Re: ATM Homebrew Ronchi screens

["Bob May" <bobmay@nethere.com>]

One of the things that you might want to do is stop by my website (the
nav.to one will be the best) and look at the Mosby test.  There's a program
with it that will generate the Mosby patterns of the Ronchi test.  If you
don't like the relatively low resolution, you can always use the patterns as
the shape of the Ronchi line shape.
Any process that makes a good sharp edge will produce good Ronchi gratings.
I would suspect that the slidemaker will do better than the printer would
although if you took and used a large plotter and then used a regular film
camera to photograph the lines, you will get a good grating that way also.
You might want to make the lines from about 40lp/in. up to whatever the
resolution of the printing device is and that will give you a good set that
can be used for anything up to where the lines start getting ragged.
I might also add that larger sized gratings tend to be more useful as they
can be used with the double pass form where the light source is behind the
grating and shines through it thus making alignment of the grating to a
knife edge not necessary.  You don't even have to limit the number of lines
that the light is shining through either.
As to the use of the Ronchi and Foucault, the Foucault will tell you real
fast if you have a parabola vs. a hyperbola by measuring the center to edge
distance but it's poor telling you if the shape is smooth.  The Ronchi test,
on the other hand is extremely poor when it comes to telling whether the
shape is a parabola or hyperobola but it very quickly tells you if the form
of the shape is correct.
BTW, just tested a Cave mirror last weekend and the shape was nice in the
outer part but it suffered from a hole in the middle half.  The Foucault
test showed that it has a SC of about -2.5 which is a bit strong!
Bob May
My new web space address is http://webu.wigloo.com/bobmay/ or
http://nav.to/bobmay
and my new email address is bobmay@nethere.com
Bob May