PROPOSALS FOR SEVERAL interesting varieties of modified Gregorian telescopes, offered to other advanced members of the amateur telescope making fraternity by Lyle T. Johnson, Box 236, La Plata, Md., are contained in the following extracts and abstracts from several of his letters.

"As I was reading" Pickering's chapter, 'Reflectors versus Refractors,' in 'A.T.M.,' his statement, page 607, about the extensive diffraction effects due to the large size of the secondary mirrors in Cassegrain and Gregorian telescopes, led me to wonder whether some way couldn't be found to reduce these effects. I have figured out two ways of doing this for the Gregorian telescope. In the best of these (Figure 1) the small mirror is placed at the focal point of the primary, and its diameter is equal to that of the field desired at, the eyepiece divided by the amplifying power of the secondary.

"In the layout shown the elliptical flat (1/2" minor axis) will give a field of 2" diameter at the eyepiece. The cone of light (from a single star), reflected from the secondary to the eyepiece, has a diameter at the flat of 3", or six times the diameter of the flat. This means that the small flat will give the same diffraction effects and light obstruction as a 2" mirror in front of the 12" primary. I will call the diameter of this imaginary 2" mirror the 'equivalent diameter' of the flat. As the secondary mirror of the conventional 12" Gregorian would be 3-1/2" or 4" in diameter the modified Gregorian should give considerably better definition.

"As the secondary of the modified Gregorian is not in front of the primary mirror, it may be made as large as desired. That is, since there is no necessity of cutting the mirror to exact size, Pyrex blanks can be used just as they come. Also, turned edge would do no harm, whereas a slight turned edge on a conventional Cass or Greg is disastrous to definition.

"No perforation of the primary is necessary. Because the light beam doesn't have to pass through the hole, correct design would make a larger field available for photography.

"This telescope would have a rather and wider than it is long. It probably would be possible, however, to design a mounting with the light passing from the secondary through a hollow declination axis to the eyepiece at an easily accessible point.

"In the conventional Gregorian the minimum value of amplification of the secondary is about 5, but in the modified type it could be made smaller if desired.

"Although the diagonal flat is extremely small, the more advanced amateurs probably wouldn't have too much difficulty with its construction.

"As the eyepiece is at one focus of the ellipsoidal secondary, and the small flat at the other focus any dust on the surface of the flat would be sharply in focus at the eyepiece. This bad effect might be reduced or eliminated by moving the flat a little farther away from the primary. This would cut down the diameter of the field of view unless the flat were made larger- which in turn would increase the diffraction effects and light loss. The flat probably could be shifted far enough from the primary focus to throw the specks of dust out of focus without reducing the field of view too much.

"As the two beams of light cross at the position of the flat the construction of the diagonal supports is more complicated than in the Newtonian.

"An alternative method of reducing the diameter of the obstructing mirror, which was considered, required a larger flat placed between the primary and its focus. This would reflect the light from the primary to the secondary at the side of the tube and then back through the hole in the primary to the eyepiece. It, however, was seen to give only a slight reduction in the diameter of the central obstruction, hence not much would be gained. It also has the disadvantage of four reflections instead of three, and the primary must be perforated.

"The modified Greg eliminates one of the disadvantages of many Gregorians-direct light from the sky striking the eyepiece.

"Design data:

"Primary, 1/2" f/3.

"Secondary, 5" or 6" diameter.

"Equivalent focal length, 144".

"Diameter, d, of secondary necessary for rays parallel to axis is given by d/12 = 12/36, therefore d = 4".

"Diameter, D, of cone of light reflected from 4" secondary is given by D/36 = d/48, or 3".

"If equivalent diameter of flat is E, and primary mirror diameter is M, then E/M = 1/2D and E = M/2D, or 2".

"Useful diameter of field at secondary focus = 1/2" times amplification, or 2".

"This idea of mine may not be new.

"If the mounting for the modified Gregorian should prove too difficult it would be possible to make the tube more like that of the conventional telescope, at the expense of an extra reflection (Figures 2 and 3).

"Figure 4 shows the small diagonal support, which presents a minimum of obstruction to the light traveling from the concave secondary to the eyepiece. If the obstruction of light before it gets to the primary is greater than the diameter of the flat, this makes no difference as long as it is smaller than the 'equivalent diameter' of the flat. Thus, in Figure 1, the obstruction at the center of the spider could be up to 2" in diameter without obstructing more light than the flat does.

"Later, while 'pushing glass,' I thought of another modification (Figure 5) which further reduces the bad effects due to the diffraction around the secondary. Its secondary is an off-axis ellipsoid. The beam of light misses the diagonal entirely. Obstruction by the small diagonal occurs only before the light reaches the primary and. if the diagonal has a diameter of a half inch or so, the obstruction is insignificant. In fact, the diffraction due to the spider would be more serious than that due to the diagonal itself. This telescope should approach the performance of a refractor of equal aperture, more closely than any other type of reflector.

"Another advantage of this general type of Gregorian [shall it be called the crossed, or Johnson, type?-Ed.] is that there is a certain amount of interchangeability of the secondaries, so that one off-axis ellipsoid could be used as the secondary for almost any paraboloid of approximately a given diameter. On the contrary, the secondary of the conventional Cassegrain or Gregorian must be made especially for that particular diameter and focal length of primary. This quality could be made use of by a person who desired to build a second Gregorian a few inches larger than the first.

"Figuring an 'off-axis' ellipsoid would be difficult-just how difficult I don't know. The fellows who have been designing [Schell.-Ed.] and making [Beede of Youngstown, Ohio.-Ed.] off-axis paraboloids could perhaps shed a little light on the subject. It perhaps would be best to figure the mirror spherical and then arrange the pinhole mirror and knife-edge in the same positions as the primary focus which mirror and eyepiece would occupy in the telescope. Then one could see what the mirror looked like, and figure accordingly. If things permit, I may make one of these telescopes but, if you publish this, someone else may come up with some better ideas on the subject in the meantime."

Johnson's letters were shown to Porter, who commented: "That's the way -try it out and find the bugs in it, if any."

Johnson next performed an experiment to throw light on figuring the off-axis ellipsoid (Figure 5) with small polishers but the equations pertaining to it would be too detailed to publish here. Especially interested readers may borrow them, or should write to Johnson direct.

All of Johnson's data were next submitted to Norbert J. Schell, 1019 Third Avenue, Beaver Falls, Pennsylvania, one of the amateur group (Beaver County Amateur Astronomers Association) 25 miles north-west of Pittsburgh, near Ohio's border. Schell is the originator of the off-side, or unobstructed, reflector (this column, April, 1939), also of the off-axis reflector (May, 1940) and is a widely known advanced amateur designer. His invited opinions, comments:

"So far as I know, this is a new way of getting away from most of the obstruction in a compound reflector. The only other similar effort to reduce the effective size of the obstruction was that mentioned by Dall, ('A.T.M.A.,' page 584)

"Johnson's method is sufficiently different, using a Gregorian set-up and all reflecting surfaces, to give him credit for doping out something new.

"Of his designs I rather prefer the on-axis type as being somewhat more fool-proof, but the off-axis type would, or rather could, be made to get away from the somewhat awkward arrangement of the on-axis.

"The off-axis type, like all other off-axis arrangements, would result in a restricted field, as the field will be the same as the field resulting from the use of the larger imaginary surface, a side section of which is represented by the off-axis surface in use.

"The off-axis type could be designed with the secondary closer to the small flat than with the on-axis, and in this way would bring the secondary focal plane closer to the tube.

"It seems to me that, since the off-axis design leaves only the obstruction of the small flat, it n1ight be better with it to make the primary an f/5, even f/6, which would still leave a flat rather small-say an inch or so and reduce the amplification factor of the secondary accordingly. This, with moving the secondary closer to the flat would make it possible to bring the secondary focal plane about a foot or so outside the tube.

"There is another reason why I would suggest that the secondary be small. It would not be necessary to make and figure it as a section, but it could quite easily be made full size of the imaginary secondary, and thus figured symmetrically in the regular way. This large secondary would, in use, be placed so that the proper off-axis part of it came into action. I would recommend this method over either making the section only, or making several sections en-bloc. I have tried both and I am satisfied that by this method surface accuracy is more readily obtained and this is well worth the use of the over-sized surface. I would not recommend this for very large surfaces or for off-axis primaries as such, but for secondaries, by all means.

"As to the efficiency of Johnson's Gregorian, I think it should work out in the regular off-axis form at least as well as a first-class Newtonian with small flat, such as is used for higher powers; and in the off-axis form, I think it will surprise those who have had no previous experience with reflectors working with little or no central obstruction.

"There is another angle to the off-axis form, which may be found to be an improvement. To wit: Obstructions are usually out in front of the primary almost as far as the focus, and the resulting diffraction is without question worse than if there were a similar size of obstruction at the mirror surface (or at or near an object glass). In Dall's experiment to bring out effect of various sized obstructions, fro which he came to the conclusion that a 1/5-sized obstruction is as large as could be used without causing to much trouble, I don't think he brought this double distance into effect. At least he did not displace the obstruction relative to the diaphragms used. In this on-axis Gregorian of Johnson's effective obstruction is only 3/4 the distance from the focal plane to the secondary-a much shorter path for diffraction to spread out. This might b found to produce only the diffraction effect of a considerably smaller obstruction at the usual location. This is one of the reasons I prefer the on-axis design in this case-the other being the matter of field, previously mentioned.

"The job of figuring an f/3 primary must also be given consideration.

"Some trouble must be expected from maintaining good smooth coating on the small flat-and of course this must be a really good flat.

"I hope you will mention Dall in this as his scheme, referred to above, always stuck in my mind as a star toward the ideal way of reducing obstruction of a symmetrical, or off-axis, mirror."

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