Re: [ATM] Pitch flow during figuring

["Jerry" <wa4guu@verizon.net>]

Yep! Figuring is a touchy feely operation.

I envision that the lap flows, and it flexes. I think of flowing as the
pitch has been moved by some force to a position and if the force is removed
the pitch will pretty much stay where it has been moved to. Flexing is
movement under some force that if removed the pitch will spring back to (or
close to) its previous position prior to the force. Flow is a long-term
conformation to a changed mirror shape (shape changed by the work.) Flex is
a short-term conformation of the pitch to the shape of the part of the glass
it is in contact with any moment during a stroke.

As the sub-diameter lap doesn't have to flow/flex as far as a large lap
would to maintain contact, working slower gives the lap more time to
flow/flex farther (or further?) to maintain contact. I think that is one
reasonable explanation of why slow = smooth. 

It does all boil down to good contact. You can feel that too.

Jerry


-----Original Message-----
From: Mike Lockwood
 
Hi,

Jerry wrote:
> As to curves not being a prefect match as soon as you push the mirror or
lap
> off center... In optics there are degrees of "perfect".
> The 8" f:6 parabola has a P-V deviation from a sphere of about 18
millionths
> inch. That is less than 1 wave of green light. Most of the time spent
> figuring is somewhere between the sphere and parabola, The difference of
the
> partially corrected mirror surface and the well pressed lap (perfect match
> to mirror curve) pushed off center will be significantly less than that.
> Only slight flexing of the lap surface of a few millionths inch will keep
it
> in contact with the mirror.

The amazing thing is that you can FEEL that difference in shape 
(during figuring) after you get some correction into the mirror, even 
with just the ~1 wave departure from a sphere on an 8" F/6!

> Larger and/or faster mirrors will have more deviation. A 12 inch f"4
> parabola (which by the way is the same focal length as the 8 inch f:6)
would
> have 5 waves P-V difference from a sphere. You can see that the difference
> has grown significantly. You can imagine (or calculate) what happens going
> larger or faster than the 12 inch f:4. The surface of the perfect fit lap
> will have to flex a good deal more on this one.

Of course this is why the subdiameter lap is the preferred method for 
faster mirrors.  It's not easy to get pitch to flow enough to conform 
to such a mirror without very soft pitch, which itself can cause other 
problems if used improperly.

	Mike Lockwood

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