OF THE THREE ways to make optical flats, briefly outlined by Porter in "Amateur Telescope Making," page 57, the first, or three-disk method, is described at greater length by Selby in "Amateur Telescope Making-Advanced;" the second, or Ritchey, method using concave mirror and knife-edge accessories is described by Pierce and Ritchey in "A.T.M.," page 241; but the third, or borrow-a-master-flat method, is not covered in either book.

"Why keep this third method in the optical doghouse?" a few have asked, "What do you hold against it?" The method is excellent-provided the user can borrow the necessary master flat. John W. Erdman, 50 Howland Ave., North Hackensack, N. J., an amateur astronomer and telescope maker who has worked several years as a professional precision optician, now offers the rest of the fraternity the full recipe for method 3. He writes:

First, procure a flat as your testing piece-by the "beg, borrow, or steal" method if necessary. Even one which may not be absolutely flat will be of great help, but in such case some other final test would be necessary.

Use Pyrex; it is more dependable for maintaining surface than plate glass, is less likely to spring out of shape if the glass is to be blocked on either wood or metal, and is less affected by heat generated from polishing.

Choose the face having the lesser diameter for the optical flat side. The taper then will tend to prevent, rather than facilitate, accidentally dropping the disk when the hands are wet. Hence, grind the larger face first. Proceed on that side with the usual grades of emery, grinding against any suitable flat surface. A grinding tool slightly larger than the disk, roughly one tenth larger, may be used to advantage, as will be shown later. A tool made with facets is a great help in eliminating zones. Those having a lathe can make a good grinder by cutting a series of concentric rings in a metal tool.

The back (larger) face may be polished quickly and needs no figuring. All we require is clean polish and reasonable flatness. A wavy or irregular surface might, however, introduce incorrect interpretations of the interference fringes on the other side.

Now for the all-important surface on the smaller side.

Some form of monochromatic light must be provided. A fluorescent lamp may serve in the early stages of polishing, but is not suitable for the final testing.

The grinding of the second face proceeds as before, except that with each grind of emery the surface must be brought closer to flatness, testing with a straightedge and frequently inverting the tool, whether it be glass or metal, to maintain this flatness. If a straightedge is not available, the borrowed test flat will serve if moistened (for this there is nothing better than the tongue) and pressed into close contact with the surface. The moisture spreading between the glasses will show even minute deviations. The straightedge can be checked by carefully placing it on the test flat.

In the final stage of grinding, great care is necessary in bringing the surface as nearly flat as possible. Here moisture from the breath is sufficient in the test against the master flat. Better still is testing the fine grind by interference. If this grind is brought to a satiny, glossy finish, fringes may be seen by placing the two glasses in contact and viewing them at 18" to 24" from a very low and slanting angle. A fluorescent light will not serve for this test. While the view from this angle is not exactly correct, it is close enough at this stage. Move the test glass off center and study the fringes clear to the edge. Often they appear quite straight over most of the ground glass, but break off toward the edge. Continue grinding until this condition is eliminated.

Close attention must be paid to the stroke in grinding, so that if small errors are to be corrected, the least amount of effort will be required. The same type of strokes are used as in producing a spherical surface. A flat must be considered, both in grinding and in polishing, as a spherical surface. If it appears concave, it is treated exactly as a concave surface which is too deep. If it appears convex, it is treated as a convex surface which is too steep.

The work may have to be turned from bottom to top several times, before completing the grind. Never, however, attempt to polish until the interference fringes show that the surface is quite flat, for it is much simpler to grind out imperfections than to polish them out. Watch particularly for concave zones near the center, and for turned down edge. These cannot be tolerated. One wave regular curvature is acceptable 1/2 wave, or 1 ring, is still better. If the rougher grinds were kept reasonably flat, it should not be difficult to bring the fine grind this close. With the borrowed master flat as guide, making small changes in stroke, this can be attained without too much difficulty. Again it must be stressed, do not polish until the fringes show not more than two rings of regular curvature.

The first job connected with polishing is making a lap. I sometimes have found advantage in using a lap slightly larger diameter than the disk (approximately 1/2" larger for 6" work). If a slightly oversized grinding tool is used, it will do nicely in making the lap. One advantage in polished flat world is that many types of materials, only reasonably flat, will serve as backing for the lap. Thick plate glass, properly supported, will do. Often, in making a flat, a full-sized lap can be used in polishing on top, and the oversized lap on the bottom when the disk is to be inverted. Here a great deal is left to the worker and to the intricacies of each particular job.

Use a pitch somewhat on the hard side, but not too hard. Good work can be produced on pitch of varying hardness, but nothing but new difficulties will arise from using pitch harder than necessary. Keeping full contact with the glass is absolutely necessary. Pitch that is too hard often makes this difficult.

Build a small dam around the warm tool (ordinary masking tape serves nicely for this) and pour on a thin layer of pitch. A little less than 1/4" is about right for 6" work. As soon as possible, remove the dam with short, quick pulls. Raised edges of the pitch may be chopped off with a razor blade.

When the pitch begins to harden, yet is still quite warm, smear on rouge having a consistency on the pasty side, and carefully rub the lap to shape with the fine-ground flat. Rub lightly at first, and add pressure as the lap takes shape, using short strokes and small circles. Do not allow too much heat to get into the glass; remove it from time to time to cool. Continue this until the pitch is fairly cool, having assured yourself that the lap has come to full contact all over.

With a little warmth still left in the lap, clean off all the rouge and scrape small channels into the pitch with the corner of a razor blade. They should be cut clean across the lap with one quick stroke of the hand, and about 1/32" deep. Five or six strokes in one direction are crossed with as many in one or two directions, until a reasonably regular criss-cross pattern is achieved. It is not, however, necessary to space these scratches carefully, but leave no one zone untouched.

New contact must now be made with the lap, until an even, steady pull with the stroke is felt.

When the lap is fully cool, actual polishing may begin. Within a few minutes, the worker can note whether his flat has begun to polish with reasonable evenness. A spotty condition in the polish will indicate that the lap is out of contact, and re-heating is necessary. The easiest method to reheat a lap is to invert it in a pan of warm water for a minute or two cold pressing between spells is not recommended with this type of lap. Just warm it a little before the start. If an oversized lap is being used, it should remain on the bottom, even the first test shows that the work is concave. Simply proceed with short strokes for a polishing spell, and study the test piece what changes occur is first test indicates that the work is convex, proceed with longer strokes. These strokes must vary with your own judgement, while watching the response with the borrowed flat. There is no need for waiting long to test at this stage, small variations are not yet important. It is well, however, to keep in mind that heat generated from polishing will give you a reading on the convex side of the true condition. For example, if the first reading should indicate absolute flatness, the work will nearly always be at least one ring concave. If it appeared at first a little concave, it will ultimately cool off even more so. At this point we are concerned mostly with an even polish, keeping the surface as flat as possible and free from zones. If a machine is used, a slow, steady stroke should be selected.

The scratches on the lap should never be permitted to close in entirely but should be removed as soon as the worker sees that they are becoming filled. They are purposely made shallow, so that they may be changed or modified to meet the irregularities or conditions which may develop in the surface of the flat. For example, if a small hole should begin to appear near the center, new scratches can be cut in the lap at the corresponding spot. As this hole begins to disappear, these scratches need not be renewed, and will soon flow back into the lap. If the edge of the glass appears to be turned up, the scratches may be deepened more in the central section of the lap and temporarily not renewed at the edge. All normal deviations may be dealt with in this manner. However, no large zone should ever be left without any scratches at all, else serious irregularities may develop. Generally, when certain corrective scratches are made, enough former ones remain in other zones to suffice. In this fashion I have produced several flats, free from irregularities, with a single oversized lap used entirely on the bottom.

Do not be afraid that the oversized will develop a turned edge. Reasonable care with the stroke will prevent this. In fact, an undersized lap will more quickly result in a turned-up edge. In mirror making, where infinitesimal deviations of radius have no importance, a turned-up edge is easily dealt with; while in flat making, the radius being infinity and necessarily kept so, treating a turned-up edge will often produce an undesired radius in work. Frequently a finished flat, quite satisfactory as far as edge condition is concerned, will show from the fringe reading a faint blur in the outer 1/64". Since this amount will be taken with a bevel, it should not concern worker, as this sharp edge must be removed at any rate.

When an equi-sized lap is used throughout, the worker will naturally polish face up or face down, as the curve of his surface demands. Do not always, when it is found that a fairly long stroke has changed the reading from concave to convex, or vice versa, be too eager to invert the work, but try first a shorter stroke.

As in grinding, close attention must be paid to the stroke used, in order that small changes can readily be dealt with by the necessary alteration in stroke.

As each job has new characteristics, these must be dealt with through the ingenuity of the worker. Sometimes when a hole persists in the central zone, and continual renewal of the scratches fails to correct it, a hole may be cut in the center of the lap, beginning with a small one and, if necessary, enlarging it. If a turned edge should persist, cut small scallops around the edge of the lap.

By following these methods the worker should have no difficulty in making a good flat; especially through bringing the fine grind as nearly flat as possible and keeping it that way throughout.

It is well to move slowly until you are sure of yourself. Usually, time is saved in the end.

In the final stages allow from 20 to 40 minutes for cooling-depending upon the length of the polishing spell-for a good test.

A flat can be called really good if the fringes, when separated about 1-l/2'', are straight clear to the edge. Rotate the flat and view the fringes in several directions; a minute cylindrical condition will not necessarily reveal itself from a single axis.

If the flat is to be used solely as a testing piece or master flat, polishing need continue only until naked-eye evidence of the final grinding is removed. Small pits will in no way lessen its value.

In making the plane side of a lens, the same procedure may be followed. Usually one ring of regular curvature is acceptable, hence such a surface is a great deal easier to produce than the master flat. Turned edges of small degree may be ignored, since the centering of the lens will remove them. Prism faces are naturally more convenient to polish on top, and an oversized lap should always be used. The lap should have a circumference equal to or slightly larger than the longest dimension of the prism face. When a number of prisms or lenses are set, or blocked, into plaster in circular form, a lap of the same size may be used, and the work may be treated exactly as if it were a solid surface.

WORK LOVING beavers are not extinct in Beaver County, Pa., about 25 miles north-west of Pittsburgh. "The Beaver County Amateur Astronomers' Association" has just been organized there, William A. Lintz, 440 Navigation St., Beaver, Pa., being the secretary. In this organization are 13 members, a way of showing that astronomers snap their fingers at superstition. N. J. Schell, of off-axis (unobstructed) and criss-cross off-axis telescope fame, also Roelof Weertman whose 12-1/2'' turret telescope has been described in this department, are members of this new group. Paul McConnell, of Beaver, is its president and Keith Shields of Fair Oaks its vice president.

The Society for Amateur Scientists (SAS) is a nonprofit research and educational organization dedicated to helping people enrich their lives by following their passion to take part in scientific adventures of all kinds.