Re: [ATM] Not tde or tue, important oblate spheroid, ellipsoid

[Bern Kosicki <kosicki@verizon.net>]

ATM experts,

I'm working on a first mirror, and am in the final polishing/figuring stages.

I'm confused by directly contradictory statements made by two ATM people on 
achieving a sphere with the standard Tex stroke (this is defined by him as 
1/3D COC with sideways W of no more than 1/8D overhang on either side).

Tex says that doing this stroke will result in a sphere, and in fact that 
with a very slow mirror you can actually stop without even measuring it (I 
believe Dobson taught some mirror making classes years ago in which the 
students did not test their mirrors)

One ATM repeats the Tex statement,

But another says that Theory pushes the surface further into oblate territory.

Can you resolve this discrepancy for a novice?

Thanks,

Bernie Kosicki
Acton, MA

>At 2007-12-31 19:57 +1000, Thomas Janstrom wrote:
>
> >Your oblate spheroid can be corrected by doing 1/3 centre over centre
> >strokes, if you don't want to change the focal length alternate tool and
> >mirror being on top.
> >
> >This type of stroke will bring you back towards a sphere, so you have to
> >test and when you get back to a sphere (or if you're really lucky you might
> >be able to stop at a parabola) start working on getting the parabola back
> >again.

At 12:33 AM 1/1/2008, Jim Burrows wrote:

>1)  Theory says that any COC stroke, MOT or TOT, pushes the mirror
>surface further into oblate spheroid territory.
>
>2)  Assuming that's true, a parabolic surface is on the other (wrong)
>side of a sphere.
>
>I think that's why Tex chooses the 1/4 W stroke as his "standard" stroke.

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