I have read several explanations for the cause of Turned Edge, and how
to prevent and correct it. I am not able to express an opinion. However
I have had some experience with the problem.
THE CAUSE. Every mirror I made had a turned down edge and I believed
my method of polishing was the cause. All mirrors were polished at high
speed on a machine. The mirror was held face up in the bottom of a spinning
bowl which contained polishing compound. The lap was on top. A plough
directed the polishing fluid onto the edge of the lap. This I believed
caused the edge to be polished away Also a soft lap could have had something
to do with it also.
CORRECTION. It was necessary for me to work out how to correct this
problem. This is the method I devised. The Focault test diagram of the
spherical mirror with a turned down edge is illustrated in Fig. 1 The
mirror was then short stroked by hand to shalloy the curve to an oblate
spheroid. Diagram Fig.2 now shows how the mirror now appeared with the
turned edge. The mirror was now returned to the polishing machine. Using
the tips of the fingers. material was polished away as shown in Fig.3.
The Focault test showed it to be a spheroid with a hill in the centre.
The mirror was then long stroked by hand to remove the centre hill and
bring it o a spheroid. Fig4. Paraboloid stroking then brought it to
a Paraboloid with a perfect edge.
Today I have come to believe that the polishing compound directed on
the edge of the mirror was not the cause. Just too soft a lap. I now
polish mirrors in a bowl and immerced in polishing compound and have
no trouble. But I now use very hard laps.