> Alan Kilian wrote :
>
> > I took four zone readings, and went upstairs to fight it out with
> > the Texereau book.
> >
> > About 90 minutes later, I had computed the billion values in his
> > spreadsheet like form, and the result is...
>
Allan,
Why not use the five zone program on this lists FTP site and save yourself heaps of calculations. Just type in your knife values and choose a value for coefficient D which gives you a minimum value for the wavefront error. You can see just where to accent your polishing too.. ( I think it is called foucault.zip )
> This suggests that you might not doing your calculations well. A spherical
> 8" f/7 has an error of approx 1/4 wave on the surface (or 1/2 wave on
> the front).
>
> Bratislav
>
For the uninitiated, you must remove 1 wave of glass from the centre if you are leaving the edge untouched. A spherical 8" mirror at F7 will have 2 waves on the wavefront ( between the edge and centre rays ) which can be minimised to 0.5 wave by slight refocussing; that is effectively choosing your focal point at the 70% zone.
Mark