JOHNSONIAN IS THE NAME SUGGESTED by this writer for a new type of Gregorian telescope devised by Lyle T. Johnson, a physicist amateur astronomer of La Plata, Md. Johnson gave much thought to the evil effects of the conventional secondary mirrors that are used in compound telescopes, and that are best adapted to observing the planets because they magnify highly. These secondary mirrors lie squarely in the path of the incoming light. Not only do they cut off some of it, but they cause a bending of the rays that graze their edges. This diffraction renders the images less distinct and reduces planetary contrast.

Seeking ways to reduce this obstruction, Johnson proposed to take the secondary mirror entirely outside of the tube, and leave in its place only a tiny diagonal flat mirror to reflect the rays to this secondary. His final arrangement is shown in the drawing below, where it will be seen that only one off-axis side of the secondary mirror is employed. In Johnson's first solution the center of the same mirror was used, but he quickly saw that the same rays would then pass the diagonal mirror twice, thus largely annulling the reduction in diffraction due to the reduced size of the diagonal mirror. He by-passed the diagonal by using an off-axis ellipsoidal mirror-that is, the side of a larger ellipsoidal-the remaining part being left nonfunctional.

"This telescope was designed primarily for planetary observing and has an equivalent focal length of 15 feet. The square skeleton tube is mainly of aluminum and the rest of the telescope is steel. For photographic use, a 40-pound war-surplus K-19 aerial camera with 13.5-inch focal length, f3.5 Eastman anastigmat lens, is attached to the same mounting. The camera is not an integral part of the telescope. When attached it shifts the tube's center of gravity toward the eyepiece, making it easier to reach.

"For a finder, a three-inch war-surplus objective is mounted in the corner of the tube. A war-surplus flat leads the light from this to a second flat, which may be slid into position in front of the eyepiece during use of the finder.

"The telescope pier is in the center of a 12-foot-square observing floor. An aluminum-covered housing protects the instrument when it is not in use.

"This type of telescope has a number of advantages. The secondary of the conventional 10-inch Cassegrainian or Gregorian telescope is usually about three inches in diameter, and the diagonal flat of a Newtonian about two inches. This large central obstruction causes extensive diffraction effects that impair definition. The modified Gregorian, however, has a central obstruction of less than 3/4-inch diameter, reducing diffraction effects and improving definition. The loss of light due to the central obstruction is .5 per cent, compared with 5 or 10 per cent in the usual reflector. This gain, however, is approximately canceled by the loss in the third reflection. Hence the over-all transmission of light is about the same as in a conventional two-mirror telescope.

"The long focal length is especially good for planetary observing, as it makes possible high powers with medium focal length eyepieces. The slender cone of light favors the eyepiece but may increase the conspicuousness of 'ghosts' in some Ramsden and Kellner eyepieces, so I use mostly orthoscopic eyepieces and a Hastings triplet. With these I get magnifications of 180, 250 and 300 diameters and, with a Kellner eyepiece, occasionally up to 490-but at the Newtonian focus, as low as 38 diameters with two-degree field: nearly a richest-field telescope. A stop may be placed at or very near the primary focus to reduce stray light entering the eyepiece.

"The telescope may be arranged to use the Newtonian focus without disturbing the Gregorian flat in any way. The Newtonian eyepiece is so close to the Gregorian eyepiece that the same slow-motion controls may be used at either position.

"A conventional Cassegrainian or Gregorian must have a high pier, to permit comfortable access to the eyepiece at the lower end of the tube. Such an arrangement puts the Newtonian eyepiece high up where it is difficult to reach. A modified Gregorian can use a lower pier, thus making both eyepieces more accessible. With my telescope I can observe any part of the sky without climbing more than 18 inches off the observing floor, and if I had built it with a rotating tube I would never have to climb at all. The lower pier also makes possible a smaller shelter or dome.

"Each mirror, primary and secondary, may be tested by itself. For the test there is no need for a large flat, and no possibility that errors in one mirror will mask those in another.

"The paraboloidal primary may be tested by the usual methods, but must be very accurate, as any errors in it will be magnified by the ellipsoid. It may be given a greater focal length than is possible with conventional Cassegrainians or Gregorians, making its testing and figuring easier, and also making it more suitable for use at the Newtonian focus. Paraboloids of medium focal length are easier to make than those of short focal length, and ellipsoids with small amplifying ratio are easier to make and test than those of higher amplifying ratio. It follows that modified Gregorians, with primary mirrors of f6 to f8 and secondaries of amplifying ratios from two to three, are much easier to make than conventional compound types, with primaries of f3 to f5 and ellipsoids of focal ratios from three to five."

These are the advantages of the Johnsonian.

THE six-inch ellipsoidal secondary proved to be much easier to make than the 10-inch paraboloid. It was to have a radius of curvature of 10.8 inches, or a speed of f9, and almost half an inch of glass had to be removed from the center. This was done by hand tools- glass caster cups of 2 3/8-inch, three-inch and four-inch diameter, used in that sequence. Grinding was concentrated in the center until a hollow of about the right curvature developed, and then the strokes were lengthened until this hollow spread to the edge of the disk. With No. 80 Carbo 22 1/2 hours were required, and five caster cups were worn out.

"When the fine grinding was finished the mirror was given an hour's polishing and tested at the center of curvature. It was found to be a sphere. With the pinhole moved to the F₁ position and the knife-edge to the F₂ position, the apparent figure resembled that of an oblate spheroid tested at the center of curvature. The difference in knife-edge settings from center to edge was about three inches.

"Rough figuring was done with emery, alternated with short spells of polishing to permit testing. When figuring a semipolished surface with emery it is easy to tell whether the tool is grinding in the right place, as the mirror loses its polish soonest in the zone where the grinding is greatest. It was necessary to return to emery six times before the surface was close enough to the desired ellipsoid to permit figuring by polishing. Most of the rough figuring was done with a three-inch tool.

"Ceria was used for most of the polishing, but it was found that the mirror was being scratched, so the job was finished with rouge. A soft, 2 1/2-inch lap was found best for polishing so radical a departure from a sphere.

"In testing the ellipsoid the pinhole was placed at F₁ and the knife-edge at F₂. The desired ellipsoid darkens uniformly and appears flat, no zonal measurements being necessary. The pinhole was in the end of a 1/2-inch-diameter cylinder and was illuminated by a flashlight bulb, with a piece of ground glass between bulb and pinhole. Pinholes were made in heavy foil and were easily interchangeable.

"This test setup had two drawbacks. First, the mirror wasn't evenly illuminated by the pinhole, making it difficult to distinguish the shadows near the edge of the mirror. Also, convection currents that rose from the lamp housing after it had been in use for a few minutes made short periods advisable in testing. Nevertheless, the ellipsoid was figured with this setup. These drawbacks could be overcome by using the method described in Amateur Telescope Making, page 369: a drop of mercury on the end of a stick.

"As a further test a .165-inch eyepiece was placed at F to examine the image. A number of pinholes were made in aluminum foil, and the smallest was selected. This was then partly closed by pushing the ragged edges back into the hole, leaving a very irregularly shaped hole with a length of .0015 inch. Details in the jagged edges of the hole, with dimensions of less than .0001 inch, could easily be seen. The theoretical resolving power of a 10-inch telescope is .45 second, which would be .000113 inch at the primary focus. Thus the ellipsoid takes full advantage of the resolving power of the primary. Any stretching or movement of the detail as the eyepiece is moved into or out of focus may indicate astigmatism. If the direction of such movement does not rotate with the mirror, the astigmatism is in the testing apparatus.

"In making any of these tests of the ellipsoid the apparatus had to be positioned very accurately. Luckily, the mirror was so close to F₂ that it could be moved with one hand while looking through the eyepiece. Coma becomes rather evident with any slight misalignment of the apparatus.

"The same apparatus was rearranged, as shown in the drawing in Figure 2, to test the small prism that was to be used, with its hypotenuse face toward the reflected cone of rays, as a diagonal. The image was examined with the eyepiece. The prism was arranged so that almost the entire ellipsoid was illuminated. The prism-face dimensions needed were 7/16 by 3/4 inch. The components were arranged almost as they are in the completed telescope, yet the sensitivity to error was much greater in the test, because the entire aperture instead of only an off-axis portion of the ellipsoid is used, and thus the light leaves the mirror in an f4 instead of an f18 cone. Nearly the entire surface of the prism is used to form the image of a single point, while in the telescope only a 1/12-inch-diameter area is used to form the same image. The high-power (.165-inch) eyepiece used in the test would give much too high a magnification on the telescope-1,070 diameters.

"The tiny diagonal reflects the light to a two-inch portion of the six-inch ellipsoid. Actually, the flat was placed half an inch beyond the focus of the primary, r since at the focus any dust on its surface would be sharply in focus at the eyepiece. Also, if the flat is placed inside . the primary focus the ellipsoid may be so proportioned as to bring the eyepiece closer to the tube. (If it is desired to make photographs at the prime focus, h the flat may be placed far enough outside the focus so that the film holder can be attached at the proper position without disturbing it. But with the flat outside the focus collimation would be more complicated.)

"As in any compound telescope, the optics must be held in rigid alignment and must be easily adjustable. For collimation, crosslines were scratched on the cell of the ellipsoid with their intersection over the center of the mirror. A tube 1 1/4-inch in diameter, with cross-hairs in one end and a peephole in the other, was placed in the eyepiece tube. The eyepiece mount was tilted until the cross hairs, made of thread, were lined up with the cross-scratches on the cell of the ellipsoid. This tube was then removed from the eyepiece tube and a ring with cross-hairs placed over the end of the eyepiece tube.

"A frame with cross-hairs was then clamped to the diagonal mount in such a way that the cross-hairs could be adjusted until they were exactly at the primary focus. This frame was adjusted until its cross-hairs were in line with the cross-scratches on the ellipsoid cell and the cross-hairs at the end of the eyepiece tube. The cross-hairs were also adjusted until they were 1/2 inch from the center of the diagonal.

"The eyepiece mount was then focused until the distance between the two sets of threads was 14 1/2 inches, which was the distance between F₁ and F₂ as determined when testing the ellipsoid. The ellipsoid was then adjusted, longitudinally and by tilting, until the thrice-magnified image of the first set of crosshairs was coincident with the cross-hairs at the end of the eyepiece tube.

"To determine whether the image and the second set of cross-hairs were in the same plane, they were tested for parallax by watching them while moving the head from side to side. If there is no apparent movement of the image with respect to the second set of cross-threads, they are in the same plane. If the image moves in the same direction as the eye, it is farther from the eye, and the ellipsoid must be moved closer to the primary focus. If the cross-hairs move in the same direction as the eye relative to the image, the image is between them and the eye, and the ellipsoid must be moved away from the primary focus. The distance the mirror must be moved is one third the distance between the second set of crosshairs and the image of the first set, so the test is very sensitive.

"With the ellipsoid properly adjusted, the diagonal could be adjusted. It was moved longitudinally and tilted until the reflection of the lower end of the telescope tube was properly centered when looking into the eyepiece tube.

"The 10-inch paraboloid was then attached to the tube and squared approximately. The ring with cross-hairs was removed from the eyepiece tube and an eyepiece inserted. The paraboloid was moved longitudinally until the primary focus was coincident with the cross-hairs. This may be done with great accuracy by focusing the telescope on a distant object and then moving the paraboloid until the cross-hairs and the image of the distant object are in the same plane. This was tested by the parallax method, just as when adjusting the ellipsoid, except that now an eyepiece was used. If the object image appears to be beyond the cross-hair image, the mirror is too far down the tube and must be moved up.

"The mirror was first adjusted on a house two miles distant, but when turned on a house four miles distant was found to require readjustment, so the final adjustment was made on the moon.

"The paraboloid was then accurately squared on, making use of a dot of paint in its exact center. The position of the image plane was again checked. The cross-hairs were removed from the diagonal mount, as the telescope was now in collimation and ready for use.

"The parallax method of adjusting the mirror was found to be so sensitive that the focal plane of the primary could actually be located precisely between the two crossing threads, which moved in opposite directions when the observer moved his head.

"A Newtonian of the conventional f8 ratio can be made more suitable for planetary observation by converting it to an f16 or f18 modified Gregorian. To convert a 10-inch telescope from f8 to f18 or longer, the ellipsoid would be made, for example, from a 4 1/2-inch Pyrex disk. The radius of curvature would be 8 3/4inches, sagitta 1/4 inch, p would be six inches and p' 13 1/2 inches. To get a field one inch in diameter at the eyepiece a .56-inch flat would be needed. If the Newtonian does not have a first-class paraboloid, however, it would be a waste of time to convert it.

"The modified Gregorian has a doubly inverted field. North is at the top and west at the left, just as on a map.

"The following formulas may be used for determining the mirror proportions when the small flat is inside of the prime focus. The formulas are not all exact, since some approximations were made in their derivation, but they are close enough for use.

"Diameter of field at secondary focus = I

"Diameter of field at prime focus = i = I/A

"Amplification factor, ellipsoid = A = p'/p

"Mirror diameter = M

"Focal length of primary = F

"Equivalent focal length = AF

"Distance, primary focus to flat = b

"Width of flat = E = [Mb + i(F - b)] /F

"Diameter of off-axis portion of ellipsoid used = D = [MP + i(F + p) ]/F

"Diameter of cone between off-axis part of ellipsoid and eyepiece at point where it crosses optical-axis = C = [(p' p - b) (D - I) / p'] + I

"If the small diagonal were outside of the prime focus, + b would be used in the formula for C, all other formulas remaining the same.

"Distance from center of ellipsoid to edge of off-axis area of ellipsoid = d = pp' [(C + E)/2 (p + b) (p' - p)]-D/2

"Minimum radius of blank needed to make ellipsoid = r = D + d

"Radius of curvature of ellipsoid = R = 2 p'p/(p' + p)

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