Re: [ATM] Which diameter gets worked for a given stroke direction?

[Ken Hunter <atm_ken_hunter@yahoo.com>]

Jay...

Just an educated guess here. I haven't actually
figured out astigmatism with this process.

With a well pressed full size tool....

On the long axis. Mirror on top, no pressure but the
weight of the glass, 30 seconds of "W" strokes
accenting parallel to the axis.

Followed by:

On the short axis. Tool on top, no pressure but the
weight of the tool, 30 seconds of "W" strokes
accenting parallel to the axis.

This should blend both curves together rather than
just digging a trench parallel to an axis.

Clean, stabilize and test to ascertain the remaining
astigmatism (if any) and repeat, adjust if necessary.

Ken Hunter




--- J Killea <jkillea@yahoo.com> wrote:

> My 8" f/6.4 appears to have rather different ROCs
> along 2 perpendicular axes, 2604mm vs 2606mm.  More
> disturbing is that "b" is about 1.3 for the long ROC
> axis and about 1.09 for the short (Strehl is .980
> for the short ROC, and 0.891 for the long).  So, I
> would like to get things a bit more even-keeled (
> using the average of both axes, of which I took 4
> measurement sets each) this mirror is at worst 1/6
> wave (spherical wavefront error).  
>  
> So if I want to work primarily on the one diameter's
> figure, I am still wondering which direction to
> stroke.  I can see either argument - stroking
> parallel to a given diameter changes it
> preferentially, or the other argument that stroking
> perpendicular to that diameter is the one that
> changes its ROC more.  Since we generally talk in
> terms of surfaces of revolution (which apparently
> this isn't) we usually don't actually consider what
> happens in just one stroke.  
>  
> The strokes I would use would be of even, probably
> light, pressure.  Clearly when bearing down on a
> local spot, that spot gets worked more, but that
> isn't what I plan to do.
>  
> If you decide to reply (much appreciated!), please
> use unambiguous language; I think words like
> "parallel" and "perpendicular" are narrower in
> meaning than words like "across" and "along" whose
> meaning can sometimes go either way.  Sorry for the
> pickiness - just don't want to pester folks with
> avoidable followup questions.
>  
> Thanks!
> Jay
> 
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