From: "Dwight K. Elvey" 
To: nilsolof.carlin{at}telia.com
cc: jlerch1{at}tampabay.rr.com, atm{at}shore.net
Reply-To: "Dwight K. Elvey" 


>From: "Nils Olof Carlin" 
>
>James Lerch wrote:
>
>
>
>> 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
>

Hi Nils Olof
 This intensity problem is why I suggested the rate of change
method. It is a little more complicated than the method James is currently
using but should be much more accurate and less effected by issues of light
gradients and even less effected by the uniformity of the light source as
well. It also has the possibility of interpolation between readings for
improved accuracy. It is just a software change and still takes advantage
of the hardware he currently has.
 I'm not totally sure why James' method is so flawed but when
I use a slitless tester, I find that I can't get the same repeatable
numbers until I started looking at the rate change. For the eye/brain, it
is relatively easy because one sees it as the zones blinking that causes an
apparent motion. The direction of motion depends on if the KE is nearer or
farther. When both zone sides seem to blink synchronous, I am dead on. I
can see this effect to much smaller than my micrometer reads out, which is
why I think it is a superior method. Dwight

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