Re: ATM Hole in center

["Bill T." <twentiethwave@hotmail.com>]

> Bob Van Deusen
>
> I'm figuring my second 8" (this one is f/5, the first f/6) and am having
> a problem figuring. I keep digging a hole in the center. This has
> happened twice. After the first time, I went back to a sphere. I'm using
> the standard 'W' with mirror on top. I'm wondering what I could be doing
> to cause this. I'm also wondering if I need to go back to a sphere, or
> if I can get to a paraboliod from here. I'm using a ronchi grating and
> seeing the hold in the center.

The wide W stroke is intended to both dig out the center and flatten the
edge, which is the most efficient way to turn a sphere into a parabola.
Problem is, when the center of the mirror is over the edge of the lap, the
lap edge gets pressed down so fast that the edge of the mirror soon gets
tilted too far up to get good contact.  You get deepening at the center, but
nowhere near enough flattening at the edges.

For the W technique you must press the lap a lot during figuring, maybe one
minute figuring to 15 minutes of pressing.  Be sure you do at least one
orbit per each press to avoid astigmatism. The closer you get to the
parabola, the harder it is for the lap to maintain the correct shape.

Another useful technique is to first develop a hole with Mirror On Top using
classic W strokes, then go Tool On Top with vertical strokes mostly over the
center of the mirror, with very little side to side.  The long TOT strokes
will push down the edge, and may also fight turned edge development.  For
the centered TOT strokes on an 8" hang the lap as much as 2.5" to 3" over
the mirror edge.  MOT W strokes works on the center, long TOT centered
stokes work on the edge.  By alternating MOT and TOT in the right proportion
you just might get parabolized.  Try just a few minutes of each initially,
and monitor with the Ronchi and Foucault tests.  Use slow strokes.

Brad Davy's point about weight distribution is right on.  Try a 5 or 10 lb
weight-lifting weight on the back of whatever piece is on top.  Let the
weight apply the pressure, only use your fingers for guiding.  Put a (clean)
piece of felt or cloth underneath the weight for good pressure distribution.

Bill T.