Testing for Astigmatism
In testing for astigmatism, the image of the pinhole should be returned as closely as possible to the source, and in the same plane.
This avoids the possibility of any erroneous shadow appearances. If the knife-edge is vertical, any shadow movement at any setting should be in a horizontal direction; that is, if the mirror is considered to be the face of a clock, shadows should approach from the directions of 3 o'clock and 9 o'clock only.
Starting with the knife-edge either well inside or well outside of focus, and shifting it gradually and smoothly to a similar distance on the opposite side of focus, observe carefully for shadow approach from any other "o'clock." All shadow movements should be symmetrical about the horizontal diameter. Test on three diameters.
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