Often, when we have telescope makers grinding and polishing ten-inch mirror blanks
on eight-inch tools, we run into a
problem. When the mirror makers
are fine-grinding and polishing with the mirror face up, and with the tool on top,
they are asked to put pressure on the right-hand edge of the tool when it is running just barely inside the right-hand
edge of the mirror face. But sometimes, for fear of running over the edge, either
they or their helpers shy too far away
from the edge and fine-grind
a trench about an inch or so in from the edge of the mirror blank . We call this a "scaredycat
Sometimes, when the mirror is partly polished, you can see the trench as a hazy ring, less polished than
the rest. But
sometimes you don't notice it till you are reading
the mirror curve in the telescope. There it shows as what we call
doily." When the glint of sunlight on the power pole insulator is thrown out of focus, inside of focus, the
edge of the out-of-focus disc may look okay. But on the other side of focus, the disc
will show a lovely doily pattern
around the edge. The light from
the trench will be spaced out beyond the main light from the mirror, but with a narrow line
of brightness outlining its outer edge. The pattern is reminiscent of a doily because the central portion
while the outer portion looks lacy with a bright edge.
Last year Jennifer
Moody's ten-incher had such a trench. It showed up as a hazy ring a little in from the edge, and about
an inch and a half wide, which refused to polish bright. And it was only when we read the curve of the
mirror in the
telescope that we saw the doily pattern. At that time
we hadn't figured out what had happened, so we sent her back to
But again she had a trench and doily. So she held for a year, and we still hadn't figured it out. This
year we figured it out.
This year we had several doilies, and we noticed that with a hand magnifier you can
see the fine-grinding pits in the
trench. That allowed us to see
how far back in fine-grinding the grinder had shied away from the edge. It takes only a
little fine-grinding, not out to the edge, to give you a doily. Watch carefully if you have someone help!
And be sure
that you yourself don't fine-grind too far from the edge!
This year we reground
Jennifer's mirror and finished without a trench.
If the defect is minor, it may be corrected by polishing with what we call the "forbidden
stroke," with the lap out over
the edge of the mirror, and with
pressure on that edge. This is an edge defect. It's no use attacking it elsewhere.
If the defect is major, go back to fine-grinding.