The task was to make a wedge in silicon 2.362 in. long × 0.392 in. wide and only 0.032 in. thick. This was accomplished on an opposed face polishing machine whose laps rotate in opposite directions. This precludes putting the pressure all on one side as would be done if the laps rotated in the same direction. This feature has enabled very thin, highly accurate substrates to be made. It is of special value when working with friable material such as calcium fluoride. The septum must be thinner than the work and without wrinkles. By stretching the thin Mylar these wrinkles are avoided. The procedure is explained in the illustrations. Note that the usual method of planetary lapping always pushes on one edge of the crystal, which is acceptable when polishing rigid thick material. When the laps rotate in opposite directions the polishing forces are divided equally between top and bottom surfaces. Readers are also referred to our column “ About Flats and Parallels” Appl. Opt. 5, No. 7 (July 1966).
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