Re: ATM Am I done yet?

[Douglas Braun <doug@dougbraun.com>]

I am also figuring a 10 inch mirror.  According to Sixtests, I had it down to 0.993 Strehl and 6nm RMS surface
error (something like 1/40 wave P-V).  But a star test showed problems.  The cause was apparently a somewhat rough irregular figure, with some shallow but narrow "gouges" causes by over-zealous
use of a small figuring tool.  There was also a small but deep hole in the center, that was mostly but
not completely obstructed by the diagonal.

So despite the good looking numbers (which tended to change significantly if I re-tested with the mirror
in a different orientation), I decided to smooth out the mirror and re-figure it.

In a nutshell:  A Foucault test only looks at a small percentage of the surface.  If the
overall figure is not smooth, the numbers may be optimistic.

Also: GO SLOW when you are "almost there".  My mistake was to be too aggressive
in the final stages of figuring...

Doug Braun


At 11:56 AM 9/17/02 -1000, you wrote:

>A couple of days ago I started to figure my 10 inch to a parabola. (again..)
>
>After the first session I was at about 1/2 wave.
>So I did some work twice around the barrel and took 2 more readings.
>
>Suddenly I am at ~ 1/6 wave PV.
>Sixtests shows strehl of .94
>
>I often here people talk about 1/8 wave being their target or goal.  I was surprised to see .94 strehl with a 1/6 wave pv.
>
>So.. I am at the cross-roads now,  should I leave it as-is or work it a little more... I know it will be my decision in the end..