To: atm{at}shore.net
From: "Matthew L. Brown" 
Reply-To: "Matthew L. Brown" 



>
>A little update:
>  I rebuilt the lap with new pitch, to about 1/4" thickness, but chamfered
>it so the OD of the lap was very close to the OD of the mirror, about 1/16"
>under.  Hot press.  Visit with relatives. Cold press.
>
>Tool on top, +/- 1/4" strokes both fore/aft and side to side (W).  The edge
>begins to clean up, and a faint diffraction ring appears.  The hole in the
>center is still there, but is slowly widening and becoming more shallow.
>
>So I do more.  The edge starts turning up 1/4" inside the outer edge of the
>mirror, and the turned edge starts getting worse.  The diffraction ring
>disappears.
>
>So I decide to just clean up the hole in the middle and paint the TDE.  So
>I cut out an aluminum foil star about the dia of the hole, and press that
>between the lap and mirror, and start my hole recovery routine: 1/4" center
>over center (no W).
>
>And voila!  The edge sharpens up!  So I do more, and the edge sharpens
>more!  So I overdo it (by 5 minutes, I figure) and now I have a slight hill
>in the middle, a slight turned up edge, and a very, very narrow (like
>1/32") TDE outside that.
>
>Hot press, no aluminum star.  Cold press with netting.  The lap is almost
>the mirror
>diameter.  I check to make sure it isn't larger, and it isn't.
>
>OK. I know what to do about the hill in the middle.  I mark it's diameter
>on the back of the mirror, and MOT, I do long strokes making sure that that
>circle stays on the lap at all times (a W stroke that is narrow on left and
>right).  I work in 10 minute sessions, now using rouge (keeping my
>fingertips clean with a dip bowl and washing my hands with soap after each
>session, which seems to work).  Of course, I overdo it a little, and now I
>sit, with a slight
>hole in the center (about 0.15" shorter focal length than the middle) and a
>turned up edge (about 0.1" shorter focal length than the middle), and a
>barely detectable TDE.  The hole in the center is a little larger than
>the zone that the diagonal would shadow, so I think I should clean it up.
>And of course, the TUE is unacceptable.  The TDE is small enough for my
>taste.

This is the latest foucault.
http://host67.everstrive.com/~mlbrown/ASTRONOMY/Sphere20Mar03A.jpg
Although it looks like I still have a fair amount of TDE, I think that some
of this is due to the camera somehow.  I don't see it with my eye, and
Ronchi shows only a tiny bit.

>Now I am considering either:
>a) more short 1/4" TOT strokes, center over center, hoping that this will
>wear just about everywhere, and somehow magically bringing everything to
>the same sphere.
>b) trying to wear down the middle zone to match the turned up edge profile
>using medium length (+/-1") strokes.  However, I fear this will just make
>me zoney.
>
>Advice?  It is the closest to a complete sphere yet, but not 1/4 wave, I
>think (I haven't done accurate measurements).
>
>=Matt

--- BBBS/NT v4.00 MP