Re: [ATM] Turntable and weight

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

Jerry wrote:
> That was some ugly mirror. Is it aluminized? I don't know if that
> is etched.

Can't remember if that Cave mirror was coated or not when I took those
photos.

> I can think of other possible explanations. Hot pressed before
> figuring? Etched is as good as any I can think of though. But I
> have to think of how to explain why I don't have that problem and I
> take no special means to avoid it. At least I don't think I do.

I think there are several phenomena at work here involving roughness.

First, there is bad contact - this results in "gouges" in the surface
when examined under the Foucault test.  These tend to be fairly random
in pattern, but can be regular in form if they are due to one bad facet.

Second, there is "etching" as it has been called.  I believe this
happens AFTER polishing is done, when the mixture of glass/Cerox/water
sits on the surface, eating away at the glass under the facets.  This 
could be related to warmed pitch or glass.  In hundreds of presses on 
many mirrors, I have never seen this result from cold pressing BEFORE 
polishing.  (However, something like this could result if the 
cerox/water dried out, and the lap was stuck to the surface and had to 
be removed with lots of force.)  I don't know the chemistry involved, 
but the observed results seem clear to me - don't leave the lap on 
after polishing, remove it immediately and rinse the mirror.

Third, there is leftover polishing residue left on the surface.  This 
either results from incomplete washing after polishing, or from 
allowing the polishing solution to dry on the mirror before it is 
rinsed.  Both must be avoided at all costs, especially during the last 
steps of figuring, or you will have cerium residue on your finished 
mirror!

> I figure using a full size lap and for local correction accented
> pressure. So even for working a single zone the lap still works the
> entire surface. Maybe that is why I don't have a lap print. I could
> be wiping it off.

More likely, you are cleaning up properly.

1) When I conclude polishing/figuring, the first thing I do is spray 
extra water on the surface of the mirror to prevent it from drying. 
Drying can happen VERY FAST if you are working with a thin polishing 
mixture with little water in order to speed up polishing.  If the 
surface dries without rinsing, Cerium stains will be left on the 
mirror, removeable only by more polishing.  These stains WILL throw 
off Foucault readings, and are often invisible to the eye.  However, 
they CAN be felt by running a clean palm across the mirror.

2) After wetting the surface to buy time after polishing is concluded, 
I put aside the lap and rinse the mirror.  I use my hand to actually 
wipe the surface of the mirror.  I find this is NECESSARY to remove 
all the residue, and a soft cloth or paper towel could be used instead 
of a hand.  For some reason, this physical wiping of the surface is 
needed to help remove the remaining residue.

3) After rinsing, I wipe the surface dry with a clean towel, and use 
the palm of my hand to remove any remaining lint/dust.  In doing this 
I can also feel if there is any remaining residue via the friction 
between my hand and the polished surface.  This procedure leaves a 
bright, clean polished surface that will produce accurate Foucault 
test readings.

Maybe someone else can suggest a better cleaning procedure that is 
less work?

	Mike Lockwood

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