent of the correction. The same diaphrams were used that served to test the mirror.

"The lens was brought to a polish with a full lap of 1" squares of HCF, with the lens face up and the lap on top, and with no backing to the sponge rubber. A lap with the rubber base cemented to a wooden backing tended to polish some parts more than others. The unbacked lap seemed better able to conform to the curvature without changing the correction. The final figure was obtained with small laps used in the same manner as on the mirror. However, these laps were mounted on rubber the same as the full diameter lap. The last two hours were spent in polishing with the tip of my index finger, since the zones to be corrected were so narrow.

"One curious thing was noticed in testing the correcting lens. Although the difference in radii between zones was the same for all diameters, the whole set of readings fluctuated up to ten-thousandths of an inch for different diameters. That is, the readings for one diameter varied up to ten-thousandths from those of any other diameter, although the difference between zones was right. Just what this fluctuation for different diameters really means regarding the shape of the lens, I don't know, but apparently it should be avoided, since it does have a small effect on some of the images. The effect on the images, fortunately, is not troublesome and is explained in a later paragraph.

"The appearance of the mirror under test is that of a paraboloid turned inside out, and because of the 3.2 focal ratio the shadows are very pronounced. A Ronchi test shows the grid to be 'knock-kneed' in contrast to the parabolic bowing, rather gently curved near the center and the curvature becoming more pronounced as the edge is approached. With the correcting lens placed in front of the mirror, the appearance is almost exactly reversed. The knife-edge shows the familiar parabolic doughnut, although it is not quite the usual form, and the Ronchi grid is bowed, gently near the center and strongly at the edge. All the tests on the correcting lens were zonal tests of radii of curvature, the readings being calculated from the formula on page 409 of 'ATMA.' I am indebted to Mr. Wright for his counsel and advice during the figuring of the mirror and lens. He was most helpful in converting his theory into actual practice. One should be thoroughly familiar with the theory behind this camera before attempting one, and he was most patient in explaining its details to me.

"The two optical parts are now mounted in a square wooden tube made of 3/4" five-ply wood, 13" wide and 44" long. The mirror is mounted in a machined brass cell held to the end plate of the tube with three clamp and three butt screws, which serve also as collimating screws. The correcting lens is mounted in a cell of plywood. The plate holder (Figure 4) clamps to one end of a piece of 3" brass tubing, 2" from the back of the lens (Figure 3). Focusing is done by means of the collimating screws that hold the lens cell in the tube. Thus, in focusing, both the correcting lens and plate move together. In the middle of the main tube, between the plate and the mirror, is a diaphram, in front of which is a hinged flap used for opening and closing the exposure. A trap door in the side allows one to reach the plate holder. The plate holder was made especially for the camera, and takes glass plates 2 1/2" square. The exposed area is circular and 2 3/8" in diameter, which is just over 4°.

"Collimation of the two optical parts cannot be stressed too strongly. The most important thing is to have the optical axis of the mirror pass precisely through the optical center of the correcting lens. I did not realize this until after I had taken innumerable plates and found plus sign images instead of round dots. I tried all manner of means to cure the plus sign images but with no results. I finally talked it over with Mr. Wright and he suggested that the collimation be adjusted even more accurately than I had done. I did, and found that the plus sign images disappeared over most of the plate, but remained to a slight degree in two small areas diametrically opposite on the plate. By rotating the correcting lens in its cell the two small areas are found to rotate by an equal amount. This apparently means that the correcting lens is possibly not a true figure of revolution, or that the unfigured surface is slightly cylindrical in shape. There is probably some connection between the fluctuations noticed in testing the lens and the two small areas of plus sign images. However, this condition does not seriously affect the performance of the camera, since the plus sign images are burned out with normal exposure. Accordingly, there seems to be no necessity for refiguring the lens. The images (Figure 5) produced by the Wright camera are very good over 4.2° field after the collimation, focus and tilt of the plate are accurately adjusted, and have been pronounced satisfactory for photometric work, which requires extremely good images.

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