[ATM] Pitch flow during figuring

[Mike Lockwood <melockwo@uiuc.edu>]

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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