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[ATM] RE: [atm_free] zone one challenge
At least as displayed on my computer the right opening on simulation from A
to O shows the dark light dark from left to right. From D through O it is
obvious. I eliminated the possibility of optical illusion by sweeping a
small (approx 1 mm) aperture across the mask opening and can see that the
light varies dark bright dark across the right opening. In the left opening
I see that pattern but not on all and not so well that I can say for sure it
is not an illusion.
Some things worth considering (at least I think so).
If you have an opening in your mask that is large enough to easily perceive
the crest as it moves through the zone, then I would think that you may as
well go ahead and measure that zone by placing the crest in the center of
the zone. This would be the equivalent of the pin-stick test except instead
of putting the crest on the pin you put it in the middle of the mask
opening.
If you want to test by judging shades of grey, equal illumination or even
the simultaneous action in both openings of the pair when as you traverse
the slit image laterally with the knife, then the mask opening should be
smaller. If you feel you must see the crest then use a Couder mask with
staggered round holes and you can even over-lap the zones a little. A round
mask opening might be superior to the rectangle bent to the zone arc type
opening. I think the round diffraction ring would be less distracting per
square inch of opening.
Now I realize that this would be part of what you are searching for but I
don't think that equal light in the left and right is the only thing that
you look for in Foucault/Couder testing. When the opening is large I don't
think you will have equal light. I think of Foucault/Couder testing
differently depending on how close the curve is. When I start figuring I
might test just by seeking equal shading in the pair of openings. And then
when the curve gets closer you reach a point where you have to be more
precise in testing to decide what to do in figuring. Then I make the fine
determination of knife position by making microscopic back and forth lateral
movements of the knife, all the way from first contact with the returning
light bundle to the point of total obscuration. When the knife is at the
right longitudinal position (by my reckoning) if the knife moving laterally
slooowwwwly into the beam first shadow is seen at the same time in both
openings, and last light. And then half way between those lateral positions
is "for practical purposes" equally illuminated. Barring bends in the figure
in that zone they will be evenly illuminated. within reason.
This method at least gives (me) a more definite longitudinal knife position.
I wouldn't trust this method to be highly accurate if the mask opening were
large enough to easily see a slope change inside the mask opening. I know
that others use this method or something close to it. I'll bet most do but
that is just a guess.
I chose "I" as my favorite. In my defense on this error, I find it
interesting that when I look at your intensity plots across the openings,
that at least to my eye window "I" is more symmetrical. Symmetrical both in
left and right intensity curve shape and the cross-over is in the center.
.....Are you sure you didn't loose track of which is the right one?
Jerry
-----Original Message-----
From: Koehler, Steve [mailto:steve_koehler@securecomputing.com]
Sent: Wednesday, March 30, 2005 3:05 PM
To: atm_free@yahoogroups.com; atm@atmlist.net
Subject: RE: [atm_free] zone one challenge
Jerry,
> If you look at the images closely you will see that what you are seeing is
> something similar to the classical "doughnut" in each mask opening...
> separately. A doughnut, except that it is an arc. Your simulation seems
> to be showing "nulls" either side of center in each mask opening.
> Therefore the center of each of the openings is not the same as the
> average intensity. The average is located a little to the left and right
> of center in both mask openings.
This may be an optical illusion. I updated the page to add a gradient
profile
plot, so you can see better where the average intensity falls within the
opening.
> I'm thinking that the doughnut made into an arc is an artifact of the
> simulation method. Maybe, maybe not, but the shadows are showing a
> doughnut profile in each opening. So I'm thinking an opening was simulated
> on its own and somehow a doughnut like profile generated. Then that
> profile extended around the arc of the zone opening.
The simulation is a full wave-optics simulation of both openings at once. I
believe the simulation is very close to reality. I got started on this path
by
examining some data collected by Dale Eason with his robo platform about a
year
ago. I didn't show this in my report, but it clearly shows something very
strange is happening in the brightness profiles in the vicinity of zone 1
(of 9
zones, in this case). The simulations duplicate this very well.
> But the left and right openings do have a symmetry in their appearance.
> Look at simulation J, The right opening has a central area (center of the
> mask opening) that is light bounded on both sides by darker shades and on
> the left it is dark in the center and lighter to either side. Now the
> central band in each opening maybe is not the same intensity, but the
> center of symmetry in each opening is at the center of the mask openings.
> If that is not an artifact it may say something about the method of
> looking for simultaneous change in intensity in left and right openings
> when jogging the knife laterally across the slit image.
Actually, I disagree. The central bands in the two openings do have the
same
intensity (for images I and J), except that the null is displaced a tiny bit
to
the outside for J. This is because I use the equal area formula for the
center
of the zone, and derived the zero point for the simulation from that.
Perhaps
this demonstrates that the arithmetic mean of the inside and outside zone
radii
is better for the first zone.
> I suspect that the shadow contrast in the simulation is exaggerated. That
> can be hard to simulate when you don't know how I have my monitor
> adjusted. What is the radius on the mask of the outer edge of the opening?
Actually, I thought the contrast was a little low. It turns out that the
contrast increases a lot with a narrower slit, and gets worse for a wider
slit.
I chose to use an example where the contrast was a little sub-optimal. I
believe a bright source with a very narrow slit can help to get zone 1
right.
Here are the zone boundaries I used (in mm):
[1] 33.75000 85.58507 116.23488 140.34347 160.87893 179.07477 195.58506
[8] 210.80621 225.00000
-- Steve Koehler
steve_koehler@securecomputing.com
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