Re: [ATM] Phase contrast and the 10 incher.

["Arjan te Marvelde" <arjan.te.marvelde@hetnet.nl>]

Dale, Dominic-Luc,

Maybe I don't understand how the foucault test works, but as far as I know,
the intensity of the returned light is to a large extent related to
measuring in the image of the lightsource, as it is created by the mirror
surface shape. What you normally see as structures on the surface (usually
larger scale) are deviations from the surface shape.
Light that is reflected by just any randomly shaped sub-surface structure
just scatter the light, and hence will be overwhelmed by the in focus image
itself.
If this were not the case, the visibility of the structures must be
independent of foucault focus! This is an easy test to do: just move the KE
a few inches in or out, or don't use a KE at all...

So assuming this statement holds, the only structures that will become
visible in foucault focus would have to be related to the surface, being
either on the surface, in the surface or in an other way follow the shape of
the surface.

Am I making sense?

   Arjan

> I carefully focus on the front surface using a target
> placed on the mirror.  When not focued on the surface
> then features that I am sure are on the surface are
> slightly blurred.  All features show regardless of
> focus.
>
> Perhaps focus is a way to figure out the dept of them.
>  What I recall however is that the finger prints are
> usually sharp.  I will have to check that.  I know the
> finger prints are only on the back surface. I don't
> believe they are etched into the front surface
> although I know it can happen.

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