RE: [ATM] Dall Kirkham secondary update

["Rod Brackenridge" <rjbr@lycos.com>]

Thanks Scott,

I did what you suggested and the error does appear to be on the convex surface.  I tried 5 minutes with a standard 1/3 W stroke tool on top.  I think that helped a little.  When I press the edge and get 4 fringes, the outer two are near straight and the inner two have a bulge in the centre.  I will try another 5 minute session tomorrow and see if there is further progress.

You are right about the blanks when they are warm.  I warm pressed the mirror and lap tonight and the hill looked much worse when I tested soon after polishing.  Leaving it for an hour to cool made it look much smaller.

Rod.

> Rod:
> 
> If you make the test plate match with one piece off-center a bit with
> respect to the other and note the position of the zonal irregularities, you
> may be able to distinguish whether or not those zones are in the cx or the
> cc piece.  For example; make the match such that the center of the cx piece
> contacts just inside the edge of the cc, and note where the zone
> corresponding to the central hill seen in the centered match is.  If it
> stays put in the center, then you know that this zone is in the cc, but if
> it moves out to the edge, it is in the cx.
> 
> Make sure to clean off any dust particles etc before making the match, and
> don't slide the two disks across each other when in contact for any reason.
> (Scratches).  Also, I would advise waiting for any accumulated heat in one
> or both disks to dissipate fully before interpreting a match that is close
> to showing finished; pyrex test plate matches can show false astigmatism,
> power, and zonal irregularities when not at thermal equilibrium.
> 
> Scott Milligan

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