When I make a polishing lap I want it to last a long time. With 6' f/8
and 8" f/6 mirrors use a plain channelled lap. I do not interfere with
it. Using different strokes I can control the curve. However when I
began on optical flats I found the same rules did not apply. 1 found
it necessary to reduce the facet size around the centre to flatten a
convcave surface. and cut the facets away around the outside to flatten
a convex surface. I found I was cutting them one way and the other with
the result the lap didn't last long.
I obtained a piece of .030" nylon shim and cut out an eight inch circle.
On this I drew a petal pattern and cut it out with a snarp nife. I now
an inner and an outer. Depening which was to be used It was placed on
the warmed polishing lap which had beed coated with polishing compound.
A weight was placed on top. This depressed the area covered with the
nylon piece. If the lap had to be changed to get the mirror surface
to polish the other way the other nylon piece was used. When neither
was required the lap was pressed with the flat. Using this proceedure
the lap will last for a very long time.