[ATM] Ronchi profile technique

["Jerry" <wa4guu@verizon.net>]

Here is a technique to interpret the profile of a mirror surface by Ronchi
test that has worked well for me in determining how to apply local
correction of zones. This is easier to do than to say. Since there are quite
a few Ronchi images available on the ATM free group and Dale and Steve have
presented links to real and simulated Ronchi test you can try this to see
what you think and offer comment.

Of course I'm not going to add anything to the technical or mathematical
stuff of Ronchi. There are plenty of people here smarter than me on that.
Almost all of you are.

This is a practical application only.


For example imagine first that we are looking at a mirror that is about 50
percent correct to parabola but it has some zones to work out. For this
example we will be inside the COC (Center Of Curvature).  Start with a
Ronchi with several lines and look at just one Ronchi band just to the left
or right of center, so we are looking at the same one, let's use the right
side. 

The band to the right of center has a general curve like this   )

The band wiggles a bit as it traverses the zones and the grating is inside
COC.  

We can picture that the band is the solid of the mirror and the edge of the
band is the surface in profile, 
Or we can picture that the light between the bands is the solid and the edge
of that the surface in profile.

Now working either from the center or the edge, just pick one, we can see
the slope changes of the mirror. 
There is just one rule to remember:  The last slope change you interpret in
the opposite direction.
Let's say that again because it will cause grief if you forget when working
near the edge.

THE LAST SLOPE CHANGE YOU INTERPRET IN THE OPPOSITE DIRECTION!!!!

We all know enough to know if the edge is turned down or up or if the center
is a hole or a hill.


If you work from the center out and you have a hole in the center and a
turned down edge as you look at the Ronchi band in the center (meaning the
band to the right of center but look at it near the center of the mirror)
and  and see the profile of the hole in the center of the mirror and work
out all the slopes will be as seen in the bands except that the turn down
will look like a turn up so you have to reverse that last slope change in
you mind. (you would be looking at the left side of the band as the surfac
with the band being the glass)

If you work from the edge inward to the center, and you look at the side of
the band that shows a turn down profile (the right side) when you get to the
center the hole will look like a bump. So again you have to reverse the last
slope change.

The side of the band you choose to look is up to you but you will find that
depending on where the defect is you might choose to look at it different.
You look at it the way that the figuring process would push down the "high"
part of an unwanted bend into the desire curve. 

You are trying to "push" the band into the proper shape.

Once you get the gist of what I'm saying then you will find that you might
want more or fewer line on the mirror depending on where the defects are.
And if you notice on real or simulations of a perfect mirror, if you go far
enough inside COC the parabola like curve of the bands with fewer lines
turns to just a bit of a gull wing shape with the parabola like shape close
to the center but then out nearer the edge the band has a slight curve back
toward straight. You would not want to misunderstand that and figure that
out. Move the grating closer.



Anyone ever played Ronchi this way?   Comments please.

Jerry

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