Re: [ATM] figuring an ultra-thin 16" mirror

["Mel Bartels" <mbartels@bbastrodesigns.com>]

> >I have tried to figure it using the "classic" method (at least in
France):
> >full size tool, mirror on TOP, long wide strokes. But I could never
achieved
> >more than 8mm of difference of radius between inner and outer zones,
while I
> >need 11mm. The mirror may be too light, or too flexible.


You can parabolize using full sized tool, but it is difficult and taxing.
It will take many hours.  Contrast to a 6 or 8 inch where parabolization can
be put in 5 minutes.   You need to widen and lengthen your strokes to the
maximum, such that the mirror is ready to tip over the edge at the top and
bottom of every single stroke.  Then, after you put in the parabolization,
getting it equally distributed will be very challenging with the full sized
lap, though at least the surface will likely be very smooth, without the
subdiameter roughness that is typical.

Easier is to make a parabolizing lap.  Cut back the pitch increasingly
towards the edge so that the greatest concentration of pitch is in the
middle and the pitch squares near the perimeter are maybe 1/2 in dimension,
1/4 in area, compared to their former selves.  Do 1/2 long wide W strokes.
It should parabolize quickly.  You can control parabolization by varying the
length and width of the strokes.  Without the parabolizing lap, an ordinary
lap will strongly force the mirror back to spherical.  You want a lap that
will maintain parabolization while you adjust the individual zones.

Try to increase the parabolization evenly, that is, when the mirror is 50%
parabolized, make every zone 50% parabolized, and so forth.  That way you
can ease yourself into 80-90-95-97-100 percent parabolization and have a
good overall figure.

Let the mirror sit for a couple of hours before final testing.  The figure
can become fairly overcorrected if you test too early.

Mel Bartels

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