Re: [ATM] Problems With Central Hole Unchanged

[Mike Lockwood <melockwo@uiuc.edu>]

Bryan,

Sounds like you've gotten some good advice on how to fix the central 
hole.  A pitch contact problem is the likely cause.  Your pitch sounds 
like it's nice and soft, which is fine, and I'm confident that the 
hole will get fixed.  I will echo James Mulherin's advice about 
polishing pressure - you have to push down on the mirror/lap in order 
to get contact and get back to a sphere.  (It's not so true later on 
when you're figuring, and the mirror is no longer spherical.)

Anyway, my main point here is not how to remove the hole, but what 
caused the hole in the first place.  A central hole is created when 
there is too much action when the mirror is hanging over the edge of 
the lap (at the sides of the W-stroke, the parabolizing stroke).  More 
action is needed at other times during the W-stroke.  So, you end up 
doing a stroke with much less "action" (forward and backward strokes) 
when the mirror is overhanging lots, and more "action" when the mirror 
is going over the center of the lap or just an inch to the side.

Just how much more action you need to do when the mirror isn't 
overhanging much depends on the pitch hardness and your personal 
technique.  You just have to work for a while and test to see if 
you're getting the desired balance of center and edge correction.

This is a typical problem when learning figuring - a hole is dug in 
the center, and the outer zones never really gets corrected properly. 
  My advice is (after you get back to a decent sphere) to modify your 
technique and get the edge correcting first.  It's always easy to 
deepen the center later if necessary (as you have seen).  This is an 
important figuring lesson to learn - correcting the center is easy, 
correcting the edge is much more difficult.

	Mike Lockwood


Bryan Stiles wrote:
> Good Evening Everyone, I am hoping that someone out there can give me
> a hand. I am working on a 8" f/6 mirror. When I attempted to figure
> this mirror, I worked about 15-20 with a W stroke. Apparently my
> stroke was too long and this produced a nice hole in the middle of the
> mirror about 4.25" in diameter. I have worked to try to get this out
> for about 4-5 hours now and don't seem to be getting anywhere.
> 
> Here's what I have tried:
> 1. MOT 1/3 COC. This didn't really do anything that I could see.
> Quickly abandoned this method after about 30 minutes.
> 2. TOT 1/3 COC. This seemed to smooth the hole somewhat but not get rid of it.
> 3. Cut a paper disc the size of the hole and cold pressed for ~30 min.
> More TOT 1/3 COC.
> 
> The dang hole is still there. I have warm pressed, cold pressed, with
> mesh (microfacet), without mesh, more pressure, less pressure, thicker
> CeO and thinner CeO. I feel that I'm getting pretty good contact but
> I'm getting nowhere fast.
> 
> I am looking at my mirror on my Foucault Tester (Homebuilt Stellafane
> Type) with the razor edge and Ronchi grating. The blade is coming in
> from the left and I am assuming the light to come from the right (thus
> a hole not a hill). When using the Ronchi grating (100 lpi) the lines
> will hardly deflect when crossing the hole.
> 
> I am using soft burgundy and have rechanneled my lap 4 times now and
> am closing in on the fifth.
> 
> I am seriously considering going back to 500 grit and trying it that
> way. I have really enjoyed making this mirror and don't mind the extra
> work but I am losing my patience with this problem. Any helpful advice
> would be very much appreciated.
> 
> Thanks, Bryan
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> 


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