Re: ATM Robo-Foucault, Image intensity, and Changing knife edge readings

[Aplanatic@aol.com]

James,

I agree with Nils Olof.  Here's what's happening:

As you increase the exposure or the source brightness (or decrease the desired pixel value in "Shades of Grey") the lateral position of the KE must cut more deeply into the light from the mirror.  As this total exposure is increased the lateral position of the KE will need to be set increasing far from the optical axis.  Because of the asymmetry in the diffraction integral under this condition, it will force the longitudinal position of the KE inward (for a paraboloid) to achieve left-right balance.  The fix is to insist that the KE cut out half the light, not an arbitrary amount.  We seem to do this unconsciously when making a masked Foucault measurement by eyeball.

-Dave Rowe-


> > At the moment I sit here rather confused.  Robo's goal was to
> eliminate the
> > subjective measurements of "shades of gray".  However Robo was based
> on the
> > theory that a given "UN-MASKED" mirror surface had only one null
> position
> > for each zone radius, independent of light source intensity, or the
> shade of
> > gray used.  This theory seems to be flawed, which doesn't bode well
> for
> > Robo's future!
> 
> The simulation experimets I did rather a long time ago convinced me
> that the intensity profile across the mirror was quite asymmetric, and
> by no means "mirror-imaged" with respect to the zone under measure.
> So, it doesn't surprise me much that you don't get consistent results.
> 
> What *might* work, and at least might be worth trying is to use a thin
> slit instead of a "slitless" configuration, in order to get control
> over the light distribution. Then start measuring the intensity over
> the mirror with the KE well out of the way, as a reference, or
> "flat-field". Then you do that robo routine but go for an intensity of
> *half* the reference at the zone in question, not just any old value.
> With the KE cutting the diffraction peak in half, I believe the
> intensity of the zone is half the reference, but I have no proof (the
> reference image should be reasonably flat in intensity - if not, the
> light distribution from the slit should be attended to).
> 
> At least in simulations, if the KE is at the position for that zone,
> and *on* the optical axis, the light intensities are equal, possibly
> excepting the edge and center areas where diffraction tends to get
> unbalanced, but the fact that the intensities are equal does *not*
> mean that the KE position is that of the wanted zone, and that the KE
> is indeed on the optical axis. In short, I do not see why your version
> should work well unless you go for the correct fraction of the "flat
> field" reference.
> I guess this modification shouldn't be too difficult to try - just put
> an extra KE close to the one you have already, but only 
> over the light
> source, forming a thin slit there.
> 
> Nils Olof