Re single line Ronchi test

[Mark Suchting <masuch@dap.CSIRO.AU>]

On Fri, 20 Oct 1995, Johnston, Steve (S.W.)@T.H. wrote:

>   What is a single line Ronchi grating? It is simply the Foucault knife-edge
> placed somewhat inside or outside the radius of curvature of the mirror. The
> knife-edge acts like a single line Ronchi grating.  Although the mirror edge
> diffraction and multiple overlaping diffraction fringes of a real Ronchi 
> grating
> can make TDE difficult to judge, a single line Ronchi pattern avoids these
> problems while retaining the simplicity of interpretation inherent to Ronchi
> patterns.


Texereau also provides a nice photo of a quantitative test for the turned 
edge. The first diffraction line beside the defocussed knife edge is 
`spaced' at about 2/3 wave. Any zonal iregularities show as `wiggles' in 
the the knife edge and diffraction line. A perturbation of of the width 
of the knife edge to first diff. line is then of course a 2/3 wave 
surface error. This test works very nice ely with laser  light and a null 
test, as you get quantitative information on zonal errors as you approach 
perfection. The mathematics of this effect comes under the heading of 
Fraunhoffer or Fresnel diffraction ( someone refresh my memory ).

A variation of the Ronchi single wire test with even greater sensitivity 
is the Line Diffraction Interferometer published in Applied Optics by the 
late Bill James. A fine transparent line is scribed in a semi transparent 
thin film ( ideally 10% transmission(?). The plate is placed at focus,and the 
spherical diffraction wave created by light incidentally passing the 
`slit' interferes with the wavefront coming directly from the optics 
through the semi transparent film. You see a pair of dark fringes close
to focus and some finer fringes on the 
left and right of the mirror. The fringes have an identical appearance to 
those from a shearing interferometer. 

The test is very sensitive, but 
like the ronchi test is fairly insensitive to all but gross astigmatism. 
However, folks this is a real INTERFEROMETER, and is extremely easy to 
set up and fairly insensitive to vibration. The quantitative aspects of 
the fringes are not so simple but I think the first order fringes are 
similar to a normal interferogram ie 1 fringe = 0.5 wave. Don't be 
fooled, these are true interference fringes not just Frounhoffer 
diffraction like the single wire test. I'll dig out the refernce for 
anyody who is interested. I've been using the line diffraction 
interferometer along with other tests for five years now...

Mark