Re: [ATM] Tom's back - Parabolizing Plate Glass 12.5" x 0.73" f/5.5

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

> In response to your later message about a specially carved
> parabolizing lap, all I can say is it can definitely work, but you'd
> be off on your own working out your own procedures, which is not a bad
> thing, it's just not easy.  I've never used this type of lap on a
> parabola, but I have used "formatted" laps or petal laps on Cassegrain
> secondaries.


It's best if people learn the standard techniques first.

I've used parabolizing laps of 4 types.  Let me discuss 3 types at first.  I
shape the pitch based on Ronchigrams, of which there are three variations:
inside of radius of curvature such that the pitch is heavy towards the
center and lighter towards the edge, outside of radius of curvature such
that the pitch looks like a mirror image of the former, and finally at the
radius of curvature where the pitch looks like a mirror image of a donut.

The outside of RC in particular yielded beautiful surfaces because the edge
is under heavy work.  However, all three are rather difficult to control in
arriving at 100% parabolization across all zones at the same time.

Hence, my fourth type of parabolizing laps which I've used in my mirror
making classes.  This is actually a partial parabolizing lap built in the
inside of RC style mentioned above with the aim to help the student
parabolize more quickly.  This calls for a bit of trimming of the pitch
squares nearer the edge.

Mel Bartels


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