Re: [ATM] Problems With Central Hole Unchanged

[Dale Eason <atmpob@yahoo.com>]

This will not answer your central question however I
think it is time to start making Foucault measurments
and using the data reduction programs.  Then you will
know if it is indeed a hole or a hill for sure.

If the ronchii lines are straight then that area is a
sphere and the correction is going into the edge not
the center.  Which does not match with what should
happen if you worked 15-20 minutes with mirror on top
and a Porabolizing W. You should have gotten a hole in
the center and not a sphere I think.

So better to start measuring.   You need to anyway in
the long run.

Dale Eason

--- Bryan Stiles <bryan.stiles@gmail.com> 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
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
> 



		
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